Oracle — SQLAlchemy 2.0 Documentation

Release: 2.0.41 | Release Date: May 14, 2025

# SQLAlchemy 2.0 Documentation

# Oracle

Support for the Oracle Database database.

The following table summarizes current support levels for database release versions.

Supported Oracle Database versions
Support type | Versions
--- | ---
Supported version | 11+
Best effort | 9+

## DBAPI Support

The following dialect/DBAPI options are available. Please refer to individual DBAPI sections for connect information.

- python-oracledb

- cx-Oracle

## Auto Increment Behavior

SQLAlchemy Table objects which include integer primary keys are usually assumed to have “autoincrementing” behavior, meaning they can generate their own primary key values upon INSERT. For use within Oracle Database, two options are available, which are the use of IDENTITY columns (Oracle Database 12 and above only) or the association of a SEQUENCE with the column.

### Specifying GENERATED AS IDENTITY (Oracle Database 12 and above)

Starting from version 12, Oracle Database can make use of identity columns using the `Identity` to specify the autoincrementing behavior:

    t = Table(
        "mytable",
        metadata,
        Column("id", Integer, Identity(start=3), primary_key=True),
        Column(...),
        ...,
    )

The CREATE TABLE for the above `Table` object would be:

    CREATE TABLE mytable (
        id INTEGER GENERATED BY DEFAULT AS IDENTITY (START WITH 3),
        ...,
        PRIMARY KEY (id)
    )

The `Identity` object support many options to control the “autoincrementing” behavior of the column, like the starting value, the incrementing value, etc. In addition to the standard options, Oracle Database supports setting `Identity.always` to `None` to use the default generated mode, rendering GENERATED AS IDENTITY in the DDL. It also supports setting `Identity.on_null` to `True` to specify ON NULL in conjunction with a ‘BY DEFAULT’ identity column.

### Using a SEQUENCE (all Oracle Database versions)

Older version of Oracle Database had no “autoincrement” feature: SQLAlchemy relies upon sequences to produce these values. With the older Oracle Database versions, a sequence must always be explicitly specified to enable autoincrement. This is divergent with the majority of documentation examples which assume the usage of an autoincrement-capable database. To specify sequences, use the sqlalchemy.schema.Sequence object which is passed to a Column construct:

    t = Table(
        "mytable",
        metadata,
        Column("id", Integer, Sequence("id_seq", start=1), primary_key=True),
        Column(...),
        ...,
    )

This step is also required when using table reflection, i.e. autoload_with=engine:

    t = Table(
        "mytable",
        metadata,
        Column("id", Integer, Sequence("id_seq", start=1), primary_key=True),
        autoload_with=engine,
    )

Changed in version 1.4: Added `Identity` construct in a `Column` to specify the option of an autoincrementing column.

## Transaction Isolation Level / Autocommit

Oracle Database supports “READ COMMITTED” and “SERIALIZABLE” modes of isolation. The AUTOCOMMIT isolation level is also supported by the python-oracledb and cx_Oracle dialects.

To set using per-connection execution options:

    connection = engine.connect()
    connection = connection.execution_options(isolation_level="AUTOCOMMIT")

For `READ COMMITTED` and `SERIALIZABLE`, the Oracle Database dialects sets the level at the session level using `ALTER SESSION`, which is reverted back to its default setting when the connection is returned to the connection pool.

Valid values for `isolation_level` include:

- `READ COMMITTED`

- `AUTOCOMMIT`

- `SERIALIZABLE`

Note

The implementation for the `Connection.get_isolation_level()` method as implemented by the Oracle Database dialects necessarily force the start of a transaction using the Oracle Database DBMS_TRANSACTION.LOCAL_TRANSACTION_ID function; otherwise no level is normally readable.

Additionally, the `Connection.get_isolation_level()` method will raise an exception if the `v$transaction` view is not available due to permissions or other reasons, which is a common occurrence in Oracle Database installations.

The python-oracledb and cx_Oracle dialects attempt to call the `Connection.get_isolation_level()` method when the dialect makes its first connection to the database in order to acquire the “default”isolation level. This default level is necessary so that the level can be reset on a connection after it has been temporarily modified using `Connection.execution_options()` method. In the common event that the `Connection.get_isolation_level()` method raises an exception due to `v$transaction` not being readable as well as any other database-related failure, the level is assumed to be “READ COMMITTED”. No warning is emitted for this initial first-connect condition as it is expected to be a common restriction on Oracle databases.

New in version 1.3.16: added support for AUTOCOMMIT to the cx_Oracle dialect as well as the notion of a default isolation level

New in version 1.3.21: Added support for SERIALIZABLE as well as live reading of the isolation level.

Changed in version 1.3.22: In the event that the default isolation level cannot be read due to permissions on the v$transaction view as is common in Oracle installations, the default isolation level is hardcoded to “READ COMMITTED” which was the behavior prior to 1.3.21.

See also

Setting Transaction Isolation Levels including DBAPI Autocommit

## Identifier Casing

In Oracle Database, the data dictionary represents all case insensitive identifier names using UPPERCASE text. This is in contradiction to the expectations of SQLAlchemy, which assume a case insensitive name is represented as lowercase text.

As an example of case insensitive identifier names, consider the following table:

    CREATE TABLE MyTable (Identifier INTEGER PRIMARY KEY)

If you were to ask Oracle Database for information about this table, the table name would be reported as `MYTABLE` and the column name would be reported as `IDENTIFIER`. Compare to most other databases such as PostgreSQL and MySQL which would report these names as `mytable` and `identifier`. The names are not quoted, therefore are case insensitive. The special casing of `MyTable` and `Identifier` would only be maintained if they were quoted in the table definition:

    CREATE TABLE "MyTable" ("Identifier" INTEGER PRIMARY KEY)

When constructing a SQLAlchemy `Table` object, an all lowercase name is considered to be case insensitive. So the following table assumes case insensitive names:

    Table("mytable", metadata, Column("identifier", Integer, primary_key=True))

Whereas when mixed case or UPPERCASE names are used, case sensitivity is assumed:

    Table("MyTable", metadata, Column("Identifier", Integer, primary_key=True))

A similar situation occurs at the database driver level when emitting a textual SQL SELECT statement and looking at column names in the DBAPI `cursor.description` attribute. A database like PostgreSQL will normalize case insensitive names to be lowercase:

    >>> pg_engine = create_engine("postgresql://scott:tiger@localhost/test")
    >>> pg_connection = pg_engine.connect()
    >>> result = pg_connection.exec_driver_sql("SELECT 1 AS SomeName")
    >>> result.cursor.description
    (Column(name='somename', type_code=23),)

Whereas Oracle normalizes them to UPPERCASE:

    >>> oracle_engine = create_engine("oracle+oracledb://scott:tiger@oracle18c/xe")
    >>> oracle_connection = oracle_engine.connect()
    >>> result = oracle_connection.exec_driver_sql(
    ...     "SELECT 1 AS SomeName FROM DUAL"
    ... )
    >>> result.cursor.description
    [('SOMENAME', <DbType DB_TYPE_NUMBER>, 127, None, 0, -127, True)]

In order to achieve cross-database parity for the two cases of a. table reflection and b. textual-only SQL statement round trips, SQLAlchemy performs a step called name normalization when using the Oracle dialect. This process may also apply to other third party dialects that have similar UPPERCASE handling of case insensitive names.

When using name normalization, SQLAlchemy attempts to detect if a name is case insensitive by checking if all characters are UPPERCASE letters only; if so, then it assumes this is a case insensitive name and is delivered as a lowercase name.

For table reflection, a tablename that is seen represented as all UPPERCASE in Oracle Database’s catalog tables will be assumed to have a case insensitive name. This is what allows the `Table` definition to use lower case names and be equally compatible from a reflection point of view on Oracle Database and all other databases such as PostgreSQL and MySQL:

    # matches a table created with CREATE TABLE mytable
    Table("mytable", metadata, autoload_with=some_engine)

Above, the all lowercase name `"mytable"` is case insensitive; it will match a table reported by PostgreSQL as `"mytable"` and a table reported by Oracle as `"MYTABLE"`. If name normalization were not present, it would not be possible for the above `Table` definition to be introspectable in a cross-database way, since we are dealing with a case insensitive name that is not reported by each database in the same way.

Case sensitivity can be forced on in this case, such as if we wanted to represent the quoted tablename `"MYTABLE"` with that exact casing, most simply by using that casing directly, which will be seen as a case sensitive name:

    # matches a table created with CREATE TABLE "MYTABLE"
    Table("MYTABLE", metadata, autoload_with=some_engine)

For the unusual case of a quoted all-lowercase name, the `quoted_name` construct may be used:

    from sqlalchemy import quoted_name

    # matches a table created with CREATE TABLE "mytable"
    Table(
        quoted_name("mytable", quote=True), metadata, autoload_with=some_engine
    )

Name normalization also takes place when handling result sets from purely textual SQL strings, that have no other `Table` or `Column` metadata associated with them. This includes SQL strings executed using `Connection.exec_driver_sql()` and SQL strings executed using the `text()` construct which do not include `Column` metadata.

Returning to the Oracle Database SELECT statement, we see that even though `cursor.description` reports the column name as `SOMENAME`, SQLAlchemy name normalizes this to `somename`:

    >>> oracle_engine = create_engine("oracle+oracledb://scott:tiger@oracle18c/xe")
    >>> oracle_connection = oracle_engine.connect()
    >>> result = oracle_connection.exec_driver_sql(
    ...     "SELECT 1 AS SomeName FROM DUAL"
    ... )
    >>> result.cursor.description
    [('SOMENAME', <DbType DB_TYPE_NUMBER>, 127, None, 0, -127, True)]
    >>> result.keys()
    RMKeyView(['somename'])

The single scenario where the above behavior produces inaccurate results is when using an all-uppercase, quoted name. SQLAlchemy has no way to determine that a particular name in `cursor.description` was quoted, and is therefore case sensitive, or was not quoted, and should be name normalized:

    >>> result = oracle_connection.exec_driver_sql(
    ...     'SELECT 1 AS "SOMENAME" FROM DUAL'
    ... )
    >>> result.cursor.description
    [('SOMENAME', <DbType DB_TYPE_NUMBER>, 127, None, 0, -127, True)]
    >>> result.keys()
    RMKeyView(['somename'])

For this case, a new feature will be available in SQLAlchemy 2.1 to disable the name normalization behavior in specific cases.

## Maximum Identifier Lengths

SQLAlchemy is sensitive to the maximum identifier length supported by Oracle Database. This affects generated SQL label names as well as the generation of constraint names, particularly in the case where the constraint naming convention feature described at Configuring Constraint Naming Conventions is being used.

Oracle Database 12.2 increased the default maximum identifier length from 30 to 128. As of SQLAlchemy 1.4, the default maximum identifier length for the Oracle dialects is 128 characters. Upon first connection, the maximum length actually supported by the database is obtained. In all cases, setting the `create_engine.max_identifier_length` parameter will bypass this change and the value given will be used as is:

    engine = create_engine(
        "oracle+oracledb://scott:tiger@localhost:1521?service_name=freepdb1",
        max_identifier_length=30,
    )

If `create_engine.max_identifier_length` is not set, the oracledb dialect internally uses the `max_identifier_length` attribute available on driver connections since python-oracledb version 2.5. When using an older driver version, or using the cx_Oracle dialect, SQLAlchemy will instead attempt to use the query `SELECT value FROM v$parameter WHERE name = 'compatible'` upon first connect in order to determine the effective compatibility version of the database. The “compatibility” version is a version number that is independent of the actual database version. It is used to assist database migration. It is configured by an Oracle Database initialization parameter. The compatibility version then determines the maximum allowed identifier length for the database. If the V$ view is not available, the database version information is used instead.

The maximum identifier length comes into play both when generating anonymized SQL labels in SELECT statements, but more crucially when generating constraint names from a naming convention. It is this area that has created the need for SQLAlchemy to change this default conservatively. For example, the following naming convention produces two very different constraint names based on the identifier length:

    from sqlalchemy import Column
    from sqlalchemy import Index
    from sqlalchemy import Integer
    from sqlalchemy import MetaData
    from sqlalchemy import Table
    from sqlalchemy.dialects import oracle
    from sqlalchemy.schema import CreateIndex

    m = MetaData(naming_convention={"ix": "ix_%(column_0N_name)s"})

    t = Table(
        "t",
        m,
        Column("some_column_name_1", Integer),
        Column("some_column_name_2", Integer),
        Column("some_column_name_3", Integer),
    )

    ix = Index(
        None,
        t.c.some_column_name_1,
        t.c.some_column_name_2,
        t.c.some_column_name_3,
    )

    oracle_dialect = oracle.dialect(max_identifier_length=30)
    print(CreateIndex(ix).compile(dialect=oracle_dialect))

With an identifier length of 30, the above CREATE INDEX looks like:

    CREATE INDEX ix_some_column_name_1s_70cd ON t
    (some_column_name_1, some_column_name_2, some_column_name_3)

However with length of 128, it becomes:

    .. sourcecode:: sql

> CREATE INDEX ix_some_column_name_1some_column_name_2some_column_name_3 ON t (some_column_name_1, some_column_name_2, some_column_name_3)

Applications which have run versions of SQLAlchemy prior to 1.4 on Oracle Database version 12.2 or greater are therefore subject to the scenario of a database migration that wishes to “DROP CONSTRAINT” on a name that was previously generated with the shorter length. This migration will fail when the identifier length is changed without the name of the index or constraint first being adjusted. Such applications are strongly advised to make use of `create_engine.max_identifier_length` in order to maintain control of the generation of truncated names, and to fully review and test all database migrations in a staging environment when changing this value to ensure that the impact of this change has been mitigated.

Changed in version 1.4: the default max_identifier_length for Oracle Database is 128 characters, which is adjusted down to 30 upon first connect if the Oracle Database, or its compatibility setting, are lower than version 12.2.

## LIMIT/OFFSET/FETCH Support

Methods like `Select.limit()` and `Select.offset()` make use of `FETCH FIRST N ROW / OFFSET N ROWS` syntax assuming Oracle Database 12c or above, and assuming the SELECT statement is not embedded within a compound statement like UNION. This syntax is also available directly by using the `Select.fetch()` method.

Changed in version 2.0: the Oracle Database dialects now use `FETCH FIRST N ROW / OFFSET N ROWS` for all `Select.limit()` and `Select.offset()` usage including within the ORM and legacy `Query`. To force the legacy behavior using window functions, specify the `enable_offset_fetch=False` dialect parameter to `create_engine()`.

The use of `FETCH FIRST / OFFSET` may be disabled on any Oracle Database version by passing `enable_offset_fetch=False` to `create_engine()`, which will force the use of “legacy” mode that makes use of window functions. This mode is also selected automatically when using a version of Oracle Database prior to 12c.

When using legacy mode, or when a `Select` statement with limit/offset is embedded in a compound statement, an emulated approach for LIMIT / OFFSET based on window functions is used, which involves creation of a subquery using `ROW_NUMBER` that is prone to performance issues as well as SQL construction issues for complex statements. However, this approach is supported by all Oracle Database versions. See notes below.

### Notes on LIMIT / OFFSET emulation (when fetch() method cannot be used)

If using `Select.limit()` and `Select.offset()`, or with the ORM the `Query.limit()` and `Query.offset()` methods on an Oracle Database version prior to 12c, the following notes apply:

- SQLAlchemy currently makes use of ROWNUM to achieve LIMIT/OFFSET; the exact methodology is taken from https://blogs.oracle.com/oraclemagazine/on-rownum-and-limiting-results .

- the “FIRST_ROWS()” optimization keyword is not used by default. To enable the usage of this optimization directive, specify `optimize_limits=True` to `create_engine()`.

  Changed in version 1.4: The Oracle Database dialect renders limit/offset integer values using a “post compile” scheme which renders the integer directly before passing the statement to the cursor for execution. The `use_binds_for_limits` flag no longer has an effect.

  See also

  New “post compile” bound parameters used for LIMIT/OFFSET in Oracle, SQL Server.

## RETURNING Support

Oracle Database supports RETURNING fully for INSERT, UPDATE and DELETE statements that are invoked with a single collection of bound parameters (that is, a `cursor.execute()` style statement; SQLAlchemy does not generally support RETURNING with executemany statements). Multiple rows may be returned as well.

Changed in version 2.0: the Oracle Database backend has full support for RETURNING on parity with other backends.

## ON UPDATE CASCADE

Oracle Database doesn’t have native ON UPDATE CASCADE functionality. A trigger based solution is available at https://web.archive.org/web/20090317041251/https://asktom.oracle.com/tkyte/update_cascade/index.html

When using the SQLAlchemy ORM, the ORM has limited ability to manually issue cascading updates - specify ForeignKey objects using the “deferrable=True, initially=’deferred’” keyword arguments, and specify “passive_updates=False” on each relationship().

## Oracle Database 8 Compatibility

Warning

The status of Oracle Database 8 compatibility is not known for SQLAlchemy 2.0.

When Oracle Database 8 is detected, the dialect internally configures itself to the following behaviors:

- the use_ansi flag is set to False. This has the effect of converting all JOIN phrases into the WHERE clause, and in the case of LEFT OUTER JOIN makes use of Oracle’s (+) operator.

- the NVARCHAR2 and NCLOB datatypes are no longer generated as DDL when the `Unicode` is used - VARCHAR2 and CLOB are issued instead. This because these types don’t seem to work correctly on Oracle 8 even though they are available. The `NVARCHAR` and `NCLOB` types will always generate NVARCHAR2 and NCLOB.

## Synonym/DBLINK Reflection

When using reflection with Table objects, the dialect can optionally search for tables indicated by synonyms, either in local or remote schemas or accessed over DBLINK, by passing the flag `oracle_resolve_synonyms=True` as a keyword argument to the `Table` construct:

    some_table = Table(
        "some_table", autoload_with=some_engine, oracle_resolve_synonyms=True
    )

When this flag is set, the given name (such as `some_table` above) will be searched not just in the `ALL_TABLES` view, but also within the `ALL_SYNONYMS` view to see if this name is actually a synonym to another name. If the synonym is located and refers to a DBLINK, the Oracle Database dialects know how to locate the table’s information using DBLINK syntax(e.g. `@dblink`).

`oracle_resolve_synonyms` is accepted wherever reflection arguments are accepted, including methods such as `MetaData.reflect()` and `Inspector.get_columns()`.

If synonyms are not in use, this flag should be left disabled.

## Constraint Reflection

The Oracle Database dialects can return information about foreign key, unique, and CHECK constraints, as well as indexes on tables.

Raw information regarding these constraints can be acquired using `Inspector.get_foreign_keys()`, `Inspector.get_unique_constraints()`, `Inspector.get_check_constraints()`, and `Inspector.get_indexes()`.

Changed in version 1.2: The Oracle Database dialect can now reflect UNIQUE and CHECK constraints.

When using reflection at the `Table` level, the `Table` will also include these constraints.

Note the following caveats:

- When using the `Inspector.get_check_constraints()` method, Oracle Database builds a special “IS NOT NULL” constraint for columns that specify “NOT NULL”. This constraint is not returned by default; to include the “IS NOT NULL” constraints, pass the flag `include_all=True`:

      from sqlalchemy import create_engine, inspect

      engine = create_engine(
          "oracle+oracledb://scott:tiger@localhost:1521?service_name=freepdb1"
      )
      inspector = inspect(engine)
      all_check_constraints = inspector.get_check_constraints(
          "some_table", include_all=True
      )

- in most cases, when reflecting a `Table`, a UNIQUE constraint will not be available as a `UniqueConstraint` object, as Oracle Database mirrors unique constraints with a UNIQUE index in most cases (the exception seems to be when two or more unique constraints represent the same columns); the `Table` will instead represent these using `Index` with the `unique=True` flag set.

- Oracle Database creates an implicit index for the primary key of a table; this index is excluded from all index results.

- the list of columns reflected for an index will not include column names that start with SYS_NC.

## Table names with SYSTEM/SYSAUX tablespaces

The `Inspector.get_table_names()` and `Inspector.get_temp_table_names()` methods each return a list of table names for the current engine. These methods are also part of the reflection which occurs within an operation such as `MetaData.reflect()`. By default, these operations exclude the `SYSTEM` and `SYSAUX` tablespaces from the operation. In order to change this, the default list of tablespaces excluded can be changed at the engine level using the `exclude_tablespaces` parameter:

    # exclude SYSAUX and SOME_TABLESPACE, but not SYSTEM
    e = create_engine(
        "oracle+oracledb://scott:tiger@localhost:1521/?service_name=freepdb1",
        exclude_tablespaces=["SYSAUX", "SOME_TABLESPACE"],
    )

## FLOAT / DOUBLE Support and Behaviors

The SQLAlchemy `Float` and `Double` datatypes are generic datatypes that resolve to the “least surprising” datatype for a given backend. For Oracle Database, this means they resolve to the `FLOAT` and `DOUBLE` types:

    >>> from sqlalchemy import cast, literal, Float
    >>> from sqlalchemy.dialects import oracle
    >>> float_datatype = Float()
    >>> print(cast(literal(5.0), float_datatype).compile(dialect=oracle.dialect()))
    CAST(:param_1 AS FLOAT)

Oracle’s `FLOAT` / `DOUBLE` datatypes are aliases for `NUMBER`. Oracle Database stores `NUMBER` values with full precision, not floating point precision, which means that `FLOAT` / `DOUBLE` do not actually behave like native FP values. Oracle Database instead offers special datatypes `BINARY_FLOAT` and `BINARY_DOUBLE` to deliver real 4- and 8- byte FP values.

SQLAlchemy supports these datatypes directly using `BINARY_FLOAT` and `BINARY_DOUBLE`. To use the `Float` or `Double` datatypes in a database agnostic way, while allowing Oracle backends to utilize one of these types, use the `TypeEngine.with_variant()` method to set up a variant:

    >>> from sqlalchemy import cast, literal, Float
    >>> from sqlalchemy.dialects import oracle
    >>> float_datatype = Float().with_variant(oracle.BINARY_FLOAT(), "oracle")
    >>> print(cast(literal(5.0), float_datatype).compile(dialect=oracle.dialect()))
    CAST(:param_1 AS BINARY_FLOAT)

E.g. to use this datatype in a `Table` definition:

    my_table = Table(
        "my_table",
        metadata,
        Column(
            "fp_data", Float().with_variant(oracle.BINARY_FLOAT(), "oracle")
        ),
    )

## DateTime Compatibility

Oracle Database has no datatype known as `DATETIME`, it instead has only `DATE`, which can actually store a date and time value. For this reason, the Oracle Database dialects provide a type `DATE` which is a subclass of `DateTime`. This type has no special behavior, and is only present as a “marker” for this type; additionally, when a database column is reflected and the type is reported as `DATE`, the time-supporting `DATE` type is used.

## Oracle Database Table Options

The CREATE TABLE phrase supports the following options with Oracle Database dialects in conjunction with the `Table` construct:

- `ON COMMIT`:

      Table(
          "some_table",
          metadata,
          ...,
          prefixes=["GLOBAL TEMPORARY"],
          oracle_on_commit="PRESERVE ROWS",
      )

- `COMPRESS`:

      Table(
          "mytable", metadata, Column("data", String(32)), oracle_compress=True
      )

      Table("mytable", metadata, Column("data", String(32)), oracle_compress=6)

  The `oracle_compress` parameter accepts either an integer compression level, or `True` to use the default compression level.

- `TABLESPACE`:

      Table("mytable", metadata, ..., oracle_tablespace="EXAMPLE_TABLESPACE")

  The `oracle_tablespace` parameter specifies the tablespace in which the table is to be created. This is useful when you want to create a table in a tablespace other than the default tablespace of the user.

  New in version 2.0.37.

## Oracle Database Specific Index Options

### Bitmap Indexes

You can specify the `oracle_bitmap` parameter to create a bitmap index instead of a B-tree index:

    Index("my_index", my_table.c.data, oracle_bitmap=True)

Bitmap indexes cannot be unique and cannot be compressed. SQLAlchemy will not check for such limitations, only the database will.

### Index compression

Oracle Database has a more efficient storage mode for indexes containing lots of repeated values. Use the `oracle_compress` parameter to turn on key compression:

    Index("my_index", my_table.c.data, oracle_compress=True)

    Index(
        "my_index",
        my_table.c.data1,
        my_table.c.data2,
        unique=True,
        oracle_compress=1,
    )

The `oracle_compress` parameter accepts either an integer specifying the number of prefix columns to compress, or `True` to use the default (all columns for non-unique indexes, all but the last column for unique indexes).

## VECTOR Datatype

Oracle Database 23ai introduced a new VECTOR datatype for artificial intelligence and machine learning search operations. The VECTOR datatype is a homogeneous array of 8-bit signed integers, 8-bit unsigned integers (binary), 32-bit floating-point numbers, or 64-bit floating-point numbers.

See also

Using VECTOR Data - in the documentation for the python-oracledb driver.

New in version 2.0.41.

### CREATE TABLE support for VECTOR

With the `VECTOR` datatype, you can specify the dimension for the data and the storage format. Valid values for storage format are enum values from `VectorStorageFormat`. To create a table that includes a `VECTOR` column:

    from sqlalchemy.dialects.oracle import VECTOR, VectorStorageFormat

    t = Table(
        "t1",
        metadata,
        Column("id", Integer, primary_key=True),
        Column(
            "embedding",
            VECTOR(dim=3, storage_format=VectorStorageFormat.FLOAT32),
        ),
        Column(...),
        ...,
    )

Vectors can also be defined with an arbitrary number of dimensions and formats. This allows you to specify vectors of different dimensions with the various storage formats mentioned above.

Examples

- In this case, the storage format is flexible, allowing any vector type data to be inserted, such as INT8 or BINARY etc:

      vector_col: Mapped[array.array] = mapped_column(VECTOR(dim=3))

- The dimension is flexible in this case, meaning that any dimension vector can be used:

      vector_col: Mapped[array.array] = mapped_column(
          VECTOR(storage_format=VectorStorageType.INT8)
      )

- Both the dimensions and the storage format are flexible:

      vector_col: Mapped[array.array] = mapped_column(VECTOR)

### Python Datatypes for VECTOR

VECTOR data can be inserted using Python list or Python `array.array()` objects. Python arrays of type FLOAT (32-bit), DOUBLE (64-bit), or INT (8-bit signed integer) are used as bind values when inserting VECTOR columns:

    from sqlalchemy import insert, select

    with engine.begin() as conn:
        conn.execute(
            insert(t1),
            {"id": 1, "embedding": [1, 2, 3]},
        )

### VECTOR Indexes

The VECTOR feature supports an Oracle-specific parameter `oracle_vector` on the `Index` construct, which allows the construction of VECTOR indexes.

To utilize VECTOR indexing, set the `oracle_vector` parameter to True to use the default values provided by Oracle. HNSW is the default indexing method:

    from sqlalchemy import Index

    Index(
        "vector_index",
        t1.c.embedding,
        oracle_vector=True,
    )

The full range of parameters for vector indexes are available by using the `VectorIndexConfig` dataclass in place of a boolean; this dataclass allows full configuration of the index:

    Index(
        "hnsw_vector_index",
        t1.c.embedding,
        oracle_vector=VectorIndexConfig(
            index_type=VectorIndexType.HNSW,
            distance=VectorDistanceType.COSINE,
            accuracy=90,
            hnsw_neighbors=5,
            hnsw_efconstruction=20,
            parallel=10,
        ),
    )

    Index(
        "ivf_vector_index",
        t1.c.embedding,
        oracle_vector=VectorIndexConfig(
            index_type=VectorIndexType.IVF,
            distance=VectorDistanceType.DOT,
            accuracy=90,
            ivf_neighbor_partitions=5,
        ),
    )

For complete explanation of these parameters, see the Oracle documentation linked below.

See also

CREATE VECTOR INDEX - in the Oracle documentation

### Similarity Searching

When using the `VECTOR` datatype with a `Column` or similar ORM mapped construct, additional comparison functions are available, including:

- `l2_distance`

- `cosine_distance`

- `inner_product`

Example Usage:

    result_vector = connection.scalars(
        select(t1).order_by(t1.embedding.l2_distance([2, 3, 4])).limit(3)
    )

    for user in vector:
        print(user.id, user.embedding)

### FETCH APPROXIMATE support

Approximate vector search can only be performed when all syntax and semantic rules are satisfied, the corresponding vector index is available, and the query optimizer determines to perform it. If any of these conditions are unmet, then an approximate search is not performed. In this case the query returns exact results.

To enable approximate searching during similarity searches on VECTORS, the `oracle_fetch_approximate` parameter may be used with the `Select.fetch()` clause to add `FETCH APPROX` to the SELECT statement:

    select(users_table).fetch(5, oracle_fetch_approximate=True)

## Oracle Database Data Types

As with all SQLAlchemy dialects, all UPPERCASE types that are known to be valid with Oracle Database are importable from the top level dialect, whether they originate from `sqlalchemy.types` or from the local dialect:

    from sqlalchemy.dialects.oracle import (
        BFILE,
        BLOB,
        CHAR,
        CLOB,
        DATE,
        DOUBLE_PRECISION,
        FLOAT,
        INTERVAL,
        LONG,
        NCLOB,
        NCHAR,
        NUMBER,
        NVARCHAR,
        NVARCHAR2,
        RAW,
        TIMESTAMP,
        VARCHAR,
        VARCHAR2,
        VECTOR,
    )

New in version 1.2.19: Added `NCHAR` to the list of datatypes exported by the Oracle dialect.

Types which are specific to Oracle Database, or have Oracle-specific construction arguments, are as follows:

Object Name | Description
--- | ---
BFILE |
BINARY_DOUBLE | Implement the Oracle `BINARY_DOUBLE` datatype.
BINARY_FLOAT | Implement the Oracle `BINARY_FLOAT` datatype.
DATE | Provide the Oracle Database DATE type.
FLOAT | Oracle Database FLOAT.
INTERVAL |
LONG |
NCLOB |
NUMBER |
NVARCHAR2 | alias of `NVARCHAR`
RAW |
ROWID | Oracle Database ROWID type.
TIMESTAMP | Oracle Database implementation of `TIMESTAMP`, which supports additional Oracle Database-specific modes
VECTOR | Oracle VECTOR datatype.
VectorDistanceType | Enum representing different types of vector distance metrics.
VectorIndexConfig | Define the configuration for Oracle VECTOR Index.
VectorIndexType | Enum representing different types of VECTOR index structures.
VectorStorageFormat | Enum representing the data format used to store vector components.

class sqlalchemy.dialects.oracle.BFILE

  Members

  __init__()

  Class signature

  class `sqlalchemy.dialects.oracle.BFILE` (`sqlalchemy.types.LargeBinary`)

  method `sqlalchemy.dialects.oracle.BFILE.` __init__(length: int | None = None)

    inherited from the `sqlalchemy.types.LargeBinary.__init__` method of `LargeBinary`

    Construct a LargeBinary type.

    Parameters:

      length – optional, a length for the column for use in DDL statements, for those binary types that accept a length, such as the MySQL BLOB type.

class sqlalchemy.dialects.oracle.BINARY_DOUBLE

  Implement the Oracle `BINARY_DOUBLE` datatype.

  This datatype differs from the Oracle `DOUBLE` datatype in that it delivers a true 8-byte FP value. The datatype may be combined with a generic `Double` datatype using `TypeEngine.with_variant()`.

  See also

  FLOAT / DOUBLE Support and Behaviors

  Members

  __init__()

  Class signature

  class `sqlalchemy.dialects.oracle.BINARY_DOUBLE` (`sqlalchemy.types.Double`)

  method `sqlalchemy.dialects.oracle.BINARY_DOUBLE.` __init__(precision: int | None = None, asdecimal: bool = False, decimal_return_scale: int | None = None)

    inherited from the `sqlalchemy.types.Float.__init__` method of `Float`

    Construct a Float.

    Parameters:

      - precision –

        the numeric precision for use in DDL `CREATE TABLE`. Backends should attempt to ensure this precision indicates a number of digits for the generic `Float` datatype.

        Note

        For the Oracle Database backend, the `Float.precision` parameter is not accepted when rendering DDL, as Oracle Database does not support float precision specified as a number of decimal places. Instead, use the Oracle Database-specific `FLOAT` datatype and specify the `FLOAT.binary_precision` parameter. This is new in version 2.0 of SQLAlchemy.

        To create a database agnostic `Float` that separately specifies binary precision for Oracle Database, use `TypeEngine.with_variant()` as follows:

            from sqlalchemy import Column
            from sqlalchemy import Float
            from sqlalchemy.dialects import oracle

            Column(
                "float_data",
                Float(5).with_variant(oracle.FLOAT(binary_precision=16), "oracle"),
            )

      - asdecimal – the same flag as that of `Numeric`, but defaults to `False`. Note that setting this flag to `True` results in floating point conversion.

      - decimal_return_scale – Default scale to use when converting from floats to Python decimals. Floating point values will typically be much longer due to decimal inaccuracy, and most floating point database types don’t have a notion of “scale”, so by default the float type looks for the first ten decimal places when converting. Specifying this value will override that length. Note that the MySQL float types, which do include “scale”, will use “scale” as the default for decimal_return_scale, if not otherwise specified.

class sqlalchemy.dialects.oracle.BINARY_FLOAT

  Implement the Oracle `BINARY_FLOAT` datatype.

  This datatype differs from the Oracle `FLOAT` datatype in that it delivers a true 4-byte FP value. The datatype may be combined with a generic `Float` datatype using `TypeEngine.with_variant()`.

  See also

  FLOAT / DOUBLE Support and Behaviors

  Members

  __init__()

  Class signature

  class `sqlalchemy.dialects.oracle.BINARY_FLOAT` (`sqlalchemy.types.Float`)

  method `sqlalchemy.dialects.oracle.BINARY_FLOAT.` __init__(precision: int | None = None, asdecimal: bool = False, decimal_return_scale: int | None = None)

    inherited from the `sqlalchemy.types.Float.__init__` method of `Float`

    Construct a Float.

    Parameters:

      - precision –

        the numeric precision for use in DDL `CREATE TABLE`. Backends should attempt to ensure this precision indicates a number of digits for the generic `Float` datatype.

        Note

        For the Oracle Database backend, the `Float.precision` parameter is not accepted when rendering DDL, as Oracle Database does not support float precision specified as a number of decimal places. Instead, use the Oracle Database-specific `FLOAT` datatype and specify the `FLOAT.binary_precision` parameter. This is new in version 2.0 of SQLAlchemy.

        To create a database agnostic `Float` that separately specifies binary precision for Oracle Database, use `TypeEngine.with_variant()` as follows:

            from sqlalchemy import Column
            from sqlalchemy import Float
            from sqlalchemy.dialects import oracle

            Column(
                "float_data",
                Float(5).with_variant(oracle.FLOAT(binary_precision=16), "oracle"),
            )

      - asdecimal – the same flag as that of `Numeric`, but defaults to `False`. Note that setting this flag to `True` results in floating point conversion.

      - decimal_return_scale – Default scale to use when converting from floats to Python decimals. Floating point values will typically be much longer due to decimal inaccuracy, and most floating point database types don’t have a notion of “scale”, so by default the float type looks for the first ten decimal places when converting. Specifying this value will override that length. Note that the MySQL float types, which do include “scale”, will use “scale” as the default for decimal_return_scale, if not otherwise specified.

class sqlalchemy.dialects.oracle.DATE

  Provide the Oracle Database DATE type.

  This type has no special Python behavior, except that it subclasses `DateTime`; this is to suit the fact that the Oracle Database `DATE` type supports a time value.

  Members

  __init__()

  Class signature

  class `sqlalchemy.dialects.oracle.DATE` (`sqlalchemy.dialects.oracle.types._OracleDateLiteralRender`, `sqlalchemy.types.DateTime`)

  method `sqlalchemy.dialects.oracle.DATE.` __init__(timezone: bool = False)

    inherited from the `sqlalchemy.types.DateTime.__init__` method of `DateTime`

    Construct a new `DateTime`.

    Parameters:

      timezone – boolean. Indicates that the datetime type should enable timezone support, if available on the base date/time-holding type only. It is recommended to make use of the `TIMESTAMP` datatype directly when using this flag, as some databases include separate generic date/time-holding types distinct from the timezone-capable TIMESTAMP datatype, such as Oracle Database.

class sqlalchemy.dialects.oracle.FLOAT

  Oracle Database FLOAT.

  This is the same as `FLOAT` except that an Oracle Database -specific `FLOAT.binary_precision` parameter is accepted, and the `Float.precision` parameter is not accepted.

  Oracle Database FLOAT types indicate precision in terms of “binary precision”, which defaults to 126. For a REAL type, the value is 63. This parameter does not cleanly map to a specific number of decimal places but is roughly equivalent to the desired number of decimal places divided by 0.3103.

  New in version 2.0.

  Members

  __init__()

  Class signature

  class `sqlalchemy.dialects.oracle.FLOAT` (`sqlalchemy.types.FLOAT`)

  method `sqlalchemy.dialects.oracle.FLOAT.` __init__(binary_precision=None, asdecimal=False, decimal_return_scale=None)

    Construct a FLOAT

    Parameters:

      - binary_precision – Oracle Database binary precision value to be rendered in DDL. This may be approximated to the number of decimal characters using the formula “decimal precision = 0.30103 * binary precision”. The default value used by Oracle Database for FLOAT / DOUBLE PRECISION is 126.

      - asdecimal – See `Float.asdecimal`

      - decimal_return_scale – See `Float.decimal_return_scale`

class sqlalchemy.dialects.oracle.INTERVAL

  Members

  __init__()

  Class signature

  class `sqlalchemy.dialects.oracle.INTERVAL` (`sqlalchemy.types.NativeForEmulated`, `sqlalchemy.types._AbstractInterval`)

  method `sqlalchemy.dialects.oracle.INTERVAL.` __init__(day_precision=None, second_precision=None)

    Construct an INTERVAL.

    Note that only DAY TO SECOND intervals are currently supported. This is due to a lack of support for YEAR TO MONTH intervals within available DBAPIs.

    Parameters:

      - day_precision – the day precision value. this is the number of digits to store for the day field. Defaults to “2”

      - second_precision – the second precision value. this is the number of digits to store for the fractional seconds field. Defaults to “6”.

class sqlalchemy.dialects.oracle.NCLOB

  Members

  __init__()

  Class signature

  class `sqlalchemy.dialects.oracle.NCLOB` (`sqlalchemy.types.Text`)

  method `sqlalchemy.dialects.oracle.NCLOB.` __init__(length: int | None = None, collation: str | None = None)

    inherited from the `sqlalchemy.types.String.__init__` method of `String`

    Create a string-holding type.

    Parameters:

      - length – optional, a length for the column for use in DDL and CAST expressions. May be safely omitted if no `CREATE TABLE` will be issued. Certain databases may require a `length` for use in DDL, and will raise an exception when the `CREATE TABLE` DDL is issued if a `VARCHAR` with no length is included. Whether the value is interpreted as bytes or characters is database specific.

      - collation –

        Optional, a column-level collation for use in DDL and CAST expressions. Renders using the COLLATE keyword supported by SQLite, MySQL, and PostgreSQL. E.g.:

            >>> from sqlalchemy import cast, select, String
            >>> print(select(cast("some string", String(collation="utf8"))))
            SELECT CAST(:param_1 AS VARCHAR COLLATE utf8) AS anon_1

        Note

        In most cases, the `Unicode` or `UnicodeText` datatypes should be used for a `Column` that expects to store non-ascii data. These datatypes will ensure that the correct types are used on the database.

attribute `sqlalchemy.dialects.oracle..`sqlalchemy.dialects.oracle.NVARCHAR2

  alias of `NVARCHAR`

class sqlalchemy.dialects.oracle.NUMBER

  Class signature

  class `sqlalchemy.dialects.oracle.NUMBER` (`sqlalchemy.types.Numeric`, `sqlalchemy.types.Integer`)

class sqlalchemy.dialects.oracle.LONG

  Members

  __init__()

  Class signature

  class `sqlalchemy.dialects.oracle.LONG` (`sqlalchemy.types.Text`)

  method `sqlalchemy.dialects.oracle.LONG.` __init__(length: int | None = None, collation: str | None = None)

    inherited from the `sqlalchemy.types.String.__init__` method of `String`

    Create a string-holding type.

    Parameters:

      - length – optional, a length for the column for use in DDL and CAST expressions. May be safely omitted if no `CREATE TABLE` will be issued. Certain databases may require a `length` for use in DDL, and will raise an exception when the `CREATE TABLE` DDL is issued if a `VARCHAR` with no length is included. Whether the value is interpreted as bytes or characters is database specific.

      - collation –

        Optional, a column-level collation for use in DDL and CAST expressions. Renders using the COLLATE keyword supported by SQLite, MySQL, and PostgreSQL. E.g.:

            >>> from sqlalchemy import cast, select, String
            >>> print(select(cast("some string", String(collation="utf8"))))
            SELECT CAST(:param_1 AS VARCHAR COLLATE utf8) AS anon_1

        Note

        In most cases, the `Unicode` or `UnicodeText` datatypes should be used for a `Column` that expects to store non-ascii data. These datatypes will ensure that the correct types are used on the database.

class sqlalchemy.dialects.oracle.RAW

  Class signature

  class `sqlalchemy.dialects.oracle.RAW` (`sqlalchemy.types._Binary`)

class sqlalchemy.dialects.oracle.ROWID

  Oracle Database ROWID type.

  When used in a cast() or similar, generates ROWID.

  Class signature

  class `sqlalchemy.dialects.oracle.ROWID` (`sqlalchemy.types.TypeEngine`)

class sqlalchemy.dialects.oracle.TIMESTAMP

  Oracle Database implementation of `TIMESTAMP`, which supports additional Oracle Database-specific modes

  New in version 2.0.

  Members

  __init__()

  Class signature

  class `sqlalchemy.dialects.oracle.TIMESTAMP` (`sqlalchemy.types.TIMESTAMP`)

  method `sqlalchemy.dialects.oracle.TIMESTAMP.` __init__(timezone: bool = False, local_timezone: bool = False)

    Construct a new `TIMESTAMP`.

    Parameters:

      - timezone – boolean. Indicates that the TIMESTAMP type should use Oracle Database’s `TIMESTAMP WITH TIME ZONE` datatype.

      - local_timezone – boolean. Indicates that the TIMESTAMP type should use Oracle Database’s `TIMESTAMP WITH LOCAL TIME ZONE` datatype.

class sqlalchemy.dialects.oracle.VECTOR

  Oracle VECTOR datatype.

  For complete background on using this type, see VECTOR Datatype.

  New in version 2.0.41.

  Members

  __init__()

  Class signature

  class `sqlalchemy.dialects.oracle.VECTOR` (`sqlalchemy.types.TypeEngine`)

  method `sqlalchemy.dialects.oracle.VECTOR.` __init__(dim=None, storage_format=None)

    Construct a VECTOR.

    Parameters:

      - dim – integer. The dimension of the VECTOR datatype. This should be an integer value.

      - storage_format – VectorStorageFormat. The VECTOR storage type format. This may be Enum values form `VectorStorageFormat` INT8, BINARY, FLOAT32, or FLOAT64.

class sqlalchemy.dialects.oracle.VectorIndexType

  Enum representing different types of VECTOR index structures.

  See VECTOR Datatype for background.

  New in version 2.0.41.

  Members

  HNSW, IVF

  Class signature

  class `sqlalchemy.dialects.oracle.VectorIndexType` (`enum.Enum`)

  attribute `sqlalchemy.dialects.oracle.VectorIndexType.` HNSW = 'HNSW'

    The HNSW (Hierarchical Navigable Small World) index type.

  attribute `sqlalchemy.dialects.oracle.VectorIndexType.` IVF = 'IVF'

    The IVF (Inverted File Index) index type

class sqlalchemy.dialects.oracle.VectorIndexConfig

  Define the configuration for Oracle VECTOR Index.

  See VECTOR Datatype for background.

  New in version 2.0.41.

  Members

  accuracy, distance, hnsw_efconstruction, hnsw_neighbors, index_type, ivf_min_vectors_per_partition, ivf_neighbor_partitions, ivf_sample_per_partition, parallel

  Parameters:

    - index_type – Enum value from `VectorIndexType` Specifies the indexing method. For HNSW, this must be `VectorIndexType.HNSW`.

    - distance – Enum value from `VectorDistanceType` specifies the metric for calculating distance between VECTORS.

    - accuracy – interger. Should be in the range 0 to 100 Specifies the accuracy of the nearest neighbor search during query execution.

    - parallel – integer. Specifies degree of parallelism.

    - hnsw_neighbors – interger. Should be in the range 0 to 2048. Specifies the number of nearest neighbors considered during the search. The attribute `VectorIndexConfig.hnsw_neighbors` is HNSW index specific.

    - hnsw_efconstruction – integer. Should be in the range 0 to 65535. Controls the trade-off between indexing speed and recall quality during index construction. The attribute `VectorIndexConfig.hnsw_efconstruction` is HNSW index specific.

    - ivf_neighbor_partitions – integer. Should be in the range 0 to 10,000,000. Specifies the number of partitions used to divide the dataset. The attribute `VectorIndexConfig.ivf_neighbor_partitions` is IVF index specific.

    - ivf_sample_per_partition – integer. Should be between 1 and `num_vectors / neighbor partitions`. Specifies the number of samples used per partition. The attribute `VectorIndexConfig.ivf_sample_per_partition` is IVF index specific.

    - ivf_min_vectors_per_partition – integer. From 0 (no trimming) to the total number of vectors (results in 1 partition). Specifies the minimum number of vectors per partition. The attribute `VectorIndexConfig.ivf_min_vectors_per_partition` is IVF index specific.

  attribute `sqlalchemy.dialects.oracle.VectorIndexConfig.` accuracy: int | None = None
  attribute `sqlalchemy.dialects.oracle.VectorIndexConfig.` distance: VectorDistanceType | None = None
  attribute `sqlalchemy.dialects.oracle.VectorIndexConfig.` hnsw_efconstruction: int | None = None
  attribute `sqlalchemy.dialects.oracle.VectorIndexConfig.` hnsw_neighbors: int | None = None
  attribute `sqlalchemy.dialects.oracle.VectorIndexConfig.` index_type: VectorIndexType = 'HNSW'
  attribute `sqlalchemy.dialects.oracle.VectorIndexConfig.` ivf_min_vectors_per_partition: int | None = None
  attribute `sqlalchemy.dialects.oracle.VectorIndexConfig.` ivf_neighbor_partitions: int | None = None
  attribute `sqlalchemy.dialects.oracle.VectorIndexConfig.` ivf_sample_per_partition: int | None = None
  attribute `sqlalchemy.dialects.oracle.VectorIndexConfig.` parallel: int | None = None

class sqlalchemy.dialects.oracle.VectorStorageFormat

  Enum representing the data format used to store vector components.

  See VECTOR Datatype for background.

  New in version 2.0.41.

  Members

  BINARY, FLOAT32, FLOAT64, INT8

  Class signature

  class `sqlalchemy.dialects.oracle.VectorStorageFormat` (`enum.Enum`)

  attribute `sqlalchemy.dialects.oracle.VectorStorageFormat.` BINARY = 'BINARY'

    Binary format.

  attribute `sqlalchemy.dialects.oracle.VectorStorageFormat.` FLOAT32 = 'FLOAT32'

    32-bit floating-point format.

  attribute `sqlalchemy.dialects.oracle.VectorStorageFormat.` FLOAT64 = 'FLOAT64'

    64-bit floating-point format.

  attribute `sqlalchemy.dialects.oracle.VectorStorageFormat.` INT8 = 'INT8'

    8-bit integer format.

class sqlalchemy.dialects.oracle.VectorDistanceType

  Enum representing different types of vector distance metrics.

  See VECTOR Datatype for background.

  New in version 2.0.41.

  Members

  COSINE, DOT, EUCLIDEAN, MANHATTAN

  Class signature

  class `sqlalchemy.dialects.oracle.VectorDistanceType` (`enum.Enum`)

  attribute `sqlalchemy.dialects.oracle.VectorDistanceType.` COSINE = 'COSINE'

    Cosine similarity.

    Measures the cosine of the angle between two vectors.

  attribute `sqlalchemy.dialects.oracle.VectorDistanceType.` DOT = 'DOT'

    Dot product similarity.

    Measures the algebraic similarity between two vectors.

  attribute `sqlalchemy.dialects.oracle.VectorDistanceType.` EUCLIDEAN = 'EUCLIDEAN'

    Euclidean distance (L2 norm).

    Measures the straight-line distance between two vectors in space.

  attribute `sqlalchemy.dialects.oracle.VectorDistanceType.` MANHATTAN = 'MANHATTAN'

    Manhattan distance (L1 norm).

    Calculates the sum of absolute differences across dimensions.

## python-oracledb

Support for the Oracle Database database via the python-oracledb driver.

### DBAPI

Documentation and download information (if applicable) for python-oracledb is available at: https://oracle.github.io/python-oracledb/

### Connecting

Connect String:

    oracle+oracledb://user:pass@hostname:port[/dbname][?service_name=<service>[&key=value&key=value...]]

### Description

Python-oracledb is the Oracle Database driver for Python. It features a default “thin” client mode that requires no dependencies, and an optional “thick” mode that uses Oracle Client libraries. It supports SQLAlchemy features including two phase transactions and Asyncio.

Python-oracle is the renamed, updated cx_Oracle driver. Oracle is no longer doing any releases in the cx_Oracle namespace.

The SQLAlchemy `oracledb` dialect provides both a sync and an async implementation under the same dialect name. The proper version is selected depending on how the engine is created:

- calling `create_engine()` with `oracle+oracledb://...` will automatically select the sync version:

      from sqlalchemy import create_engine

      sync_engine = create_engine(
          "oracle+oracledb://scott:tiger@localhost?service_name=FREEPDB1"
      )

- calling `create_async_engine()` with `oracle+oracledb://...` will automatically select the async version:

      from sqlalchemy.ext.asyncio import create_async_engine

      asyncio_engine = create_async_engine(
          "oracle+oracledb://scott:tiger@localhost?service_name=FREEPDB1"
      )

  The asyncio version of the dialect may also be specified explicitly using the `oracledb_async` suffix:

      from sqlalchemy.ext.asyncio import create_async_engine

      asyncio_engine = create_async_engine(
          "oracle+oracledb_async://scott:tiger@localhost?service_name=FREEPDB1"
      )

New in version 2.0.25: added support for the async version of oracledb.

### Thick mode support

By default, the python-oracledb driver runs in a “thin” mode that does not require Oracle Client libraries to be installed. The driver also supports a “thick” mode that uses Oracle Client libraries to get functionality such as Oracle Application Continuity.

To enable thick mode, call oracledb.init_oracle_client() explicitly, or pass the parameter `thick_mode=True` to `create_engine()`. To pass custom arguments to `init_oracle_client()`, like the `lib_dir` path, a dict may be passed, for example:

    engine = sa.create_engine(
        "oracle+oracledb://...",
        thick_mode={
            "lib_dir": "/path/to/oracle/client/lib",
            "config_dir": "/path/to/network_config_file_directory",
            "driver_name": "my-app : 1.0.0",
        },
    )

Note that passing a `lib_dir` path should only be done on macOS or Windows. On Linux it does not behave as you might expect.

See also

python-oracledb documentation Enabling python-oracledb Thick mode

### Connecting to Oracle Database

python-oracledb provides several methods of indicating the target database. The dialect translates from a series of different URL forms.

Given the hostname, port and service name of the target database, you can connect in SQLAlchemy using the `service_name` query string parameter:

    engine = create_engine(
        "oracle+oracledb://scott:tiger@hostname:port?service_name=myservice"
    )

#### Connecting with Easy Connect strings

You can pass any valid python-oracledb connection string as the `dsn` key value in a `create_engine.connect_args` dictionary. See python-oracledb documentation Oracle Net Services Connection Strings.

For example to use an Easy Connect string with a timeout to prevent connection establishment from hanging if the network transport to the database cannot be establishd in 30 seconds, and also setting a keep-alive time of 60 seconds to stop idle network connections from being terminated by a firewall:

    e = create_engine(
        "oracle+oracledb://@",
        connect_args={
            "user": "scott",
            "password": "tiger",
            "dsn": "hostname:port/myservice?transport_connect_timeout=30&expire_time=60",
        },
    )

The Easy Connect syntax has been enhanced during the life of Oracle Database. Review the documentation for your database version. The current documentation is at Understanding the Easy Connect Naming Method.

The general syntax is similar to:

    [[protocol:]//]host[:port][/[service_name]][?parameter_name=value{&parameter_name=value}]

Note that although the SQLAlchemy URL syntax `hostname:port/dbname` looks like Oracle’s Easy Connect syntax, it is different. SQLAlchemy’s URL requires a system identifier (SID) for the `dbname` component:

    engine = create_engine("oracle+oracledb://scott:tiger@hostname:port/sid")

Easy Connect syntax does not support SIDs. It uses services names, which are the preferred choice for connecting to Oracle Database.

#### Passing python-oracledb connect arguments

Other python-oracledb driver connection options can be passed in `connect_args`. For example:

    e = create_engine(
        "oracle+oracledb://@",
        connect_args={
            "user": "scott",
            "password": "tiger",
            "dsn": "hostname:port/myservice",
            "events": True,
            "mode": oracledb.AUTH_MODE_SYSDBA,
        },
    )

#### Connecting with tnsnames.ora TNS aliases

If no port, database name, or service name is provided, the dialect will use an Oracle Database DSN “connection string”. This takes the “hostname” portion of the URL as the data source name. For example, if the `tnsnames.ora` file contains a TNS Alias of `myalias` as below:

    myalias =
      (DESCRIPTION =
        (ADDRESS = (PROTOCOL = TCP)(HOST = mymachine.example.com)(PORT = 1521))
        (CONNECT_DATA =
          (SERVER = DEDICATED)
          (SERVICE_NAME = orclpdb1)
        )
      )

The python-oracledb dialect connects to this database service when `myalias` is the hostname portion of the URL, without specifying a port, database name or `service_name`:

    engine = create_engine("oracle+oracledb://scott:tiger@myalias")

#### Connecting to Oracle Autonomous Database

Users of Oracle Autonomous Database should use either use the TNS Alias URL shown above, or pass the TNS Alias as the `dsn` key value in a `create_engine.connect_args` dictionary.

If Oracle Autonomous Database is configured for mutual TLS (“mTLS”) connections, then additional configuration is required as shown in Connecting to Oracle Cloud Autonomous Databases. In summary, Thick mode users should configure file locations and set the wallet path in `sqlnet.ora` appropriately:

    e = create_engine(
        "oracle+oracledb://@",
        thick_mode={
            # directory containing tnsnames.ora and cwallet.so
            "config_dir": "/opt/oracle/wallet_dir",
        },
        connect_args={
            "user": "scott",
            "password": "tiger",
            "dsn": "mydb_high",
        },
    )

Thin mode users of mTLS should pass the appropriate directories and PEM wallet password when creating the engine, similar to:

    e = create_engine(
        "oracle+oracledb://@",
        connect_args={
            "user": "scott",
            "password": "tiger",
            "dsn": "mydb_high",
            "config_dir": "/opt/oracle/wallet_dir",  # directory containing tnsnames.ora
            "wallet_location": "/opt/oracle/wallet_dir",  # directory containing ewallet.pem
            "wallet_password": "top secret",  # password for the PEM file
        },
    )

Typically `config_dir` and `wallet_location` are the same directory, which is where the Oracle Autonomous Database wallet zip file was extracted. Note this directory should be protected.

### Connection Pooling

Applications with multiple concurrent users should use connection pooling. A minimal sized connection pool is also beneficial for long-running, single-user applications that do not frequently use a connection.

The python-oracledb driver provides its own connection pool implementation that may be used in place of SQLAlchemy’s pooling functionality. The driver pool gives support for high availability features such as dead connection detection, connection draining for planned database downtime, support for Oracle Application Continuity and Transparent Application Continuity, and gives support for Database Resident Connection Pooling (DRCP).

To take advantage of python-oracledb’s pool, use the `create_engine.creator` parameter to provide a function that returns a new connection, along with setting `create_engine.pool_class` to `NullPool` to disable SQLAlchemy’s pooling:

    import oracledb
    from sqlalchemy import create_engine
    from sqlalchemy import text
    from sqlalchemy.pool import NullPool

    # Uncomment to use the optional python-oracledb Thick mode.
    # Review the python-oracledb doc for the appropriate parameters
    # oracledb.init_oracle_client(<your parameters>)

    pool = oracledb.create_pool(
        user="scott",
        password="tiger",
        dsn="localhost:1521/freepdb1",
        min=1,
        max=4,
        increment=1,
    )
    engine = create_engine(
        "oracle+oracledb://", creator=pool.acquire, poolclass=NullPool
    )

The above engine may then be used normally. Internally, python-oracledb handles connection pooling:

    with engine.connect() as conn:
        print(conn.scalar(text("select 1 from dual")))

Refer to the python-oracledb documentation for oracledb.create_pool() for the arguments that can be used when creating a connection pool.

### Using Oracle Database Resident Connection Pooling (DRCP)

When using Oracle Database’s Database Resident Connection Pooling (DRCP), the best practice is to specify a connection class and “purity”. Refer to the python-oracledb documentation on DRCP. For example:

    import oracledb
    from sqlalchemy import create_engine
    from sqlalchemy import text
    from sqlalchemy.pool import NullPool

    # Uncomment to use the optional python-oracledb Thick mode.
    # Review the python-oracledb doc for the appropriate parameters
    # oracledb.init_oracle_client(<your parameters>)

    pool = oracledb.create_pool(
        user="scott",
        password="tiger",
        dsn="localhost:1521/freepdb1",
        min=1,
        max=4,
        increment=1,
        cclass="MYCLASS",
        purity=oracledb.PURITY_SELF,
    )
    engine = create_engine(
        "oracle+oracledb://", creator=pool.acquire, poolclass=NullPool
    )

The above engine may then be used normally where python-oracledb handles application connection pooling and Oracle Database additionally uses DRCP:

    with engine.connect() as conn:
        print(conn.scalar(text("select 1 from dual")))

If you wish to use different connection classes or purities for different connections, then wrap `pool.acquire()`:

    import oracledb
    from sqlalchemy import create_engine
    from sqlalchemy import text
    from sqlalchemy.pool import NullPool

    # Uncomment to use python-oracledb Thick mode.
    # Review the python-oracledb doc for the appropriate parameters
    # oracledb.init_oracle_client(<your parameters>)

    pool = oracledb.create_pool(
        user="scott",
        password="tiger",
        dsn="localhost:1521/freepdb1",
        min=1,
        max=4,
        increment=1,
        cclass="MYCLASS",
        purity=oracledb.PURITY_SELF,
    )

    def creator():
        return pool.acquire(cclass="MYOTHERCLASS", purity=oracledb.PURITY_NEW)

    engine = create_engine(
        "oracle+oracledb://", creator=creator, poolclass=NullPool
    )

### Engine Options consumed by the SQLAlchemy oracledb dialect outside of the driver

There are also options that are consumed by the SQLAlchemy oracledb dialect itself. These options are always passed directly to `create_engine()`, such as:

    e = create_engine("oracle+oracledb://user:pass@tnsalias", arraysize=500)

The parameters accepted by the oracledb dialect are as follows:

- `arraysize` - set the driver cursor.arraysize value. It defaults to `None`, indicating that the driver default value of 100 should be used. This setting controls how many rows are buffered when fetching rows, and can have a significant effect on performance if increased for queries that return large numbers of rows.

  Changed in version 2.0.26: - changed the default value from 50 to None, to use the default value of the driver itself.

- `auto_convert_lobs` - defaults to True; See LOB Datatypes.

- `coerce_to_decimal` - see Precision Numerics for detail.

- `encoding_errors` - see Encoding Errors for detail.

### Unicode

As is the case for all DBAPIs under Python 3, all strings are inherently Unicode strings.

#### Ensuring the Correct Client Encoding

In python-oracledb, the encoding used for all character data is “UTF-8”.

#### Unicode-specific Column datatypes

The Core expression language handles unicode data by use of the `Unicode` and `UnicodeText` datatypes. These types correspond to the VARCHAR2 and CLOB Oracle Database datatypes by default. When using these datatypes with Unicode data, it is expected that the database is configured with a Unicode-aware character set so that the VARCHAR2 and CLOB datatypes can accommodate the data.

In the case that Oracle Database is not configured with a Unicode character set, the two options are to use the `NCHAR` and `NCLOB` datatypes explicitly, or to pass the flag `use_nchar_for_unicode=True` to `create_engine()`, which will cause the SQLAlchemy dialect to use NCHAR/NCLOB for the `Unicode` / `UnicodeText` datatypes instead of VARCHAR/CLOB.

Changed in version 1.3: The `Unicode` and `UnicodeText` datatypes now correspond to the `VARCHAR2` and `CLOB` Oracle Database datatypes unless the `use_nchar_for_unicode=True` is passed to the dialect when `create_engine()` is called.

#### Encoding Errors

For the unusual case that data in Oracle Database is present with a broken encoding, the dialect accepts a parameter `encoding_errors` which will be passed to Unicode decoding functions in order to affect how decoding errors are handled. The value is ultimately consumed by the Python decode function, and is passed both via python-oracledb’s `encodingErrors` parameter consumed by `Cursor.var()`, as well as SQLAlchemy’s own decoding function, as the python-oracledb dialect makes use of both under different circumstances.

New in version 1.3.11.

### Fine grained control over python-oracledb data binding with setinputsizes

The python-oracle DBAPI has a deep and fundamental reliance upon the usage of the DBAPI `setinputsizes()` call. The purpose of this call is to establish the datatypes that are bound to a SQL statement for Python values being passed as parameters. While virtually no other DBAPI assigns any use to the `setinputsizes()` call, the python-oracledb DBAPI relies upon it heavily in its interactions with the Oracle Database, and in some scenarios it is not possible for SQLAlchemy to know exactly how data should be bound, as some settings can cause profoundly different performance characteristics, while altering the type coercion behavior at the same time.

Users of the oracledb dialect are strongly encouraged to read through python-oracledb’s list of built-in datatype symbols at Database Types Note that in some cases, significant performance degradation can occur when using these types vs. not.

On the SQLAlchemy side, the `DialectEvents.do_setinputsizes()` event can be used both for runtime visibility (e.g. logging) of the setinputsizes step as well as to fully control how `setinputsizes()` is used on a per-statement basis.

New in version 1.2.9: Added `DialectEvents.setinputsizes()`

#### Example 1 - logging all setinputsizes calls

The following example illustrates how to log the intermediary values from a SQLAlchemy perspective before they are converted to the raw `setinputsizes()` parameter dictionary. The keys of the dictionary are `BindParameter` objects which have a `.key` and a `.type` attribute:

    from sqlalchemy import create_engine, event

    engine = create_engine(
        "oracle+oracledb://scott:tiger@localhost:1521?service_name=freepdb1"
    )

    @event.listens_for(engine, "do_setinputsizes")
    def _log_setinputsizes(inputsizes, cursor, statement, parameters, context):
        for bindparam, dbapitype in inputsizes.items():
            log.info(
                "Bound parameter name: %s  SQLAlchemy type: %r DBAPI object: %s",
                bindparam.key,
                bindparam.type,
                dbapitype,
            )

#### Example 2 - remove all bindings to CLOB

For performance, fetching LOB datatypes from Oracle Database is set by default for the `Text` type within SQLAlchemy. This setting can be modified as follows:

    from sqlalchemy import create_engine, event
    from oracledb import CLOB

    engine = create_engine(
        "oracle+oracledb://scott:tiger@localhost:1521?service_name=freepdb1"
    )

    @event.listens_for(engine, "do_setinputsizes")
    def _remove_clob(inputsizes, cursor, statement, parameters, context):
        for bindparam, dbapitype in list(inputsizes.items()):
            if dbapitype is CLOB:
                del inputsizes[bindparam]

### LOB Datatypes

LOB datatypes refer to the “large object” datatypes such as CLOB, NCLOB and BLOB. Oracle Database can efficiently return these datatypes as a single buffer. SQLAlchemy makes use of type handlers to do this by default.

To disable the use of the type handlers and deliver LOB objects as classic buffered objects with a `read()` method, the parameter `auto_convert_lobs=False` may be passed to `create_engine()`.

### RETURNING Support

The oracledb dialect implements RETURNING using OUT parameters. The dialect supports RETURNING fully.

### Two Phase Transaction Support

Two phase transactions are fully supported with python-oracledb. (Thin mode requires python-oracledb 2.3). APIs for two phase transactions are provided at the Core level via `Connection.begin_twophase()` and `Session.twophase` for transparent ORM use.

Changed in version 2.0.32: added support for two phase transactions

### Precision Numerics

SQLAlchemy’s numeric types can handle receiving and returning values as Python `Decimal` objects or float objects. When a `Numeric` object, or a subclass such as `Float`, `DOUBLE_PRECISION` etc. is in use, the `Numeric.asdecimal` flag determines if values should be coerced to `Decimal` upon return, or returned as float objects. To make matters more complicated under Oracle Database, the `NUMBER` type can also represent integer values if the “scale” is zero, so the Oracle Database-specific `NUMBER` type takes this into account as well.

The oracledb dialect makes extensive use of connection- and cursor-level “outputtypehandler” callables in order to coerce numeric values as requested. These callables are specific to the specific flavor of `Numeric` in use, as well as if no SQLAlchemy typing objects are present. There are observed scenarios where Oracle Database may send incomplete or ambiguous information about the numeric types being returned, such as a query where the numeric types are buried under multiple levels of subquery. The type handlers do their best to make the right decision in all cases, deferring to the underlying python-oracledb DBAPI for all those cases where the driver can make the best decision.

When no typing objects are present, as when executing plain SQL strings, a default “outputtypehandler” is present which will generally return numeric values which specify precision and scale as Python `Decimal` objects. To disable this coercion to decimal for performance reasons, pass the flag `coerce_to_decimal=False` to `create_engine()`:

    engine = create_engine(
        "oracle+oracledb://scott:tiger@tnsalias", coerce_to_decimal=False
    )

The `coerce_to_decimal` flag only impacts the results of plain string SQL statements that are not otherwise associated with a `Numeric` SQLAlchemy type (or a subclass of such).

Changed in version 1.2: The numeric handling system for the oracle dialects has been reworked to take advantage of newer driver features as well as better integration of outputtypehandlers.

New in version 2.0.0: added support for the python-oracledb driver.

## cx_Oracle

Support for the Oracle Database database via the cx-Oracle driver.

### DBAPI

Documentation and download information (if applicable) for cx-Oracle is available at: https://oracle.github.io/python-cx_Oracle/

### Connecting

Connect String:

    oracle+cx_oracle://user:pass@hostname:port[/dbname][?service_name=<service>[&key=value&key=value...]]

### Description

cx_Oracle was the original driver for Oracle Database. It was superseded by python-oracledb which should be used instead.

### DSN vs. Hostname connections

cx_Oracle provides several methods of indicating the target database. The dialect translates from a series of different URL forms.

#### Hostname Connections with Easy Connect Syntax

Given a hostname, port and service name of the target database, for example from Oracle Database’s Easy Connect syntax then connect in SQLAlchemy using the `service_name` query string parameter:

    engine = create_engine(
        "oracle+cx_oracle://scott:tiger@hostname:port?service_name=myservice&encoding=UTF-8&nencoding=UTF-8"
    )

Note that the default driver value for encoding and nencoding was changed to “UTF-8” in cx_Oracle 8.0 so these parameters can be omitted when using that version, or later.

To use a full Easy Connect string, pass it as the `dsn` key value in a `create_engine.connect_args` dictionary:

    import cx_Oracle

    e = create_engine(
        "oracle+cx_oracle://@",
        connect_args={
            "user": "scott",
            "password": "tiger",
            "dsn": "hostname:port/myservice?transport_connect_timeout=30&expire_time=60",
        },
    )

#### Connections with tnsnames.ora or to Oracle Autonomous Database

Alternatively, if no port, database name, or service name is provided, the dialect will use an Oracle Database DSN “connection string”. This takes the “hostname” portion of the URL as the data source name. For example, if the `tnsnames.ora` file contains a TNS Alias of `myalias` as below:

    myalias =
      (DESCRIPTION =
        (ADDRESS = (PROTOCOL = TCP)(HOST = mymachine.example.com)(PORT = 1521))
        (CONNECT_DATA =
          (SERVER = DEDICATED)
          (SERVICE_NAME = orclpdb1)
        )
      )

The cx_Oracle dialect connects to this database service when `myalias` is the hostname portion of the URL, without specifying a port, database name or `service_name`:

    engine = create_engine("oracle+cx_oracle://scott:tiger@myalias")

Users of Oracle Autonomous Database should use this syntax. If the database is configured for mutural TLS (“mTLS”), then you must also configure the cloud wallet as shown in cx_Oracle documentation Connecting to Autononmous Databases.

#### SID Connections

To use Oracle Database’s obsolete System Identifier connection syntax, the SID can be passed in a “database name” portion of the URL:

    engine = create_engine(
        "oracle+cx_oracle://scott:tiger@hostname:port/dbname"
    )

Above, the DSN passed to cx_Oracle is created by `cx_Oracle.makedsn()` as follows:

    >>> import cx_Oracle
    >>> cx_Oracle.makedsn("hostname", 1521, sid="dbname")
    '(DESCRIPTION=(ADDRESS=(PROTOCOL=TCP)(HOST=hostname)(PORT=1521))(CONNECT_DATA=(SID=dbname)))'

Note that although the SQLAlchemy syntax `hostname:port/dbname` looks like Oracle’s Easy Connect syntax it is different. It uses a SID in place of the service name required by Easy Connect. The Easy Connect syntax does not support SIDs.

### Passing cx_Oracle connect arguments

Additional connection arguments can usually be passed via the URL query string; particular symbols like `SYSDBA` are intercepted and converted to the correct symbol:

    e = create_engine(
        "oracle+cx_oracle://user:pass@dsn?encoding=UTF-8&nencoding=UTF-8&mode=SYSDBA&events=true"
    )

Changed in version 1.3: the cx_Oracle dialect now accepts all argument names within the URL string itself, to be passed to the cx_Oracle DBAPI. As was the case earlier but not correctly documented, the `create_engine.connect_args` parameter also accepts all cx_Oracle DBAPI connect arguments.

To pass arguments directly to `.connect()` without using the query string, use the `create_engine.connect_args` dictionary. Any cx_Oracle parameter value and/or constant may be passed, such as:

    import cx_Oracle

    e = create_engine(
        "oracle+cx_oracle://user:pass@dsn",
        connect_args={
            "encoding": "UTF-8",
            "nencoding": "UTF-8",
            "mode": cx_Oracle.SYSDBA,
            "events": True,
        },
    )

Note that the default driver value for `encoding` and `nencoding` was changed to “UTF-8” in cx_Oracle 8.0 so these parameters can be omitted when using that version, or later.

### Options consumed by the SQLAlchemy cx_Oracle dialect outside of the driver

There are also options that are consumed by the SQLAlchemy cx_oracle dialect itself. These options are always passed directly to `create_engine()` , such as:

    e = create_engine(
        "oracle+cx_oracle://user:pass@dsn", coerce_to_decimal=False
    )

The parameters accepted by the cx_oracle dialect are as follows:

- `arraysize` - set the cx_oracle.arraysize value on cursors; defaults to `None`, indicating that the driver default should be used (typically the value is 100). This setting controls how many rows are buffered when fetching rows, and can have a significant effect on performance when modified.

  Changed in version 2.0.26: - changed the default value from 50 to None, to use the default value of the driver itself.

- `auto_convert_lobs` - defaults to True; See LOB Datatypes.

- `coerce_to_decimal` - see Precision Numerics for detail.

- `encoding_errors` - see Encoding Errors for detail.

### Using cx_Oracle SessionPool

The cx_Oracle driver provides its own connection pool implementation that may be used in place of SQLAlchemy’s pooling functionality. The driver pool supports Oracle Database features such dead connection detection, connection draining for planned database downtime, support for Oracle Application Continuity and Transparent Application Continuity, and gives support for Database Resident Connection Pooling (DRCP).

Using the driver pool can be achieved by using the `create_engine.creator` parameter to provide a function that returns a new connection, along with setting `create_engine.pool_class` to `NullPool` to disable SQLAlchemy’s pooling:

    import cx_Oracle
    from sqlalchemy import create_engine
    from sqlalchemy.pool import NullPool

    pool = cx_Oracle.SessionPool(
        user="scott",
        password="tiger",
        dsn="orclpdb",
        min=1,
        max=4,
        increment=1,
        threaded=True,
        encoding="UTF-8",
        nencoding="UTF-8",
    )

    engine = create_engine(
        "oracle+cx_oracle://", creator=pool.acquire, poolclass=NullPool
    )

The above engine may then be used normally where cx_Oracle’s pool handles connection pooling:

    with engine.connect() as conn:
        print(conn.scalar("select 1 from dual"))

As well as providing a scalable solution for multi-user applications, the cx_Oracle session pool supports some Oracle features such as DRCP and Application Continuity.

Note that the pool creation parameters `threaded`, `encoding` and `nencoding` were deprecated in later cx_Oracle releases.

### Using Oracle Database Resident Connection Pooling (DRCP)

When using Oracle Database’s DRCP, the best practice is to pass a connection class and “purity” when acquiring a connection from the SessionPool. Refer to the cx_Oracle DRCP documentation.

This can be achieved by wrapping `pool.acquire()`:

    import cx_Oracle
    from sqlalchemy import create_engine
    from sqlalchemy.pool import NullPool

    pool = cx_Oracle.SessionPool(
        user="scott",
        password="tiger",
        dsn="orclpdb",
        min=2,
        max=5,
        increment=1,
        threaded=True,
        encoding="UTF-8",
        nencoding="UTF-8",
    )

    def creator():
        return pool.acquire(
            cclass="MYCLASS", purity=cx_Oracle.ATTR_PURITY_SELF
        )

    engine = create_engine(
        "oracle+cx_oracle://", creator=creator, poolclass=NullPool
    )

The above engine may then be used normally where cx_Oracle handles session pooling and Oracle Database additionally uses DRCP:

    with engine.connect() as conn:
        print(conn.scalar("select 1 from dual"))

### Unicode

As is the case for all DBAPIs under Python 3, all strings are inherently Unicode strings. In all cases however, the driver requires an explicit encoding configuration.

#### Ensuring the Correct Client Encoding

The long accepted standard for establishing client encoding for nearly all Oracle Database related software is via the NLS_LANG environment variable. Older versions of cx_Oracle use this environment variable as the source of its encoding configuration. The format of this variable is Territory_Country.CharacterSet; a typical value would be `AMERICAN_AMERICA.AL32UTF8`. cx_Oracle version 8 and later use the character set “UTF-8” by default, and ignore the character set component of NLS_LANG.

The cx_Oracle driver also supported a programmatic alternative which is to pass the `encoding` and `nencoding` parameters directly to its `.connect()` function. These can be present in the URL as follows:

    engine = create_engine(
        "oracle+cx_oracle://scott:tiger@tnsalias?encoding=UTF-8&nencoding=UTF-8"
    )

For the meaning of the `encoding` and `nencoding` parameters, please consult Characters Sets and National Language Support (NLS).

See also

Characters Sets and National Language Support (NLS) - in the cx_Oracle documentation.

#### Unicode-specific Column datatypes

The Core expression language handles unicode data by use of the `Unicode` and `UnicodeText` datatypes. These types correspond to the VARCHAR2 and CLOB Oracle Database datatypes by default. When using these datatypes with Unicode data, it is expected that the database is configured with a Unicode-aware character set, as well as that the `NLS_LANG` environment variable is set appropriately (this applies to older versions of cx_Oracle), so that the VARCHAR2 and CLOB datatypes can accommodate the data.

In the case that Oracle Database is not configured with a Unicode character set, the two options are to use the `NCHAR` and `NCLOB` datatypes explicitly, or to pass the flag `use_nchar_for_unicode=True` to `create_engine()`, which will cause the SQLAlchemy dialect to use NCHAR/NCLOB for the `Unicode` / `UnicodeText` datatypes instead of VARCHAR/CLOB.

Changed in version 1.3: The `Unicode` and `UnicodeText` datatypes now correspond to the `VARCHAR2` and `CLOB` Oracle Database datatypes unless the `use_nchar_for_unicode=True` is passed to the dialect when `create_engine()` is called.

#### Encoding Errors

For the unusual case that data in Oracle Database is present with a broken encoding, the dialect accepts a parameter `encoding_errors` which will be passed to Unicode decoding functions in order to affect how decoding errors are handled. The value is ultimately consumed by the Python decode function, and is passed both via cx_Oracle’s `encodingErrors` parameter consumed by `Cursor.var()`, as well as SQLAlchemy’s own decoding function, as the cx_Oracle dialect makes use of both under different circumstances.

New in version 1.3.11.

### Fine grained control over cx_Oracle data binding performance with setinputsizes

The cx_Oracle DBAPI has a deep and fundamental reliance upon the usage of the DBAPI `setinputsizes()` call. The purpose of this call is to establish the datatypes that are bound to a SQL statement for Python values being passed as parameters. While virtually no other DBAPI assigns any use to the `setinputsizes()` call, the cx_Oracle DBAPI relies upon it heavily in its interactions with the Oracle Database client interface, and in some scenarios it is not possible for SQLAlchemy to know exactly how data should be bound, as some settings can cause profoundly different performance characteristics, while altering the type coercion behavior at the same time.

Users of the cx_Oracle dialect are strongly encouraged to read through cx_Oracle’s list of built-in datatype symbols at https://cx-oracle.readthedocs.io/en/latest/api_manual/module.html#database-types. Note that in some cases, significant performance degradation can occur when using these types vs. not, in particular when specifying `cx_Oracle.CLOB`.

On the SQLAlchemy side, the `DialectEvents.do_setinputsizes()` event can be used both for runtime visibility (e.g. logging) of the setinputsizes step as well as to fully control how `setinputsizes()` is used on a per-statement basis.

New in version 1.2.9: Added `DialectEvents.setinputsizes()`

#### Example 1 - logging all setinputsizes calls

The following example illustrates how to log the intermediary values from a SQLAlchemy perspective before they are converted to the raw `setinputsizes()` parameter dictionary. The keys of the dictionary are `BindParameter` objects which have a `.key` and a `.type` attribute:

    from sqlalchemy import create_engine, event

    engine = create_engine("oracle+cx_oracle://scott:tiger@host/xe")

    @event.listens_for(engine, "do_setinputsizes")
    def _log_setinputsizes(inputsizes, cursor, statement, parameters, context):
        for bindparam, dbapitype in inputsizes.items():
            log.info(
                "Bound parameter name: %s  SQLAlchemy type: %r DBAPI object: %s",
                bindparam.key,
                bindparam.type,
                dbapitype,
            )

#### Example 2 - remove all bindings to CLOB

The `CLOB` datatype in cx_Oracle incurs a significant performance overhead, however is set by default for the `Text` type within the SQLAlchemy 1.2 series. This setting can be modified as follows:

    from sqlalchemy import create_engine, event
    from cx_Oracle import CLOB

    engine = create_engine("oracle+cx_oracle://scott:tiger@host/xe")

    @event.listens_for(engine, "do_setinputsizes")
    def _remove_clob(inputsizes, cursor, statement, parameters, context):
        for bindparam, dbapitype in list(inputsizes.items()):
            if dbapitype is CLOB:
                del inputsizes[bindparam]

### LOB Datatypes

LOB datatypes refer to the “large object” datatypes such as CLOB, NCLOB and BLOB. Modern versions of cx_Oracle is optimized for these datatypes to be delivered as a single buffer. As such, SQLAlchemy makes use of these newer type handlers by default.

To disable the use of newer type handlers and deliver LOB objects as classic buffered objects with a `read()` method, the parameter `auto_convert_lobs=False` may be passed to `create_engine()`, which takes place only engine-wide.

### RETURNING Support

The cx_Oracle dialect implements RETURNING using OUT parameters. The dialect supports RETURNING fully.

### Two Phase Transactions Not Supported

Two phase transactions are not supported under cx_Oracle due to poor driver support. The newer python-oracledb dialect however does support two phase transactions.

### Precision Numerics

SQLAlchemy’s numeric types can handle receiving and returning values as Python `Decimal` objects or float objects. When a `Numeric` object, or a subclass such as `Float`, `DOUBLE_PRECISION` etc. is in use, the `Numeric.asdecimal` flag determines if values should be coerced to `Decimal` upon return, or returned as float objects. To make matters more complicated under Oracle Database, the `NUMBER` type can also represent integer values if the “scale” is zero, so the Oracle Database-specific `NUMBER` type takes this into account as well.

The cx_Oracle dialect makes extensive use of connection- and cursor-level “outputtypehandler” callables in order to coerce numeric values as requested. These callables are specific to the specific flavor of `Numeric` in use, as well as if no SQLAlchemy typing objects are present. There are observed scenarios where Oracle Database may send incomplete or ambiguous information about the numeric types being returned, such as a query where the numeric types are buried under multiple levels of subquery. The type handlers do their best to make the right decision in all cases, deferring to the underlying cx_Oracle DBAPI for all those cases where the driver can make the best decision.

When no typing objects are present, as when executing plain SQL strings, a default “outputtypehandler” is present which will generally return numeric values which specify precision and scale as Python `Decimal` objects. To disable this coercion to decimal for performance reasons, pass the flag `coerce_to_decimal=False` to `create_engine()`:

    engine = create_engine("oracle+cx_oracle://dsn", coerce_to_decimal=False)

The `coerce_to_decimal` flag only impacts the results of plain string SQL statements that are not otherwise associated with a `Numeric` SQLAlchemy type (or a subclass of such).

Changed in version 1.2: The numeric handling system for cx_Oracle has been reworked to take advantage of newer cx_Oracle features as well as better integration of outputtypehandlers.