Tips — APSW 3.8.11.1-r1 documentation

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# Tips

These tips are based on mailing list postings. You are recommended to read all the documentation as well.

## SQLite is different

While SQLite provides a SQL database like many others out there, it is also unique in many ways. Read about the unique features at the SQLite website.

## Cursors

SQLite only calculates each result row as you request it. For example if your query returns 10 million rows SQLite will not calculate all 10 million up front. Instead the next row will be calculated as you ask for it.

Cursors on the same Connection are not isolated from each other. Anything done on one cursor is immediately visible to all other Cursors on the same connection. This still applies if you start transactions. Connections are isolated from each other.

Read more about Cursors.

## Bindings

When using a cursor, always use bindings. String interpolation may seem more convenient but you will encounter difficulties. You may feel that you have complete control over all data accessed but if your code is at all useful then you will find it being used more and more widely. The computer will always be better than you at parsing SQL and the bad guys have years of experience finding and using SQL injection attacks in ways you never even thought possible.

The documentation gives many examples of how to use various forms of bindings.

## Unicode

SQLite only stores text as Unicode. However it relies on SQLite API users to provide valid UTF-8 and does not double check. (APSW only provides valid UTF-8). It is possible using other wrappers and tools to cause invalid UTF-8 to appear in the database which will then cause retrieval errors. You can work around this by using the SQL CAST operator. For example:

    SELECT id, CAST(label AS blob) from table

Then proceed to give the Joel Unicode article to all people involved.

## Diagnostics

Both SQLite and APSW provide detailed diagnostic information. Errors will be signalled via an exception.

APSW ensures you have detailed information both in the stack trace as well as what data APSW/SQLite was operating on.

SQLite has a warning/error logging facility. To set your own logger use:

    def handler(errcode, message):
        errstr=apsw.mapping_result_codes[errcode & 255]
        extended=errcode & ~ 255
        print "SQLITE_LOG: %s (%d) %s %s" % (message, errcode, errstr, apsw.mapping_extended_result_codes.get(extended, ""))

    apsw.config(apsw.SQLITE_CONFIG_LOG, handler)

Note

The handler must be set before any other calls to SQLite. Once SQLite is initialised you cannot change the logger - a `MisuseError` will happen (this restriction is in SQLite not APSW).

This is an example of what gets printed when I use `/dev/null` as the database name in the `Connection` and then tried to create a table.:

    SQLITE_LOG: cannot open file at line 28729 of [7dd4968f23] (14) SQLITE_CANTOPEN
    SQLITE_LOG: os_unix.c:28729: (2) open(/dev/null-journal) - No such file or directory (14) SQLITE_CANTOPEN
    SQLITE_LOG: statement aborts at 38: [create table foo(x,y);] unable to open database file (14) SQLITE_CANTOPEN

## Parsing SQL

Sometimes you want to know what a particular SQL statement does. The SQLite query parser directly generates VDBE byte code and cannot be hooked into. There is however an easier way.

Make a new `Connection` object making sure the statement cache is disabled (size zero). Install an execution tracer that returns `apsw.SQLITE_DENY` which will prevent any queries from running. Install an `authorizer`.

Then call `Cursor.execute()` on your query. Your authorizer will then be called (multiple times if necessary) with details of what the query does including expanding views and triggers that fire. Finally the execution tracer will fire. If the query string had multiple statements then the execution tracer lets you know how long the first statement was.

## Unexpected behaviour

Occasionally you may get different results than you expected. Before littering your code with print, try apswtrace with all options turned on to see exactly what is going on. You can also use the SQLite shell to dump the contents of your database to a text file. For example you could dump it before and after a run to see what changed.

One fairly common gotcha is using double quotes instead of single quotes. (This wouldn’t be a problem if you use bindings!) SQL strings use single quotes. If you use double quotes then it will mostly appear to work, but they are intended to be used for identifiers such as column names. For example if you have a column named `a b` (a space b) then you would need to use:

    SELECT "a b" from table

If you use double quotes and happen to use a string whose contents are the same as a table, alias, column etc then unexpected results will occur.

## Customizing cursors

Some developers want to customize the behaviour of cursors. An example would be wanting a rowcount or batching returned rows. (These don’t make any sense with SQLite but the desire may be to make the code source compatible with other database drivers).

APSW does not provide a way to subclass the cursor class or any other form of factory. Consequently you will have to subclass the `Connection` and provide an alternate implementation of `Connection.cursor()`. You should encapsulate the APSW cursor - ie store it as a member of your cursor class and forward calls as appropriate. The cursor only has two important methods - `Cursor.execute()` and `Cursor.executemany()`.

If you want to change the rows returned then use a row tracer. For example you could call `Cursor.getdescription()` and return a dictionary instead of a tuple:

    def row_factory(cursor, row):
        return {k[0]: row[i] for i, k in enumerate(cursor.getdescription())}

    # You can also set this on just a cursor
    connection.setrowtrace(row_factory)

## Busy handling

SQLite uses locks to coordinate access to the database by multiple connections (within the same process or in a different process). The general goal is to have the locks be as lax as possible (allowing concurrency) and when using more restrictive locks to keep them for as short a time as possible. See the SQLite documentation for more details.

By default you will get a `BusyError` if a lock cannot be acquired. You can set a `timeout` which will keep retrying or a `callback` where you decide what to do.

## Database schema

When starting a new database, it can be quite difficult to decide what tables and fields to have and how to link them. The technique used to design SQL schemas is called normalization. The page also shows common pitfalls if you don’t normalize your schema.

## Shared Cache Mode

SQLite supports a shared cache mode where multiple connections to the same database can share a cache instead of having their own. It is not recommended that you use this mode.

A big issue is that busy handling is not done the same way. The timeouts and handlers are ignored and instead `SQLITE_LOCKED_SHAREDCACHE` extended error is returned. Consequently you will have to do your own busy handling. (SQLite ticket, APSW issue 59)

The amount of memory and I/O saved is trivial compared to Python’s overal memory and I/O consumption. You may also need to tune the shared cache’s memory back up to what it would have been with seperate connections to get the same performance.

The shared cache mode is targetted at embedded systems where every byte of memory and I/O matters. For example an MP3 player may only have kilobytes of memory available for SQLite.

## Write Ahead Logging

SQLite 3.7 introduces write ahead logging which has several benefits, but also some drawbacks as the page documents. WAL mode is off by default. In addition to turning it on manually for each database, you can also turn it on for all opened databases by using `connection_hooks`:

    def setwal(db):
        db.cursor().execute("pragma journal_mode=wal")
        # custom auto checkpoint interval (use zero to disable)
        db.wal_autocheckpoint(10)

    apsw.connection_hooks.append(setwal)

Note that if wal mode can’t be set (eg the database is in memory or temporary) then the attempt to set wal mode will be ignored. The pragma will return the mode in effect. It is also harmless to call functions like `Connection.wal_autocheckpoint()` on connections that are not in wal mode.

If you write your own VFS, then inheriting from an existing VFS that supports WAL will make your VFS support the extra WAL methods too. (Your VFS will point directly to the base methods - there is no indirect call via Python.)

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