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# Extensions

SQLite includes a number of extensions providing additional functionality. All extensions are disabled by default and you need to take steps to have them available at compilation time, to enable them and then to use them.

## Asynchronous VFS

This extension does SQLite I/O in a background thread processing a queue of requests. To enable it you must have used fetch --asyncvfs to `setup.py` at some point. It is enabled by the downloaded file `sqlite3async.c` being present in the same directory as `setup.py`. See the SQLite documentation.

To use you must first call `apsw.async_initialize()` which will register the VFS. If you didn’t make it the default VFS then you need to specify it when opening your database. From this point on, any operations you perform that require writing to the database will be queued and the database locked. You should start a background thread to perform the write operations calling `apsw.async_run()` to do the work. You can call `apsw.async_control()` to set and get various things (eg adding delays, controlling locking) as well as telling it when the writer code should exit. This is a simple example:

    # Inherit from default vfs, do not make this the new default
    asyncvfsname=apsw.async_initialize("", False)
    # Open database
    db=apsw.Connection("database", vfs=asyncvfsname)
    # Make a worker thread
    t=threading.Thread(target=apsw.async_run)
    t.start()
    # do some work
    cur=db.cursor()
    cur.execute("..")
    # Close db
    db.close()
    # Tell worker to quit when queue is empty
    apsw.async_control(apsw.SQLITEASYNC_HALT, apsw.SQLITEASYNC_HALT_IDLE)

## FTS3/4

This is the third version of the full text search extension. It makes it easy to find words in multi-word text fields. You must enable the extension via setup.py build flags before it will work. There are no additional APIs and the documented SQL works as is.

Note that FTS4 is some augmentations to FTS3 and are enabled whenever FTS3 is enabled as described in the documentation

## ICU

The ICU extension provides an International Components for Unicode interface, in particular enabling you do sorting and regular expressions in a locale aware way. The documentation shows how to use it.

## RTree

The RTree extension provides a spatial table - see the documentation. You must enable the extension via setup.py build flags before it will work. There are no additional APIs and the documented SQL works as is.

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