# Tips

These tips are based on mailing list postings, issues, and emails. 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 and quirks.

Best practice is recommended.

## Transactions

Transactions are the changes applied to a database file as a whole. They either happen completely, or not at all. SQLite notes all the changes made during a transaction, and at the end when you do a commit will cause them to permanently end up in the database. If you do not commit, or just exit, then other/new connections will not see the changes and SQLite handles tidying up the work in progress automatically.

Committing a transaction can be quite time consuming. SQLite uses a robust multi-step process that has to handle errors that can occur at any point, and asks the operating system to ensure that data is on storage and would survive a power cycle. This will limit the rate at which you can do transactions.

If you do nothing, then each statement is a single transaction:

    # this will be 3 separate transactions
    db.execute("INSERT ...")
    db.execute("INSERT ...")
    db.execute("INSERT ...")

You can use BEGIN/COMMIT to set the transaction boundary:

    # this will be one transaction
    db.execute("BEGIN")
    db.execute("INSERT ...")
    db.execute("INSERT ...")
    db.execute("INSERT ...")
    db.execute("COMMIT")

However that is extra effort, and also requires error handling. For example if the second INSERT failed then you likely want to ROLLBACK the incomplete transaction, so that additional work on the same connection doesn't see the partial data.

If you use `with Connection` then the transaction will be automatically started, and committed on success or rolled back if exceptions occur:

    # this will be one transaction with automatic commit and rollback
    with db:
        db.execute("INSERT ...")
        db.execute("INSERT ...")
        db.execute("INSERT ...")

There are technical details at the SQLite site.

## 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.

Cursor objects are obtained by `Connection.cursor()` and are very cheap. It is best practise to not re-use them, and instead get a new one each time. If you don't, code refactoring and nested loops can unintentionally use the same cursor object which will not crash but will cause hard to diagnose behaviour in your program.

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.

## 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. You can call `apsw.ext.log_sqlite()` which installs a handler that forwards SQLite messages to the `logging module`.`

To do it yourself:

    def handler(errcode, message):
        errstr=apsw.mapping_result_codes[errcode & 255]
        print (f"SQLITE_LOG: { message } ({ errcode }) { errstr } "
               + apsw.mapping_extended_result_codes.get(errcode, ""))

    apsw.config(apsw.SQLITE_CONFIG_LOG, handler)

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

## Managing and updating your schema

If your program uses SQLite for data then you'll need to manage and update your schema. The hard way of doing this is to test for the existence of tables and their columns, and doing that maintenance programmatically. The easy way is to use pragma user_version as in this example:

    def ensure_schema(db):
      # a new database starts at user_version 0
      if db.pragma("user_version") == 0:
        with db:
          db.execute("""
            CREATE TABLE IF NOT EXISTS foo(x,y,z);
            CREATE TABLE IF NOT EXISTS bar(x,y,z);
            PRAGMA user_version = 1;""")

      if db.pragma("user_version") == 1:
        with db:
          db.execute("""
          CREATE TABLE IF NOT EXISTS baz(x,y,z);
          CREATE INDEX ....
          PRAGMA user_version = 2;""")

      if db.pragma("user_version") == 2:
        with db:
          db.execute("""
          ALTER TABLE .....
          PRAGMA user_version = 3;""")

This approach will automatically upgrade the schema as you expect. You can also use pragma application_id to mark the database as made by your application.

## Parsing SQL

Sometimes you want to know what a particular SQL statement does. Use `apsw.ext.query_info()` which will provide as much detail as you need.

## 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 Connections

`apsw.connection_hooks` is a list of callbacks for when each `Connection` is created. They are called in turn, with the new connection as the only parameter.

For example if you wanted to add an executescript method to Connections that is like `Connection.execute()` but ignores all returned rows:

    def executescript(self, sql, bindings=None):
      for _ in self.execute(sql, bindings):
        pass

    def my_hook(connection):
      connection.executescript = executescript

    apsw.connection_hooks.append(my_hook)

## Customizing Cursors

You can 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).

Set `Connection.cursor_factory` to any callable, which will be called with the connection as the only parameter, and return the object to use as a cursor.

For example instead of returning rows as tuples, we can return them as dictionaries using a row tracer with `Cursor.getdescription()`:

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

    def my_factory(connection):
      cursor = apsw.Cursor(connection)
      cursor.rowtrace = dict_row
      return cursor

    connection.cursor_factory = my_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 overall 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 separate connections to get the same performance.

The shared cache mode is targeted 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 introduced 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.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.)