Tips¶
These tips are based on mailing list postings, issues, and emails. You are recommended to read all the documentation as well.
About Python, APSW, and SQLite versions¶
SQLite has approximately quarterly releases. These include tweaks, bug fixes, and new functionality based on the billions of SQLite databases in use, and the many many programs that use SQLite (eg almost every browser, mail client, photo library, mobile and desktop OS). Despite these changes, SQLite retains backwards and forwards compatibility with the file format and APIs.
APSW wraps the SQLite C API. That means when SQLite adds new constant or API, then so does APSW. You can think of APSW as the Python expression of SQLite’s C API. You can lookup SQLite APIs to find which APSW functions and attributes call them.
Consequently the APSW version mirrors the SQLite version. You can use APSW with the corresponding version of SQLite, or any newer version of SQLite. You could use the original 2004 release of APSW with today’s SQLite just fine, although it wouldn’t know about the new APIs and constants.
APSW has compatibility with a broad range of Python versions. This is so that you can update the SQLite version you use, access new constants and APIs (if desired), all without having to change your Python version.
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.
Note
SQLite 3 has been available for two decades, improving and adding
features over time. Because of strong compatibility guarantees, you
may need to opt-in to some like foreign key enforcement. It is a good idea to
review the pragmas and
consider using apsw.connection_hooks to configure each
Connection.
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/END 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. To set your own logger use:
def handler(errcode, message):
errstr=apsw.mapping_result_codes[errcode & 255]
print ("SQLITE_LOG: %s (%d) %s %s" % (message, errcode, errstr, apsw.mapping_extended_result_codes.get(errcode, "")))
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
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 user_version(db):
return db.execute("pragma user_version").fetchall()[0][0]
def ensure_schema(db):
if user_version(db)==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 user_version(db)==1:
with db:
db.execute("""
CREATE TABLE IF NOT EXISTS baz(x,y,z);
CREATE INDEX ....
PRAGMA user_version=2;""")
if user_version(con)==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.
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.)