Firebird — SQLAlchemy 0.7.1 Documentation

# SQLAlchemy 0.7.1 Documentation

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Version: 0.7.1 Last Updated: 06/05/2011 13:13:59

# Firebird

Support for the Firebird database.

Connectivity is usually supplied via the kinterbasdb DBAPI module.

## Dialects

Firebird offers two distinct dialects (not to be confused with a SQLAlchemy `Dialect`):

dialect 1
  This is the old syntax and behaviour, inherited from Interbase pre-6.0.
dialect 3
  This is the newer and supported syntax, introduced in Interbase 6.0.

The SQLAlchemy Firebird dialect detects these versions and adjusts its representation of SQL accordingly. However, support for dialect 1 is not well tested and probably has incompatibilities.

## Locking Behavior

Firebird locks tables aggressively. For this reason, a DROP TABLE may hang until other transactions are released. SQLAlchemy does its best to release transactions as quickly as possible. The most common cause of hanging transactions is a non-fully consumed result set, i.e.:

    result = engine.execute("select * from table")
    row = result.fetchone()
    return

Where above, the `ResultProxy` has not been fully consumed. The connection will be returned to the pool and the transactional state rolled back once the Python garbage collector reclaims the objects which hold onto the connection, which often occurs asynchronously. The above use case can be alleviated by calling `first()` on the `ResultProxy` which will fetch the first row and immediately close all remaining cursor/connection resources.

## RETURNING support

Firebird 2.0 supports returning a result set from inserts, and 2.1 extends that to deletes and updates. This is generically exposed by the SQLAlchemy `returning()` method, such as:

    # INSERT..RETURNING
    result = table.insert().returning(table.c.col1, table.c.col2).\
                   values(name='foo')
    print result.fetchall()

    # UPDATE..RETURNING
    raises = empl.update().returning(empl.c.id, empl.c.salary).\
                  where(empl.c.sales>100).\
                  values(dict(salary=empl.c.salary * 1.1))
    print raises.fetchall()

## kinterbasdb

The most common way to connect to a Firebird engine is implemented by kinterbasdb, currently maintained directly by the Firebird people.

The connection URL is of the form `firebird[+kinterbasdb]://user:password@host:port/path/to/db[?key=value&key=value...]`.

Kinterbasedb backend specific keyword arguments are:

- type_conv - select the kind of mapping done on the types: by default SQLAlchemy uses 200 with Unicode, datetime and decimal support (see details).

- concurrency_level - set the backend policy with regards to threading issues: by default SQLAlchemy uses policy 1 (see details).

- enable_rowcount - True by default, setting this to False disables the usage of “cursor.rowcount” with the Kinterbasdb dialect, which SQLAlchemy ordinarily calls upon automatically after any UPDATE or DELETE statement. When disabled, SQLAlchemy’s ResultProxy will return -1 for result.rowcount. The rationale here is that Kinterbasdb requires a second round trip to the database when .rowcount is called - since SQLA’s resultproxy automatically closes the cursor after a non-result-returning statement, rowcount must be called, if at all, before the result object is returned. Additionally, cursor.rowcount may not return correct results with older versions of Firebird, and setting this flag to False will also cause the SQLAlchemy ORM to ignore its usage. The behavior can also be controlled on a per-execution basis using the enable_rowcount option with `execution_options()`:

      conn = engine.connect().execution_options(enable_rowcount=True)
      r = conn.execute(stmt)
      print r.rowcount