Oracle — SQLAlchemy 0.6.0 Documentation

# SQLAlchemy 0.6.0 Documentation

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Version: 0.6.0 Last Updated: 04/18/2010 19:16:18

# Oracle

Support for the Oracle database.

Oracle version 8 through current (11g at the time of this writing) are supported.

For information on connecting via specific drivers, see the documentation for that driver.

## Connect Arguments

The dialect supports several `create_engine()` arguments which affect the behavior of the dialect regardless of driver in use.

- use_ansi - Use ANSI JOIN constructs (see the section on Oracle 8). Defaults to `True`. If `False`, Oracle-8 compatible constructs are used for joins.
- optimize_limits - defaults to `False`. see the section on LIMIT/OFFSET.

## Auto Increment Behavior

SQLAlchemy Table objects which include integer primary keys are usually assumed to have “autoincrementing” behavior, meaning they can generate their own primary key values upon INSERT. Since Oracle has no “autoincrement” feature, SQLAlchemy relies upon sequences to produce these values. With the Oracle dialect, a sequence must always be explicitly specified to enable autoincrement. This is divergent with the majority of documentation examples which assume the usage of an autoincrement-capable database. To specify sequences, use the sqlalchemy.schema.Sequence object which is passed to a Column construct:

    t = Table('mytable', metadata,
          Column('id', Integer, Sequence('id_seq'), primary_key=True),
          Column(...), ...
    )

This step is also required when using table reflection, i.e. autoload=True:

    t = Table('mytable', metadata,
          Column('id', Integer, Sequence('id_seq'), primary_key=True),
          autoload=True
    )

## Identifier Casing

In Oracle, the data dictionary represents all case insensitive identifier names using UPPERCASE text. SQLAlchemy on the other hand considers an all-lower case identifier name to be case insensitive. The Oracle dialect converts all case insensitive identifiers to and from those two formats during schema level communication, such as reflection of tables and indexes. Using an UPPERCASE name on the SQLAlchemy side indicates a case sensitive identifier, and SQLAlchemy will quote the name - this will cause mismatches against data dictionary data received from Oracle, so unless identifier names have been truly created as case sensitive (i.e. using quoted names), all lowercase names should be used on the SQLAlchemy side.

## Unicode

SQLAlchemy 0.6 uses the “native unicode” mode provided as of cx_oracle 5. cx_oracle 5.0.2 or greater is recommended for support of NCLOB. If not using cx_oracle 5, the NLS_LANG environment variable needs to be set in order for the oracle client library to use proper encoding, such as “AMERICAN_AMERICA.UTF8”.

Also note that Oracle supports unicode data through the NVARCHAR and NCLOB data types. When using the SQLAlchemy Unicode and UnicodeText types, these DDL types will be used within CREATE TABLE statements. Usage of VARCHAR2 and CLOB with unicode text still requires NLS_LANG to be set.

## LIMIT/OFFSET Support

Oracle has no support for the LIMIT or OFFSET keywords. Whereas previous versions of SQLAlchemy used the “ROW NUMBER OVER...” construct to simulate LIMIT/OFFSET, SQLAlchemy 0.5 now uses a wrapped subquery approach in conjunction with ROWNUM. The exact methodology is taken from http://www.oracle.com/technology/oramag/oracle/06-sep/o56asktom.html . Note that the “FIRST ROWS()” optimization keyword mentioned is not used by default, as the user community felt this was stepping into the bounds of optimization that is better left on the DBA side, but this prefix can be added by enabling the optimize_limits=True flag on create_engine().

## ON UPDATE CASCADE

Oracle doesn’t have native ON UPDATE CASCADE functionality. A trigger based solution is available at http://asktom.oracle.com/tkyte/update_cascade/index.html .

When using the SQLAlchemy ORM, the ORM has limited ability to manually issue cascading updates - specify ForeignKey objects using the “deferrable=True, initially=’deferred’” keyword arguments, and specify “passive_updates=False” on each relationship().

## Oracle 8 Compatibility

When using Oracle 8, a “use_ansi=False” flag is available which converts all JOIN phrases into the WHERE clause, and in the case of LEFT OUTER JOIN makes use of Oracle’s (+) operator.

## Synonym/DBLINK Reflection

When using reflection with Table objects, the dialect can optionally search for tables indicated by synonyms that reference DBLINK-ed tables by passing the flag oracle_resolve_synonyms=True as a keyword argument to the Table construct. If DBLINK is not in use this flag should be left off.

## cx_Oracle Notes

Support for the Oracle database via the cx_oracle driver.

### Driver

The Oracle dialect uses the cx_oracle driver, available at http://cx-oracle.sourceforge.net/ . The dialect has several behaviors which are specifically tailored towards compatibility with this module. Version 5.0 or greater is strongly recommended, as SQLAlchemy makes extensive use of the cx_oracle output converters for numeric and string conversions.

### Connecting

Connecting with create_engine() uses the standard URL approach of `oracle://user:pass@host:port/dbname[?key=value&key=value...]`. If dbname is present, the host, port, and dbname tokens are converted to a TNS name using the cx_oracle `makedsn()` function. Otherwise, the host token is taken directly as a TNS name.

Additional arguments which may be specified either as query string arguments on the URL, or as keyword arguments to `create_engine()` are:

- allow_twophase - enable two-phase transactions. Defaults to `True`.
- arraysize - set the cx_oracle.arraysize value on cursors, in SQLAlchemy it defaults to 50. See the section on “LOB Objects” below.
- auto_convert_lobs - defaults to True, see the section on LOB objects.
- auto_setinputsizes - the cx_oracle.setinputsizes() call is issued for all bind parameters. This is required for LOB datatypes but can be disabled to reduce overhead. Defaults to `True`.
- mode - This is given the string value of SYSDBA or SYSOPER, or alternatively an integer value. This value is only available as a URL query string argument.
- threaded - enable multithreaded access to cx_oracle connections. Defaults to `True`. Note that this is the opposite default of cx_oracle itself.

### Unicode

cx_oracle 5 fully supports Python unicode objects. SQLAlchemy will pass all unicode strings directly to cx_oracle, and additionally uses an output handler so that all string based result values are returned as unicode as well.

### LOB Objects

cx_oracle returns oracle LOBs using the cx_oracle.LOB object. SQLAlchemy converts these to strings so that the interface of the Binary type is consistent with that of other backends, and so that the linkage to a live cursor is not needed in scenarios like result.fetchmany() and result.fetchall(). This means that by default, LOB objects are fully fetched unconditionally by SQLAlchemy, and the linkage to a live cursor is broken.

To disable this processing, pass `auto_convert_lobs=False` to `create_engine()`.

### Two Phase Transaction Support

Two Phase transactions are implemented using XA transactions. Success has been reported with this feature but it should be regarded as experimental.

## zxjdbc Notes

Support for the Oracle database via the zxjdbc JDBC connector.

### JDBC Driver

The official Oracle JDBC driver is at http://www.oracle.com/technology/software/tech/java/sqlj_jdbc/index.html.