Oracle

Support for the Oracle database.

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

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.

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.
• 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.
• 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
)

LOB Objects

cx_oracle presents some challenges when fetching LOB objects. A LOB object in a result set is presented by cx_oracle as a cx_oracle.LOB object which has a read() method. By default, SQLAlchemy converts these LOB objects into Python strings. This is for two reasons. First, the LOB object requires an active cursor association, meaning if you were to fetch many rows at once such that cx_oracle had to go back to the database and fetch a new batch of rows, the LOB objects in the already-fetched rows are now unreadable and will raise an error. SQLA “pre-reads” all LOBs so that their data is fetched before further rows are read. The size of a “batch of rows” is controlled by the cursor.arraysize value, which SQLAlchemy defaults to 50 (cx_oracle normally defaults this to one).

Secondly, the LOB object is not a standard DBAPI return value so SQLAlchemy seeks to “normalize” the results to look more like other DBAPIs.

The conversion of LOB objects by this dialect is unique in SQLAlchemy in that it takes place for all statement executions, even plain string-based statements for which SQLA has no awareness of result typing. This is so that calls like fetchmany() and fetchall() can work in all cases without raising cursor errors. The conversion of LOB in all cases, as well as the “prefetch” of LOB objects, can be disabled using auto_convert_lobs=False.

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().

Two Phase Transaction Support

Two Phase transactions are implemented using XA transactions. Success has been reported of them working successfully but this should be regarded as an experimental feature.

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.