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://python.net/crew/atuining/cx_Oracle/ . 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() include:
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.
allow_twophase - enable two-phase transactions. This feature is not yet supported.
threaded - defaults to True with SQLAlchemy, enable multithreaded access to cx_oracle connections.
use_ansi - defaults to True, use ANSI JOIN constructs (see the section on Oracle 8).
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.
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 partially implemented using XA transactions but at the time of this writing have not been successfully tested. The author of cx_oracle also stated that he's never seen them work so this may be a cx_oracle issue.
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.
Oracle compiler modifies the lexical structure of Select statements to work under non-ANSI configured Oracle databases, if the use_ansi flag is False.
Called when a SELECT statement has no froms, and no FROM clause is to be appended.
The Oracle compiler tacks a "FROM DUAL" to the statement.
Oracle doesn't like FROM table AS alias. Is the AS standard SQL??
Look for LIMIT and OFFSET in a select statement, and if so tries to wrap it in a subquery with rownum criterion.
Construct a new OracleDialect.
create a two-phase transaction ID.
this id will be passed to do_begin_twophase(), do_rollback_twophase(), do_commit_twophase(). its format is unspecified.