sqlalchemy.dialects.oracle.base

.. dialect:: oracle :name: Oracle :full_support: 11.2, 18c :normal_support: 11+ :best_effort: 9+ 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. For use within Oracle, two options are available, which are the use of IDENTITY columns (Oracle 12 and above only) or the association of a SEQUENCE with the column. Specifying GENERATED AS IDENTITY (Oracle 12 and above) ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Starting from version 12 Oracle can make use of identity columns using the :class:`_sql.Identity` to specify the autoincrementing behavior:: t = Table('mytable', metadata, Column('id', Integer, Identity(start=3), primary_key=True), Column(...), ... ) The CREATE TABLE for the above :class:`_schema.Table` object would be: .. sourcecode:: sql CREATE TABLE mytable ( id INTEGER GENERATED BY DEFAULT AS IDENTITY (START WITH 3), ..., PRIMARY KEY (id) ) The :class:`_schema.Identity` object support many options to control the "autoincrementing" behavior of the column, like the starting value, the incrementing value, etc. In addition to the standard options, Oracle supports setting :paramref:`_schema.Identity.always` to ``None`` to use the default generated mode, rendering GENERATED AS IDENTITY in the DDL. It also supports setting :paramref:`_schema.Identity.on_null` to ``True`` to specify ON NULL in conjunction with a 'BY DEFAULT' identity column. Using a SEQUENCE (all Oracle versions) ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Older version of Oracle had no "autoincrement" feature, SQLAlchemy relies upon sequences to produce these values. With the older Oracle versions, *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', start=1), primary_key=True), Column(...), ... ) This step is also required when using table reflection, i.e. autoload_with=engine:: t = Table('mytable', metadata, Column('id', Integer, Sequence('id_seq', start=1), primary_key=True), autoload_with=engine ) .. versionchanged:: 1.4 Added :class:`_schema.Identity` construct in a :class:`_schema.Column` to specify the option of an autoincrementing column. .. _oracle_isolation_level: Transaction Isolation Level / Autocommit ---------------------------------------- The Oracle database supports "READ COMMITTED" and "SERIALIZABLE" modes of isolation. The AUTOCOMMIT isolation level is also supported by the cx_Oracle dialect. To set using per-connection execution options:: connection = engine.connect() connection = connection.execution_options( isolation_level="AUTOCOMMIT" ) For ``READ COMMITTED`` and ``SERIALIZABLE``, the Oracle dialect sets the level at the session level using ``ALTER SESSION``, which is reverted back to its default setting when the connection is returned to the connection pool. Valid values for ``isolation_level`` include: * ``READ COMMITTED`` * ``AUTOCOMMIT`` * ``SERIALIZABLE`` .. note:: The implementation for the :meth:`_engine.Connection.get_isolation_level` method as implemented by the Oracle dialect necessarily forces the start of a transaction using the Oracle LOCAL_TRANSACTION_ID function; otherwise no level is normally readable. Additionally, the :meth:`_engine.Connection.get_isolation_level` method will raise an exception if the ``v$transaction`` view is not available due to permissions or other reasons, which is a common occurrence in Oracle installations. The cx_Oracle dialect attempts to call the :meth:`_engine.Connection.get_isolation_level` method when the dialect makes its first connection to the database in order to acquire the "default"isolation level. This default level is necessary so that the level can be reset on a connection after it has been temporarily modified using :meth:`_engine.Connection.execution_options` method. In the common event that the :meth:`_engine.Connection.get_isolation_level` method raises an exception due to ``v$transaction`` not being readable as well as any other database-related failure, the level is assumed to be "READ COMMITTED". No warning is emitted for this initial first-connect condition as it is expected to be a common restriction on Oracle databases. .. versionadded:: 1.3.16 added support for AUTOCOMMIT to the cx_oracle dialect as well as the notion of a default isolation level .. versionadded:: 1.3.21 Added support for SERIALIZABLE as well as live reading of the isolation level. .. versionchanged:: 1.3.22 In the event that the default isolation level cannot be read due to permissions on the v$transaction view as is common in Oracle installations, the default isolation level is hardcoded to "READ COMMITTED" which was the behavior prior to 1.3.21. .. seealso:: :ref:`dbapi_autocommit` Identifier Casing ----------------- In Oracle, the data dictionary represents all case insensitive identifier names using UPPERCASE text. SQLAlchemy on the other hand considers an all-lower case identifier name to be case insensitive. The Oracle dialect converts all case insensitive identifiers to and from those two formats during schema level communication, such as reflection of tables and indexes. Using an UPPERCASE name on the SQLAlchemy side indicates a case sensitive identifier, and SQLAlchemy will quote the name - this will cause mismatches against data dictionary data received from Oracle, so unless identifier names have been truly created as case sensitive (i.e. using quoted names), all lowercase names should be used on the SQLAlchemy side. .. _oracle_max_identifier_lengths: Max Identifier Lengths ---------------------- Oracle has changed the default max identifier length as of Oracle Server version 12.2. Prior to this version, the length was 30, and for 12.2 and greater it is now 128. This change impacts SQLAlchemy in the area of generated SQL label names as well as the generation of constraint names, particularly in the case where the constraint naming convention feature described at :ref:`constraint_naming_conventions` is being used. To assist with this change and others, Oracle includes the concept of a "compatibility" version, which is a version number that is independent of the actual server version in order to assist with migration of Oracle databases, and may be configured within the Oracle server itself. This compatibility version is retrieved using the query ``SELECT value FROM v$parameter WHERE name = 'compatible';``. The SQLAlchemy Oracle dialect, when tasked with determining the default max identifier length, will attempt to use this query upon first connect in order to determine the effective compatibility version of the server, which determines what the maximum allowed identifier length is for the server. If the table is not available, the server version information is used instead. As of SQLAlchemy 1.4, the default max identifier length for the Oracle dialect is 128 characters. Upon first connect, the compatibility version is detected and if it is less than Oracle version 12.2, the max identifier length is changed to be 30 characters. In all cases, setting the :paramref:`_sa.create_engine.max_identifier_length` parameter will bypass this change and the value given will be used as is:: engine = create_engine( "oracle+cx_oracle://scott:tiger@oracle122", max_identifier_length=30) The maximum identifier length comes into play both when generating anonymized SQL labels in SELECT statements, but more crucially when generating constraint names from a naming convention. It is this area that has created the need for SQLAlchemy to change this default conservatively. For example, the following naming convention produces two very different constraint names based on the identifier length:: from sqlalchemy import Column from sqlalchemy import Index from sqlalchemy import Integer from sqlalchemy import MetaData from sqlalchemy import Table from sqlalchemy.dialects import oracle from sqlalchemy.schema import CreateIndex m = MetaData(naming_convention={"ix": "ix_%(column_0N_name)s"}) t = Table( "t", m, Column("some_column_name_1", Integer), Column("some_column_name_2", Integer), Column("some_column_name_3", Integer), ) ix = Index( None, t.c.some_column_name_1, t.c.some_column_name_2, t.c.some_column_name_3, ) oracle_dialect = oracle.dialect(max_identifier_length=30) print(CreateIndex(ix).compile(dialect=oracle_dialect)) With an identifier length of 30, the above CREATE INDEX looks like:: CREATE INDEX ix_some_column_name_1s_70cd ON t (some_column_name_1, some_column_name_2, some_column_name_3) However with length=128, it becomes:: CREATE INDEX ix_some_column_name_1some_column_name_2some_column_name_3 ON t (some_column_name_1, some_column_name_2, some_column_name_3) Applications which have run versions of SQLAlchemy prior to 1.4 on an Oracle server version 12.2 or greater are therefore subject to the scenario of a database migration that wishes to "DROP CONSTRAINT" on a name that was previously generated with the shorter length. This migration will fail when the identifier length is changed without the name of the index or constraint first being adjusted. Such applications are strongly advised to make use of :paramref:`_sa.create_engine.max_identifier_length` in order to maintain control of the generation of truncated names, and to fully review and test all database migrations in a staging environment when changing this value to ensure that the impact of this change has been mitigated. .. versionchanged:: 1.4 the default max_identifier_length for Oracle is 128 characters, which is adjusted down to 30 upon first connect if an older version of Oracle server (compatibility version < 12.2) is detected. LIMIT/OFFSET/FETCH Support -------------------------- Methods like :meth:`_sql.Select.limit` and :meth:`_sql.Select.offset` make use of ``FETCH FIRST N ROW / OFFSET N ROWS`` syntax assuming Oracle 12c or above, and assuming the SELECT statement is not embedded within a compound statement like UNION. This syntax is also available directly by using the :meth:`_sql.Select.fetch` method. .. versionchanged:: 2.0 the Oracle dialect now uses ``FETCH FIRST N ROW / OFFSET N ROWS`` for all :meth:`_sql.Select.limit` and :meth:`_sql.Select.offset` usage including within the ORM and legacy :class:`_orm.Query`. To force the legacy behavior using window functions, specify the ``enable_offset_fetch=False`` dialect parameter to :func:`_sa.create_engine`. The use of ``FETCH FIRST / OFFSET`` may be disabled on any Oracle version by passing ``enable_offset_fetch=False`` to :func:`_sa.create_engine`, which will force the use of "legacy" mode that makes use of window functions. This mode is also selected automatically when using a version of Oracle prior to 12c. When using legacy mode, or when a :class:`.Select` statement with limit/offset is embedded in a compound statement, an emulated approach for LIMIT / OFFSET based on window functions is used, which involves creation of a subquery using ``ROW_NUMBER`` that is prone to performance issues as well as SQL construction issues for complex statements. However, this approach is supported by all Oracle versions. See notes below. Notes on LIMIT / OFFSET emulation (when fetch() method cannot be used) ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ If using :meth:`_sql.Select.limit` and :meth:`_sql.Select.offset`, or with the ORM the :meth:`_orm.Query.limit` and :meth:`_orm.Query.offset` methods on an Oracle version prior to 12c, the following notes apply: * SQLAlchemy currently makes use of ROWNUM to achieve LIMIT/OFFSET; the exact methodology is taken from https://blogs.oracle.com/oraclemagazine/on-rownum-and-limiting-results . * the "FIRST_ROWS()" optimization keyword is not used by default. To enable the usage of this optimization directive, specify ``optimize_limits=True`` to :func:`_sa.create_engine`. .. versionchanged:: 1.4 The Oracle dialect renders limit/offset integer values using a "post compile" scheme which renders the integer directly before passing the statement to the cursor for execution. The ``use_binds_for_limits`` flag no longer has an effect. .. seealso:: :ref:`change_4808`. .. _oracle_returning: RETURNING Support ----------------- The Oracle database supports RETURNING fully for INSERT, UPDATE and DELETE statements that are invoked with a single collection of bound parameters (that is, a ``cursor.execute()`` style statement; SQLAlchemy does not generally support RETURNING with :term:`executemany` statements). Multiple rows may be returned as well. .. versionchanged:: 2.0 the Oracle backend has full support for RETURNING on parity with other backends. ON UPDATE CASCADE ----------------- Oracle doesn't have native ON UPDATE CASCADE functionality. A trigger based solution is available at https://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 ---------------------- .. warning:: The status of Oracle 8 compatibility is not known for SQLAlchemy 2.0. When Oracle 8 is detected, the dialect internally configures itself to the following behaviors: * the use_ansi flag is set to False. This has the effect of converting all JOIN phrases into the WHERE clause, and in the case of LEFT OUTER JOIN makes use of Oracle's (+) operator. * the NVARCHAR2 and NCLOB datatypes are no longer generated as DDL when the :class:`~sqlalchemy.types.Unicode` is used - VARCHAR2 and CLOB are issued instead. This because these types don't seem to work correctly on Oracle 8 even though they are available. The :class:`~sqlalchemy.types.NVARCHAR` and :class:`~sqlalchemy.dialects.oracle.NCLOB` types will always generate NVARCHAR2 and NCLOB. Synonym/DBLINK Reflection ------------------------- When using reflection with Table objects, the dialect can optionally search for tables indicated by synonyms, either in local or remote schemas or accessed over DBLINK, by passing the flag ``oracle_resolve_synonyms=True`` as a keyword argument to the :class:`_schema.Table` construct:: some_table = Table('some_table', autoload_with=some_engine, oracle_resolve_synonyms=True) When this flag is set, the given name (such as ``some_table`` above) will be searched not just in the ``ALL_TABLES`` view, but also within the ``ALL_SYNONYMS`` view to see if this name is actually a synonym to another name. If the synonym is located and refers to a DBLINK, the oracle dialect knows how to locate the table's information using DBLINK syntax(e.g. ``@dblink``). ``oracle_resolve_synonyms`` is accepted wherever reflection arguments are accepted, including methods such as :meth:`_schema.MetaData.reflect` and :meth:`_reflection.Inspector.get_columns`. If synonyms are not in use, this flag should be left disabled. .. _oracle_constraint_reflection: Constraint Reflection --------------------- The Oracle dialect can return information about foreign key, unique, and CHECK constraints, as well as indexes on tables. Raw information regarding these constraints can be acquired using :meth:`_reflection.Inspector.get_foreign_keys`, :meth:`_reflection.Inspector.get_unique_constraints`, :meth:`_reflection.Inspector.get_check_constraints`, and :meth:`_reflection.Inspector.get_indexes`. .. versionchanged:: 1.2 The Oracle dialect can now reflect UNIQUE and CHECK constraints. When using reflection at the :class:`_schema.Table` level, the :class:`_schema.Table` will also include these constraints. Note the following caveats: * When using the :meth:`_reflection.Inspector.get_check_constraints` method, Oracle builds a special "IS NOT NULL" constraint for columns that specify "NOT NULL". This constraint is **not** returned by default; to include the "IS NOT NULL" constraints, pass the flag ``include_all=True``:: from sqlalchemy import create_engine, inspect engine = create_engine("oracle+cx_oracle://s:t@dsn") inspector = inspect(engine) all_check_constraints = inspector.get_check_constraints( "some_table", include_all=True) * in most cases, when reflecting a :class:`_schema.Table`, a UNIQUE constraint will **not** be available as a :class:`.UniqueConstraint` object, as Oracle mirrors unique constraints with a UNIQUE index in most cases (the exception seems to be when two or more unique constraints represent the same columns); the :class:`_schema.Table` will instead represent these using :class:`.Index` with the ``unique=True`` flag set. * Oracle creates an implicit index for the primary key of a table; this index is **excluded** from all index results. * the list of columns reflected for an index will not include column names that start with SYS_NC. Table names with SYSTEM/SYSAUX tablespaces ------------------------------------------- The :meth:`_reflection.Inspector.get_table_names` and :meth:`_reflection.Inspector.get_temp_table_names` methods each return a list of table names for the current engine. These methods are also part of the reflection which occurs within an operation such as :meth:`_schema.MetaData.reflect`. By default, these operations exclude the ``SYSTEM`` and ``SYSAUX`` tablespaces from the operation. In order to change this, the default list of tablespaces excluded can be changed at the engine level using the ``exclude_tablespaces`` parameter:: # exclude SYSAUX and SOME_TABLESPACE, but not SYSTEM e = create_engine( "oracle+cx_oracle://scott:tiger@xe", exclude_tablespaces=["SYSAUX", "SOME_TABLESPACE"]) .. versionadded:: 1.1 DateTime Compatibility ---------------------- Oracle has no datatype known as ``DATETIME``, it instead has only ``DATE``, which can actually store a date and time value. For this reason, the Oracle dialect provides a type :class:`_oracle.DATE` which is a subclass of :class:`.DateTime`. This type has no special behavior, and is only present as a "marker" for this type; additionally, when a database column is reflected and the type is reported as ``DATE``, the time-supporting :class:`_oracle.DATE` type is used. .. versionchanged:: 0.9.4 Added :class:`_oracle.DATE` to subclass :class:`.DateTime`. This is a change as previous versions would reflect a ``DATE`` column as :class:`_types.DATE`, which subclasses :class:`.Date`. The only significance here is for schemes that are examining the type of column for use in special Python translations or for migrating schemas to other database backends. .. _oracle_table_options: Oracle Table Options ------------------------- The CREATE TABLE phrase supports the following options with Oracle in conjunction with the :class:`_schema.Table` construct: * ``ON COMMIT``:: Table( "some_table", metadata, ..., prefixes=['GLOBAL TEMPORARY'], oracle_on_commit='PRESERVE ROWS') .. versionadded:: 1.0.0 * ``COMPRESS``:: Table('mytable', metadata, Column('data', String(32)), oracle_compress=True) Table('mytable', metadata, Column('data', String(32)), oracle_compress=6) The ``oracle_compress`` parameter accepts either an integer compression level, or ``True`` to use the default compression level. .. versionadded:: 1.0.0 .. _oracle_index_options: Oracle Specific Index Options ----------------------------- Bitmap Indexes ~~~~~~~~~~~~~~ You can specify the ``oracle_bitmap`` parameter to create a bitmap index instead of a B-tree index:: Index('my_index', my_table.c.data, oracle_bitmap=True) Bitmap indexes cannot be unique and cannot be compressed. SQLAlchemy will not check for such limitations, only the database will. .. versionadded:: 1.0.0 Index compression ~~~~~~~~~~~~~~~~~ Oracle has a more efficient storage mode for indexes containing lots of repeated values. Use the ``oracle_compress`` parameter to turn on key compression:: Index('my_index', my_table.c.data, oracle_compress=True) Index('my_index', my_table.c.data1, my_table.c.data2, unique=True, oracle_compress=1) The ``oracle_compress`` parameter accepts either an integer specifying the number of prefix columns to compress, or ``True`` to use the default (all columns for non-unique indexes, all but the last column for unique indexes). .. versionadded:: 1.0.0