Release: 1.3.0b1 beta release | Release Date: November 16, 2018

SQLAlchemy 1.3 Documentation

Oracle

Support for the Oracle database.

DBAPI Support

The following dialect/DBAPI options are available. Please refer to individual DBAPI sections for connect information.

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.
  • use_binds_for_limits - defaults to True. 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.

LIMIT/OFFSET Support

Oracle has no support for the LIMIT or OFFSET keywords. SQLAlchemy uses a wrapped subquery approach in conjunction with ROWNUM. The exact methodology is taken from http://www.oracle.com/technetwork/issue-archive/2006/06-sep/o56asktom-086197.html .

There are two options which affect its behavior:

  • the “FIRST ROWS()” optimization keyword is not used by default. To enable the usage of this optimization directive, specify optimize_limits=True to create_engine().
  • the values passed for the limit/offset are sent as bound parameters. Some users have observed that Oracle produces a poor query plan when the values are sent as binds and not rendered literally. To render the limit/offset values literally within the SQL statement, specify use_binds_for_limits=False to create_engine().

Some users have reported better performance when the entirely different approach of a window query is used, i.e. ROW_NUMBER() OVER (ORDER BY), to provide LIMIT/OFFSET (note that the majority of users don’t observe this). To suit this case the method used for LIMIT/OFFSET can be replaced entirely. See the recipe at http://www.sqlalchemy.org/trac/wiki/UsageRecipes/WindowFunctionsByDefault which installs a select compiler that overrides the generation of limit/offset with a window function.

RETURNING Support

The Oracle database supports a limited form of RETURNING, in order to retrieve result sets of matched rows from INSERT, UPDATE and DELETE statements. Oracle’s RETURNING..INTO syntax only supports one row being returned, as it relies upon OUT parameters in order to function. In addition, supported DBAPIs have further limitations (see RETURNING Support).

SQLAlchemy’s “implicit returning” feature, which employs RETURNING within an INSERT and sometimes an UPDATE statement in order to fetch newly generated primary key values and other SQL defaults and expressions, is normally enabled on the Oracle backend. By default, “implicit returning” typically only fetches the value of a single nextval(some_seq) expression embedded into an INSERT in order to increment a sequence within an INSERT statement and get the value back at the same time. To disable this feature across the board, specify implicit_returning=False to create_engine():

engine = create_engine("oracle://scott:tiger@dsn",
                       implicit_returning=False)

Implicit returning can also be disabled on a table-by-table basis as a table option:

# Core Table
my_table = Table("my_table", metadata, ..., implicit_returning=False)


# declarative
class MyClass(Base):
    __tablename__ = 'my_table'
    __table_args__ = {"implicit_returning": False}

See also

RETURNING Support - additional cx_oracle-specific restrictions on implicit returning.

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 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 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 NVARCHAR and NCLOB types will always generate NVARCHAR2 and NCLOB.
  • the “native unicode” mode is disabled when using cx_oracle, i.e. SQLAlchemy encodes all Python unicode objects to “string” before passing in as bind parameters.

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 Inspector.get_foreign_keys(), Inspector.get_unique_constraints(), Inspector.get_check_constraints(), and Inspector.get_indexes().

Changed in version 1.2: The Oracle dialect can now reflect UNIQUE and CHECK constraints.

When using reflection at the Table level, the Table will also include these constraints.

Note the following caveats:

  • When using the 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 Table, a UNIQUE constraint will not be available as a 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 Table will instead represent these using 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 Inspector.get_table_names() and 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 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://scott:tiger@xe",
  exclude_tablespaces=["SYSAUX", "SOME_TABLESPACE"])

New in version 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 oracle.DATE which is a subclass of 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 oracle.DATE type is used.

Changed in version 0.9.4: Added oracle.DATE to subclass DateTime. This is a change as previous versions would reflect a DATE column as types.DATE, which subclasses 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

The CREATE TABLE phrase supports the following options with Oracle in conjunction with the Table construct:

  • ON COMMIT:

    Table(
        "some_table", metadata, ...,
        prefixes=['GLOBAL TEMPORARY'], oracle_on_commit='PRESERVE ROWS')

New in version 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.

New in version 1.0.0.

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.

New in version 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 c ompression:

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).

New in version 1.0.0.

Oracle Data Types

As with all SQLAlchemy dialects, all UPPERCASE types that are known to be valid with Oracle are importable from the top level dialect, whether they originate from sqlalchemy.types or from the local dialect:

from sqlalchemy.dialects.oracle import \
            BFILE, BLOB, CHAR, CLOB, DATE, \
            DOUBLE_PRECISION, FLOAT, INTERVAL, LONG, NCLOB, \
            NUMBER, NVARCHAR, NVARCHAR2, RAW, TIMESTAMP, VARCHAR, \
            VARCHAR2

Types which are specific to Oracle, or have Oracle-specific construction arguments, are as follows:

class sqlalchemy.dialects.oracle.BFILE(length=None)

Bases: sqlalchemy.types.LargeBinary

__init__(length=None)
inherited from the __init__() method of LargeBinary

Construct a LargeBinary type.

Parameters:length – optional, a length for the column for use in DDL statements, for those binary types that accept a length, such as the MySQL BLOB type.
class sqlalchemy.dialects.oracle.DATE(timezone=False)

Bases: sqlalchemy.types.DateTime

Provide the oracle DATE type.

This type has no special Python behavior, except that it subclasses types.DateTime; this is to suit the fact that the Oracle DATE type supports a time value.

New in version 0.9.4.

__init__(timezone=False)
inherited from the __init__() method of DateTime

Construct a new DateTime.

Parameters:timezone – boolean. Indicates that the datetime type should enable timezone support, if available on the base date/time-holding type only. It is recommended to make use of the TIMESTAMP datatype directly when using this flag, as some databases include separate generic date/time-holding types distinct from the timezone-capable TIMESTAMP datatype, such as Oracle.
class sqlalchemy.dialects.oracle.DOUBLE_PRECISION(precision=None, asdecimal=False, decimal_return_scale=None, **kwargs)

Bases: sqlalchemy.types.Float

__init__(precision=None, asdecimal=False, decimal_return_scale=None, **kwargs)
inherited from the __init__() method of Float

Construct a Float.

Parameters:
  • precision – the numeric precision for use in DDL CREATE TABLE.
  • asdecimal – the same flag as that of Numeric, but defaults to False. Note that setting this flag to True results in floating point conversion.
  • decimal_return_scale

    Default scale to use when converting from floats to Python decimals. Floating point values will typically be much longer due to decimal inaccuracy, and most floating point database types don’t have a notion of “scale”, so by default the float type looks for the first ten decimal places when converting. Specfiying this value will override that length. Note that the MySQL float types, which do include “scale”, will use “scale” as the default for decimal_return_scale, if not otherwise specified.

    New in version 0.9.0.

  • **kwargs – deprecated. Additional arguments here are ignored by the default Float type. For database specific floats that support additional arguments, see that dialect’s documentation for details, such as sqlalchemy.dialects.mysql.FLOAT.
class sqlalchemy.dialects.oracle.INTERVAL(day_precision=None, second_precision=None)

Bases: sqlalchemy.types.TypeEngine

__init__(day_precision=None, second_precision=None)

Construct an INTERVAL.

Note that only DAY TO SECOND intervals are currently supported. This is due to a lack of support for YEAR TO MONTH intervals within available DBAPIs (cx_oracle and zxjdbc).

Parameters:
  • day_precision – the day precision value. this is the number of digits to store for the day field. Defaults to “2”
  • second_precision – the second precision value. this is the number of digits to store for the fractional seconds field. Defaults to “6”.
class sqlalchemy.dialects.oracle.NCLOB(length=None, collation=None, convert_unicode=False, unicode_error=None, _warn_on_bytestring=False)

Bases: sqlalchemy.types.Text

__init__(length=None, collation=None, convert_unicode=False, unicode_error=None, _warn_on_bytestring=False)
inherited from the __init__() method of String

Create a string-holding type.

Parameters:
  • length – optional, a length for the column for use in DDL and CAST expressions. May be safely omitted if no CREATE TABLE will be issued. Certain databases may require a length for use in DDL, and will raise an exception when the CREATE TABLE DDL is issued if a VARCHAR with no length is included. Whether the value is interpreted as bytes or characters is database specific.
  • collation

    Optional, a column-level collation for use in DDL and CAST expressions. Renders using the COLLATE keyword supported by SQLite, MySQL, and PostgreSQL. E.g.:

    >>> from sqlalchemy import cast, select, String
    >>> print select([cast('some string', String(collation='utf8'))])
    SELECT CAST(:param_1 AS VARCHAR COLLATE utf8) AS anon_1

    New in version 0.8: Added support for COLLATE to all string types.

  • convert_unicode

    When set to True, the String type will assume that input is to be passed as Python unicode objects, and results returned as Python unicode objects. If the DBAPI in use does not support Python unicode (which is fewer and fewer these days), SQLAlchemy will encode/decode the value, using the value of the encoding parameter passed to create_engine() as the encoding.

    When using a DBAPI that natively supports Python unicode objects, this flag generally does not need to be set. For columns that are explicitly intended to store non-ASCII data, the Unicode or UnicodeText types should be used regardless, which feature the same behavior of convert_unicode but also indicate an underlying column type that directly supports unicode, such as NVARCHAR.

    For the extremely rare case that Python unicode is to be encoded/decoded by SQLAlchemy on a backend that does natively support Python unicode, the value force can be passed here which will cause SQLAlchemy’s encode/decode services to be used unconditionally.

  • unicode_error – Optional, a method to use to handle Unicode conversion errors. Behaves like the errors keyword argument to the standard library’s string.decode() functions. This flag requires that convert_unicode is set to force - otherwise, SQLAlchemy is not guaranteed to handle the task of unicode conversion. Note that this flag adds significant performance overhead to row-fetching operations for backends that already return unicode objects natively (which most DBAPIs do). This flag should only be used as a last resort for reading strings from a column with varied or corrupted encodings.
class sqlalchemy.dialects.oracle.NUMBER(precision=None, scale=None, asdecimal=None)

Bases: sqlalchemy.types.Numeric, sqlalchemy.types.Integer

__init__(precision=None, scale=None, asdecimal=None)

Construct a Numeric.

Parameters:
  • precision – the numeric precision for use in DDL CREATE TABLE.
  • scale – the numeric scale for use in DDL CREATE TABLE.
  • asdecimal – default True. Return whether or not values should be sent as Python Decimal objects, or as floats. Different DBAPIs send one or the other based on datatypes - the Numeric type will ensure that return values are one or the other across DBAPIs consistently.
  • decimal_return_scale

    Default scale to use when converting from floats to Python decimals. Floating point values will typically be much longer due to decimal inaccuracy, and most floating point database types don’t have a notion of “scale”, so by default the float type looks for the first ten decimal places when converting. Specfiying this value will override that length. Types which do include an explicit “.scale” value, such as the base Numeric as well as the MySQL float types, will use the value of “.scale” as the default for decimal_return_scale, if not otherwise specified.

    New in version 0.9.0.

When using the Numeric type, care should be taken to ensure that the asdecimal setting is apppropriate for the DBAPI in use - when Numeric applies a conversion from Decimal->float or float-> Decimal, this conversion incurs an additional performance overhead for all result columns received.

DBAPIs that return Decimal natively (e.g. psycopg2) will have better accuracy and higher performance with a setting of True, as the native translation to Decimal reduces the amount of floating- point issues at play, and the Numeric type itself doesn’t need to apply any further conversions. However, another DBAPI which returns floats natively will incur an additional conversion overhead, and is still subject to floating point data loss - in which case asdecimal=False will at least remove the extra conversion overhead.

class sqlalchemy.dialects.oracle.LONG(length=None, collation=None, convert_unicode=False, unicode_error=None, _warn_on_bytestring=False)

Bases: sqlalchemy.types.Text

__init__(length=None, collation=None, convert_unicode=False, unicode_error=None, _warn_on_bytestring=False)
inherited from the __init__() method of String

Create a string-holding type.

Parameters:
  • length – optional, a length for the column for use in DDL and CAST expressions. May be safely omitted if no CREATE TABLE will be issued. Certain databases may require a length for use in DDL, and will raise an exception when the CREATE TABLE DDL is issued if a VARCHAR with no length is included. Whether the value is interpreted as bytes or characters is database specific.
  • collation

    Optional, a column-level collation for use in DDL and CAST expressions. Renders using the COLLATE keyword supported by SQLite, MySQL, and PostgreSQL. E.g.:

    >>> from sqlalchemy import cast, select, String
    >>> print select([cast('some string', String(collation='utf8'))])
    SELECT CAST(:param_1 AS VARCHAR COLLATE utf8) AS anon_1

    New in version 0.8: Added support for COLLATE to all string types.

  • convert_unicode

    When set to True, the String type will assume that input is to be passed as Python unicode objects, and results returned as Python unicode objects. If the DBAPI in use does not support Python unicode (which is fewer and fewer these days), SQLAlchemy will encode/decode the value, using the value of the encoding parameter passed to create_engine() as the encoding.

    When using a DBAPI that natively supports Python unicode objects, this flag generally does not need to be set. For columns that are explicitly intended to store non-ASCII data, the Unicode or UnicodeText types should be used regardless, which feature the same behavior of convert_unicode but also indicate an underlying column type that directly supports unicode, such as NVARCHAR.

    For the extremely rare case that Python unicode is to be encoded/decoded by SQLAlchemy on a backend that does natively support Python unicode, the value force can be passed here which will cause SQLAlchemy’s encode/decode services to be used unconditionally.

  • unicode_error – Optional, a method to use to handle Unicode conversion errors. Behaves like the errors keyword argument to the standard library’s string.decode() functions. This flag requires that convert_unicode is set to force - otherwise, SQLAlchemy is not guaranteed to handle the task of unicode conversion. Note that this flag adds significant performance overhead to row-fetching operations for backends that already return unicode objects natively (which most DBAPIs do). This flag should only be used as a last resort for reading strings from a column with varied or corrupted encodings.
class sqlalchemy.dialects.oracle.RAW(length=None)

Bases: sqlalchemy.types._Binary

__init__(length=None)
inherited from the __init__() method of _Binary

Initialize self. See help(type(self)) for accurate signature.

cx_Oracle

Support for the Oracle database via the cx-Oracle driver.

DBAPI

Documentation and download information (if applicable) for cx-Oracle is available at: http://cx-oracle.sourceforge.net/

Connecting

Connect String:

oracle+cx_oracle://user:pass@host:port/dbname[?key=value&key=value...]

Additional Connect Arguments

When connecting with the dbname URL token present, the hostname, port, and dbname tokens are converted to a TNS name using the cx_Oracle.makedsn() function. The URL below:

e = create_engine("oracle+cx_oracle://user:pass@hostname/dbname")

Will be used to create the DSN as follows:

>>> import cx_Oracle
>>> cx_Oracle.makedsn("hostname", 1521, sid="dbname")
'(DESCRIPTION=(ADDRESS=(PROTOCOL=TCP)(HOST=hostname)(PORT=1521))(CONNECT_DATA=(SID=dbname)))'

The service_name parameter, also consumed by cx_Oracle.makedsn(), may be specified in the URL query string, e.g. ?service_name=my_service.

If dbname is not present, then the value of hostname in the URL is used directly as the DSN passed to cx_Oracle.connect().

Additional connection arguments may be sent to the cx_Oracle.connect() function using the create_engine.connect_args dictionary. Any cx_Oracle parameter value and/or constant may be passed, such as:

import cx_Oracle
e = create_engine(
    "oracle+cx_oracle://user:pass@dsn",
    connect_args={
        "mode": cx_Oracle.SYSDBA,
        "events": True
    }
)

Alternatively, most cx_Oracle DBAPI arguments can also be encoded as strings within the URL, which includes parameters such as mode, purity, events, threaded, and others:

e = create_engine("oracle+cx_oracle://user:pass@dsn?mode=SYSDBA&events=true")

Changed in version 1.3: the cx_oracle dialect now accepts all argument names within the URL string itself, to be passed to the cx_Oracle DBAPI. As was the case earlier but not correctly documented, the create_engine.connect_args parameter also accepts all cx_Oracle DBAPI connect arguments.

There are also options that are consumed by the SQLAlchemy cx_oracle dialect itself. These options are always passed directly to create_engine(), such as:

e = create_engine("oracle+cx_oracle://user:pass@dsn", coerce_to_unicode=False)

The parameters accepted by the cx_oracle dialect are as follows:

  • arraysize - set the cx_oracle.arraysize value on cursors, defaulted to 50. This setting is significant with cx_Oracle as the contents of LOB objects are only readable within a “live” row (e.g. within a batch of 50 rows).
  • auto_convert_lobs - defaults to True; See LOB Objects.
  • coerce_to_unicode - see Unicode for detail.
  • coerce_to_decimal - see Precision Numerics for detail.

Unicode

The cx_Oracle DBAPI as of version 5 fully supports Unicode, and has the ability to return string results as Python Unicode objects natively.

Explicit Unicode support is available by using the Unicode datatype with SQLAlchemy Core expression language, as well as the UnicodeText datatype. These types correspond to the VARCHAR2 and CLOB Oracle datatypes by default. When using these datatypes with Unicode data, it is expected that the Oracle database is configured with a Unicode-aware character set, as well as that the NLS_LANG environment variable is set appropriately, so that the VARCHAR2 and CLOB datatypes can accommodate the data.

In the case that the Oracle database is not configured with a Unicode character set, the two options are to use the oracle.NCHAR and oracle.NCLOB datatypes explicitly, or to pass the flag use_nchar_for_unicode=True to create_engine(), which will cause the SQLAlchemy dialect to use NCHAR/NCLOB for the Unicode / UnicodeText datatypes instead of VARCHAR/CLOB.

Changed in version 1.3: The Unicode and UnicodeText datatypes now correspond to the VARCHAR2 and CLOB Oracle datatypes unless the use_nchar_for_unicode=True is passed to the dialect when create_engine() is called.

When result sets are fetched that include strings, under Python 3 the cx_Oracle DBAPI returns all strings as Python Unicode objects, since Python 3 only has a Unicode string type. This occurs for data fetched from datatypes such as VARCHAR2, CHAR, CLOB, NCHAR, NCLOB, etc. In order to provide cross- compatibility under Python 2, the SQLAlchemy cx_Oracle dialect will add Unicode-conversion to string data under Python 2 as well. Historically, this made use of converters that were supplied by cx_Oracle but were found to be non-performant; SQLAlchemy’s own converters are used for the string to Unicode conversion under Python 2. To disable the Python 2 Unicode conversion for VARCHAR2, CHAR, and CLOB, the flag coerce_to_unicode=False can be passed to create_engine().

Changed in version 1.3: Unicode conversion is applied to all string values by default under python 2. The coerce_to_unicode now defaults to True and can be set to False to disable the Unicode coersion of strings that are delivered as VARCHAR2/CHAR/CLOB data.

Fine grained control over cx_Oracle data binding and performance with setinputsizes

The cx_Oracle DBAPI has a deep and fundamental reliance upon the usage of the DBAPI setinputsizes() call. The purpose of this call is to establish the datatypes that are bound to a SQL statement for Python values being passed as parameters. While virtually no other DBAPI assigns any use to the setinputsizes() call, the cx_Oracle DBAPI relies upon it heavily in its interactions with the Oracle client interface, and in some scenarios it is not possible for SQLAlchemy to know exactly how data should be bound, as some settings can cause profoundly different performance characteristics, while altering the type coercion behavior at the same time.

Users of the cx_Oracle dialect are strongly encouraged to read through cx_Oracle’s list of built-in datatype symbols at http://cx-oracle.readthedocs.io/en/latest/module.html#types. Note that in some cases, signficant performance degradation can occur when using these types vs. not, in particular when specifying cx_Oracle.CLOB.

On the SQLAlchemy side, the DialectEvents.do_setinputsizes() event can be used both for runtime visibliity (e.g. logging) of the setinputsizes step as well as to fully control how setinputsizes() is used on a per-statement basis.

New in version 1.2.9: Added DialectEvents.setinputsizes()

Example 1 - logging all setinputsizes calls

The following example illustrates how to log the intermediary values from a SQLAlchemy perspective before they are converted to the raw setinputsizes() parameter dictionary. The keys of the dictionary are BindParameter objects which have a .key and a .type attribute:

from sqlalchemy import create_engine, event

engine = create_engine("oracle+cx_oracle://scott:tiger@host/xe")

@event.listens_for(engine, "do_setinputsizes")
def _log_setinputsizes(inputsizes, cursor, statement, parameters, context):
    for bindparam, dbapitype in inputsizes.items():
            log.info(
                "Bound parameter name: %s  SQLAlchemy type: %r  "
                "DBAPI object: %s",
                bindparam.key, bindparam.type, dbapitype)

Example 2 - remove all bindings to CLOB

The CLOB datatype in cx_Oracle incurs a significant performance overhead, however is set by default for the Text type within the SQLAlchemy 1.2 series. This setting can be modified as follows:

from sqlalchemy import create_engine, event
from cx_Oracle import CLOB

engine = create_engine("oracle+cx_oracle://scott:tiger@host/xe")

@event.listens_for(engine, "do_setinputsizes")
def _remove_clob(inputsizes, cursor, statement, parameters, context):
    for bindparam, dbapitype in list(inputsizes.items()):
        if dbapitype is CLOB:
            del inputsizes[bindparam]

RETURNING Support

The cx_Oracle dialect implements RETURNING using OUT parameters. The dialect supports RETURNING fully, however cx_Oracle 6 is recommended for complete support.

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, which takes place within a cx_Oracle outputtypehandler.

cx_Oracle prior to version 6 would require that LOB objects be read before a new batch of rows would be read, as determined by the cursor.arraysize. As of the 6 series, this limitation has been lifted. Nevertheless, because SQLAlchemy pre-reads these LOBs up front, this issue is avoided in any case.

To disable the auto “read()” feature of the dialect, the flag auto_convert_lobs=False may be passed to create_engine(). Under the cx_Oracle 5 series, having this flag turned off means there is the chance of reading from a stale LOB object if not read as it is fetched. With cx_Oracle 6, this issue is resolved.

Changed in version 1.2: the LOB handling system has been greatly simplified internally to make use of outputtypehandlers, and no longer makes use of alternate “buffered” result set objects.

Two Phase Transactions Not Supported

Two phase transactions are not supported under cx_Oracle due to poor driver support. As of cx_Oracle 6.0b1, the interface for two phase transactions has been changed to be more of a direct pass-through to the underlying OCI layer with less automation. The additional logic to support this system is not implemented in SQLAlchemy.

Precision Numerics

SQLAlchemy’s numeric types can handle receiving and returning values as Python Decimal objects or float objects. When a Numeric object, or a subclass such as Float, oracle.DOUBLE_PRECISION etc. is in use, the Numeric.asdecimal flag determines if values should be coerced to Decimal upon return, or returned as float objects. To make matters more complicated under Oracle, Oracle’s NUMBER type can also represent integer values if the “scale” is zero, so the Oracle-specific oracle.NUMBER type takes this into account as well.

The cx_Oracle dialect makes extensive use of connection- and cursor-level “outputtypehandler” callables in order to coerce numeric values as requested. These callables are specific to the specific flavor of Numeric in use, as well as if no SQLAlchemy typing objects are present. There are observed scenarios where Oracle may sends incomplete or ambiguous information about the numeric types being returned, such as a query where the numeric types are buried under multiple levels of subquery. The type handlers do their best to make the right decision in all cases, deferring to the underlying cx_Oracle DBAPI for all those cases where the driver can make the best decision.

When no typing objects are present, as when executing plain SQL strings, a default “outputtypehandler” is present which will generally return numeric values which specify precision and scale as Python Decimal objects. To disable this coercion to decimal for performance reasons, pass the flag coerce_to_decimal=False to create_engine():

engine = create_engine("oracle+cx_oracle://dsn", coerce_to_decimal=False)

The coerce_to_decimal flag only impacts the results of plain string SQL staements that are not otherwise associated with a Numeric SQLAlchemy type (or a subclass of such).

Changed in version 1.2: The numeric handling system for cx_Oracle has been reworked to take advantage of newer cx_Oracle features as well as better integration of outputtypehandlers.

zxjdbc

Support for the Oracle database via the zxJDBC for Jython driver.

Note

Jython is not supported by current versions of SQLAlchemy. The zxjdbc dialect should be considered as experimental.

DBAPI

Drivers for this database are available at: http://www.oracle.com/technetwork/database/features/jdbc/index-091264.html

Connecting

Connect String:

oracle+zxjdbc://user:pass@host/dbname

Previous: MySQL Next: PostgreSQL