sqlalchemy.dialects.oracle.cx_oracle

DSN vs. Hostname connections

cx_Oracle provides several methods of indicating the target database. The dialect translates from a series of different URL forms.

engine = create_engine("oracle+cx_oracle://scott:tiger@hostname:port/?service_name=myservice&encoding=UTF-8&nencoding=UTF-8")

myalias =
  (DESCRIPTION =
    (ADDRESS = (PROTOCOL = TCP)(HOST = mymachine.example.com)(PORT = 1521))
    (CONNECT_DATA =
      (SERVER = DEDICATED)
      (SERVICE_NAME = orclpdb1)
    )
  )

engine = create_engine("oracle+cx_oracle://scott:tiger@myalias/?encoding=UTF-8&nencoding=UTF-8")

engine = create_engine("oracle+cx_oracle://scott:tiger@hostname:1521/dbname?encoding=UTF-8&nencoding=UTF-8")

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

Passing cx_Oracle connect arguments

Additional connection arguments can usually be passed via the URL query string; particular symbols like cx_Oracle.SYSDBA are intercepted and converted to the correct symbol:

e = create_engine(
    "oracle+cx_oracle://user:pass@dsn?encoding=UTF-8&nencoding=UTF-8&mode=SYSDBA&events=true")

To pass arguments directly to .connect() without using the query string, use the :paramref:`_sa.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={
        "encoding": "UTF-8",
        "nencoding": "UTF-8",
        "mode": cx_Oracle.SYSDBA,
        "events": True
    }
)

Note that the default value for encoding and nencoding was changed to "UTF-8" in cx_Oracle 8.0 so these parameters can be omitted when using that version, or later.

Options consumed by the SQLAlchemy cx_Oracle dialect outside of the driver

There are also options that are consumed by the SQLAlchemy cx_oracle dialect itself. These options are always passed directly to _sa.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 :ref:`cx_oracle_lob`.
• coerce_to_unicode - see :ref:`cx_oracle_unicode` for detail.
• coerce_to_decimal - see :ref:`cx_oracle_numeric` for detail.
• encoding_errors - see :ref:`cx_oracle_unicode_encoding_errors` for detail.

Using cx_Oracle SessionPool

The cx_Oracle library provides its own connection pool implementation that may be used in place of SQLAlchemy's pooling functionality. This can be achieved by using the :paramref:`_sa.create_engine.creator` parameter to provide a function that returns a new connection, along with setting :paramref:`_sa.create_engine.pool_class` to NullPool to disable SQLAlchemy's pooling:

import cx_Oracle
from sqlalchemy import create_engine
from sqlalchemy.pool import NullPool

pool = cx_Oracle.SessionPool(
    user="scott", password="tiger", dsn="orclpdb",
    min=2, max=5, increment=1, threaded=True,
    encoding="UTF-8", nencoding="UTF-8"
)

engine = create_engine("oracle://", creator=pool.acquire, poolclass=NullPool)

The above engine may then be used normally where cx_Oracle's pool handles connection pooling:

with engine.connect() as conn:
    print(conn.scalar("select 1 FROM dual"))

As well as providing a scalable solution for multi-user applications, the cx_Oracle session pool supports some Oracle features such as DRCP and Application Continuity.

Using Oracle Database Resident Connection Pooling (DRCP)

When using Oracle's DRCP, the best practice is to pass a connection class and "purity" when acquiring a connection from the SessionPool. Refer to the cx_Oracle DRCP documentation.

This can be achieved by wrapping pool.acquire():

import cx_Oracle
from sqlalchemy import create_engine
from sqlalchemy.pool import NullPool

pool = cx_Oracle.SessionPool(
    user="scott", password="tiger", dsn="orclpdb",
    min=2, max=5, increment=1, threaded=True,
    encoding="UTF-8", nencoding="UTF-8"
)

def creator():
    return pool.acquire(cclass="MYCLASS", purity=cx_Oracle.ATTR_PURITY_SELF)

engine = create_engine("oracle://", creator=creator, poolclass=NullPool)

The above engine may then be used normally where cx_Oracle handles session pooling and Oracle Database additionally uses DRCP:

with engine.connect() as conn:
    print(conn.scalar("select 1 FROM dual"))

Unicode

As is the case for all DBAPIs under Python 3, all strings are inherently Unicode strings. Under Python 2, cx_Oracle also supports Python Unicode objects directly. In all cases however, the driver requires an explicit encoding configuration.

engine = create_engine("oracle+cx_oracle://scott:tiger@orclpdb/?encoding=UTF-8&nencoding=UTF-8")

Fine grained control over cx_Oracle data binding 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 https://cx-oracle.readthedocs.io/en/latest/api_manual/module.html#database-types. Note that in some cases, significant 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 visibility (e.g. logging) of the setinputsizes step as well as to fully control how setinputsizes() is used on a per-statement basis.

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)

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 _sa.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.

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 :paramref:`.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 _sa.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 statements that are not otherwise associated with a .Numeric SQLAlchemy type (or a subclass of such).