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python-cx_Oracle/doc/src/user_guide/sql_execution.rst
Anthony Tuininga 9377a9a0ef Documentation improvements.
2020-04-06 13:52:58 -06:00

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.. _sqlexecution:
*************
SQL Execution
*************
Executing SQL statements is the primary way in which a Python application
communicates with Oracle Database. Statements are executed using the methods
:meth:`Cursor.execute()` or :meth:`Cursor.executemany()`. Statements include
queries, Data Manipulation Language (DML), and Data Definition Language (DDL).
A few other `specialty statements
<https://www.oracle.com/pls/topic/lookup?ctx=dblatest&
id=GUID-E1749EF5-2264-44DF-99EF-AEBEB943BED6>`__ can also be executed.
PL/SQL statements are discussed in :ref:`plsqlexecution`. Other chapters
contain information on specific data types and features. See :ref:`batchstmnt`,
:ref:`lobdata`, :ref:`jsondatatype`, and :ref:`xmldatatype`.
cx_Oracle can be used to execute individual statements, one at a time. It does
not read SQL*Plus ".sql" files. To read SQL files, use a technique like the one
in ``RunSqlScript()`` in `samples/SampleEnv.py
<https://github.com/oracle/python-cx_Oracle/blob/master/samples/SampleEnv.py>`__
SQL statements should not contain a trailing semicolon (";") or forward slash
("/"). This will fail:
.. code-block:: sql
cur.execute("select * from MyTable;")
This is correct:
.. code-block:: sql
cur.execute("select * from MyTable")
SQL Queries
===========
Queries (statements beginning with SELECT or WITH) can only be executed using
the method :meth:`Cursor.execute()`. Rows can then be iterated over, or can be
fetched using one of the methods :meth:`Cursor.fetchone()`,
:meth:`Cursor.fetchmany()` or :meth:`Cursor.fetchall()`. There is a
:ref:`default type mapping <defaultfetchtypes>` to Python types that can be
optionally :ref:`overridden <outputtypehandlers>`.
.. IMPORTANT::
Interpolating or concatenating user data with SQL statements, for example
``cur.execute("SELECT * FROM mytab WHERE mycol = '" + myvar + "'")``, is a security risk
and impacts performance. Use :ref:`bind variables <bind>` instead. For
example, ``cur.execute("SELECT * FROM mytab WHERE mycol = :mybv", mybv=myvar)``.
.. _fetching:
Fetch Methods
-------------
After :meth:`Cursor.execute()`, the cursor is returned as a convenience. This
allows code to iterate over rows like:
.. code-block:: python
cur = connection.cursor()
for row in cur.execute("select * from MyTable"):
print(row)
Rows can also be fetched one at a time using the method
:meth:`Cursor.fetchone()`:
.. code-block:: python
cur = connection.cursor()
cur.execute("select * from MyTable")
while True:
row = cur.fetchone()
if row is None:
break
print(row)
If rows need to be processed in batches, the method :meth:`Cursor.fetchmany()`
can be used. The size of the batch is controlled by the ``numRows`` parameter,
which defaults to the value of :attr:`Cursor.arraysize`.
.. code-block:: python
cur = connection.cursor()
cur.execute("select * from MyTable")
numRows = 10
while True:
rows = cur.fetchmany(numRows)
if not rows:
break
for row in rows:
print(row)
If all of the rows need to be fetched, and can be contained in memory, the
method :meth:`Cursor.fetchall()` can be used.
.. code-block:: python
cur = connection.cursor()
cur.execute("select * from MyTable")
rows = cur.fetchall()
for row in rows:
print(row)
Closing Cursors
---------------
A cursor may be used to execute multiple statements. Once it is no longer
needed, it should be closed by calling :meth:`~Cursor.close()` in order to
reclaim resources in the database. It will be closed automatically when the
variable referencing it goes out of scope (and no further references are
retained). One other way to control the lifetime of a cursor is to use a "with"
block, which ensures that a cursor is closed once the block is completed. For
example:
.. code-block:: python
with connection.cursor() as cursor:
for row in cursor.execute("select * from MyTable"):
print(row)
This code ensures that, once the block is completed, the cursor is closed and
resources have been reclaimed by the database. In addition, any attempt to use
the variable ``cursor`` outside of the block will simply fail.
.. _tuningfetch:
Tuning Fetch Performance
------------------------
For best performance, the cx_Oracle :attr:`Cursor.arraysize` value should be set
before calling :meth:`Cursor.execute()`. The default value is 100. For queries
that return a large number of rows, increasing ``arraysize`` can improve
performance because it reduces the number of round-trips to the database.
However increasing this value increases the amount of memory required. The best
value for your system depends on factors like your network speed, the query row
size, and available memory. An appropriate value can be found by experimenting
with your application.
Regardless of which fetch method is used to get rows, internally all rows are
fetched in batches corresponding to the value of ``arraysize``. The size does
not affect how, or when, rows are returned to your application (other than being
used as the default size for :meth:`Cursor.fetchmany()`). It does not limit the
minimum or maximum number of rows returned by a query.
Along with tuning ``arraysize``, make sure your `SQL statements are optimal
<https://www.oracle.com/pls/topic/lookup?ctx=dblatest&id=TGSQL>`_ and avoid
selecting columns that are not required by the application. For queries that do
not need to fetch all data, use a :ref:`row limiting clause <rowlimit>` to
reduce the number of rows processed by the database.
An example of setting ``arraysize`` is:
.. code-block:: python
cur = connection.cursor()
cur.arraysize = 500
for row in cur.execute("select * from MyTable"):
print(row)
One place where increasing ``arraysize`` is particularly useful is in copying
data from one database to another:
.. code-block:: python
# setup cursors
sourceCursor = sourceConnection.cursor()
sourceCursor.arraysize = 1000
targetCursor = targetConnection.cursor()
targetCursor.arraysize = 1000
# perform fetch and bulk insertion
sourceCursor.execute("select * from MyTable")
while True:
rows = sourceCursor.fetchmany()
if not rows:
break
targetCursor.executemany("insert into MyTable values (:1, :2)", rows)
targetConnection.commit()
If you know that a query returns a small number of rows then you should reduce
the value of ``arraysize``. For example if you are fetching only one row, then
set ``arraysize`` to 1:
.. code-block:: python
cur = connection.cursor()
cur.arraysize = 1
cur.execute("select * from MyTable where id = 1"):
row = cur.fetchone()
print(row)
In cx_Oracle, the ``arraysize`` value is only examined when a statement is
executed the first time. To change the ``arraysize`` for a repeated statement,
create a new cursor:
.. code-block:: python
array_sizes = (10, 100, 1000)
for size in array_sizes:
cursor = connection.cursor()
cursor.arraysize = size
start = time.time()
cursor.execute(sql).fetchall()
elapsed = time.time() - start
print("Time for", size, elapsed, "seconds")
.. _querymetadata:
Query Column Metadata
---------------------
After executing a query, the column metadata such as column names and data types
can be obtained using :attr:`Cursor.description`:
.. code-block:: python
cur = connection.cursor()
cur.execute("select * from MyTable")
for column in cur.description:
print(column)
This could result in metadata like::
('ID', <class 'cx_Oracle.DB_TYPE_NUMBER'>, 39, None, 38, 0, 0)
('NAME', <class 'cx_Oracle.DB_TYPE_VARCHAR'>, 20, 20, None, None, 1)
.. _defaultfetchtypes:
Fetch Data Types
----------------
The following table provides a list of all of the data types that cx_Oracle
knows how to fetch. The middle column gives the type that is returned in the
:ref:`query metadata <querymetadata>`. The last column gives the type of
Python object that is returned by default. Python types can be changed with
:ref:`Output Type Handlers <outputtypehandlers>`.
.. list-table::
:header-rows: 1
:widths: 1 1 1
:align: left
* - Oracle Database Type
- cx_Oracle Database Type
- Default Python type
* - BFILE
- :attr:`cx_Oracle.DB_TYPE_BFILE`
- :ref:`cx_Oracle.LOB <lobobj>`
* - BINARY_DOUBLE
- :attr:`cx_Oracle.DB_TYPE_BINARY_DOUBLE`
- float
* - BINARY_FLOAT
- :attr:`cx_Oracle.DB_TYPE_BINARY_FLOAT`
- float
* - BLOB
- :attr:`cx_Oracle.DB_TYPE_BLOB`
- :ref:`cx_Oracle.LOB <lobobj>`
* - CHAR
- :attr:`cx_Oracle.DB_TYPE_CHAR`
- str
* - CLOB
- :attr:`cx_Oracle.DB_TYPE_CLOB`
- :ref:`cx_Oracle.LOB <lobobj>`
* - CURSOR
- :attr:`cx_Oracle.DB_TYPE_CURSOR`
- :ref:`cx_Oracle.Cursor <cursorobj>`
* - DATE
- :attr:`cx_Oracle.DB_TYPE_DATE`
- datetime.datetime
* - INTERVAL DAY TO SECOND
- :attr:`cx_Oracle.DB_TYPE_INTERVAL_DS`
- datetime.timedelta
* - LONG
- :attr:`cx_Oracle.DB_TYPE_LONG`
- str
* - LONG RAW
- :attr:`cx_Oracle.DB_TYPE_LONG_RAW`
- bytes
* - NCHAR
- :attr:`cx_Oracle.DB_TYPE_NCHAR`
- str
* - NCLOB
- :attr:`cx_Oracle.DB_TYPE_NCLOB`
- :ref:`cx_Oracle.LOB <lobobj>`
* - NUMBER
- :attr:`cx_Oracle.DB_TYPE_NUMBER`
- float or int [1]_
* - NVARCHAR2
- :attr:`cx_Oracle.DB_TYPE_NVARCHAR`
- str
* - OBJECT [3]_
- :attr:`cx_Oracle.DB_TYPE_OBJECT`
- :ref:`cx_Oracle.Object <objecttype>`
* - RAW
- :attr:`cx_Oracle.DB_TYPE_RAW`
- bytes
* - ROWID
- :attr:`cx_Oracle.DB_TYPE_ROWID`
- str
* - TIMESTAMP
- :attr:`cx_Oracle.DB_TYPE_TIMESTAMP`
- datetime.datetime
* - TIMESTAMP WITH LOCAL TIME ZONE
- :attr:`cx_Oracle.DB_TYPE_TIMESTAMP_LTZ`
- datetime.datetime [2]_
* - TIMESTAMP WITH TIME ZONE
- :attr:`cx_Oracle.DB_TYPE_TIMESTAMP_TZ`
- datetime.datetime [2]_
* - UROWID
- :attr:`cx_Oracle.DB_TYPE_ROWID`
- str
* - VARCHAR2
- :attr:`cx_Oracle.DB_TYPE_VARCHAR`
- str
.. [1] If the precision and scale obtained from query column metadata indicate
that the value can be expressed as an integer, the value will be
returned as an int. If the column is unconstrained (no precision and
scale specified), the value will be returned as a float or an int
depending on whether the value itself is an integer. In all other cases
the value is returned as a float.
.. [2] The timestamps returned are naive timestamps without any time zone
information present.
.. [3] These include all user-defined types such as VARRAY, NESTED TABLE, etc.
.. _outputtypehandlers:
Changing Fetched Data Types with Output Type Handlers
-----------------------------------------------------
Sometimes the default conversion from an Oracle Database type to a Python type
must be changed in order to prevent data loss or to fit the purposes of the
Python application. In such cases, an output type handler can be specified for
queries. Output type handlers do not affect values returned from
:meth:`Cursor.callfunc()` or :meth:`Cursor.callproc()`.
Output type handlers can be specified on the :attr:`connection
<Connection.outputtypehandler>` or on the :attr:`cursor
<Cursor.outputtypehandler>`. If specified on the cursor, fetch type handling is
only changed on that particular cursor. If specified on the connection, all
cursors created by that connection will have their fetch type handling changed.
The output type handler is expected to be a function with the following
signature::
handler(cursor, name, defaultType, size, precision, scale)
The parameters are the same information as the query column metadata found in
:attr:`Cursor.description`. The function is called once for each column that is
going to be fetched. The function is expected to return a
:ref:`variable object <varobj>` (generally by a call to :func:`Cursor.var()`)
or the value ``None``. The value ``None`` indicates that the default type
should be used.
Examples of output handlers are shown in :ref:`numberprecision` and
:ref:`directlobs`.
.. _numberprecision:
Fetched Number Precision
------------------------
One reason for using an output type handler is to ensure that numeric precision
is not lost when fetching certain numbers. Oracle Database uses decimal numbers
and these cannot be converted seamlessly to binary number representations like
Python floats. In addition, the range of Oracle numbers exceeds that of
floating point numbers. Python has decimal objects which do not have these
limitations and cx_Oracle knows how to perform the conversion between Oracle
numbers and Python decimal values if directed to do so.
The following code sample demonstrates the issue:
.. code-block:: python
cur = connection.cursor()
cur.execute("create table test_float (X number(5, 3))")
cur.execute("insert into test_float values (7.1)")
connection.commit()
cur.execute("select * from test_float")
val, = cur.fetchone()
print(val, "* 3 =", val * 3)
This displays ``7.1 * 3 = 21.299999999999997``
Using Python decimal objects, however, there is no loss of precision:
.. code-block:: python
import decimal
def NumberToDecimal(cursor, name, defaultType, size, precision, scale):
if defaultType == cx_Oracle.DB_TYPE_NUMBER:
return cursor.var(decimal.Decimal, arraysize=cursor.arraysize)
cur = connection.cursor()
cur.outputtypehandler = NumberToDecimal
cur.execute("select * from test_float")
val, = cur.fetchone()
print(val, "* 3 =", val * 3)
This displays ``7.1 * 3 = 21.3``
The Python ``decimal.Decimal`` converter gets called with the string
representation of the Oracle number. The output from ``decimal.Decimal`` is
returned in the output tuple.
.. _outconverters:
Changing Query Results with Outconverters
-----------------------------------------
cx_Oracle "outconverters" can be used with :ref:`output type handlers
<outputtypehandlers>` to change returned data.
For example, to make queries return empty strings instead of NULLs:
.. code-block:: python
def OutConverter(value):
if value is None:
return ''
return value
def OutputTypeHandler(cursor, name, defaultType, size, precision, scale):
if defaultType in (cx_Oracle.DB_TYPE_VARCHAR, cx_Oracle.DB_TYPE_CHAR):
return cursor.var(str, size, cur.arraysize, outconverter=OutConverter)
connection.outputtypehandler = OutputTypeHandler
.. _scrollablecursors:
Scrollable Cursors
------------------
Scrollable cursors enable applications to move backwards, forwards, to skip
rows, and to move to a particular row in a query result set. The result set is
cached on the database server until the cursor is closed. In contrast, regular
cursors are restricted to moving forward.
A scrollable cursor is created by setting the parameter ``scrollable=True``
when creating the cursor. The method :meth:`Cursor.scroll()` is used to move to
different locations in the result set.
Examples are:
.. code-block:: python
cursor = connection.cursor(scrollable=True)
cursor.execute("select * from ChildTable order by ChildId")
cursor.scroll(mode="last")
print("LAST ROW:", cursor.fetchone())
cursor.scroll(mode="first")
print("FIRST ROW:", cursor.fetchone())
cursor.scroll(8, mode="absolute")
print("ROW 8:", cursor.fetchone())
cursor.scroll(6)
print("SKIP 6 ROWS:", cursor.fetchone())
cursor.scroll(-4)
print("SKIP BACK 4 ROWS:", cursor.fetchone())
.. _fetchobjects:
Fetching Oracle Database Objects and Collections
------------------------------------------------
Oracle Database named object types and user-defined types can be fetched
directly in queries. Each item is represented as a :ref:`Python object
<objecttype>` corresponding to the Oracle Database object. This Python object
can be traversed to access its elements. Attributes including
:attr:`ObjectType.name` and :attr:`ObjectType.iscollection`, and methods
including :meth:`Object.aslist` and :meth:`Object.asdict` are available.
For example, if a table ``mygeometrytab`` contains a column ``geometry`` of
Oracle's predefined Spatial object type `SDO_GEOMETRY
<https://www.oracle.com/pls/topic/lookup?ctx=dblatest&id=GUID-683FF8C5-A773-4018-932D-2AF6EC8BC119>`__,
then it can be queried and printed:
.. code-block:: python
cur.execute("select geometry from mygeometrytab")
for obj, in cur:
dumpobject(obj)
Where ``dumpobject()`` is defined as:
.. code-block:: python
def dumpobject(obj, prefix = ""):
if obj.type.iscollection:
print(prefix, "[")
for value in obj.aslist():
if isinstance(value, cx_Oracle.Object):
dumpobject(value, prefix + " ")
else:
print(prefix + " ", repr(value))
print(prefix, "]")
else:
print(prefix, "{")
for attr in obj.type.attributes:
value = getattr(obj, attr.name)
if isinstance(value, cx_Oracle.Object):
print(prefix + " " + attr.name + ":")
dumpobject(value, prefix + " ")
else:
print(prefix + " " + attr.name + ":", repr(value))
print(prefix, "}")
This might produce output like::
{
SDO_GTYPE: 2003
SDO_SRID: None
SDO_POINT:
{
X: 1
Y: 2
Z: 3
}
SDO_ELEM_INFO:
[
1
1003
3
]
SDO_ORDINATES:
[
1
1
5
7
]
}
Other information on using Oracle objects is in :ref:`Using Bind Variables
<bind>`.
.. _rowlimit:
Limiting Rows
-------------
Query data is commonly broken into one or more sets:
- To give an upper bound on the number of rows that a query has to process,
which can help improve database scalability.
- To perform 'Web pagination' that allows moving from one set of rows to a
next, or previous, set on demand.
- For fetching of all data in consecutive small sets for batch processing.
This happens because the number of records is too large for Python to handle
at one time.
The latter can be handled by calling :meth:`Cursor.fetchmany()` with one
execution of the SQL query.
'Web pagination' and limiting the maximum number of rows are discussed in this
section. For each 'page' of results, a SQL query is executed to get the
appropriate set of rows from a table. Since the query may be executed more
than once, make sure to use :ref:`bind variables <bind>` for row numbers and
row limits.
Oracle Database 12c SQL introduced an ``OFFSET`` / ``FETCH`` clause which is
similar to the ``LIMIT`` keyword of MySQL. In Python you can fetch a set of
rows using:
.. code-block:: python
myoffset = 0 // do not skip any rows (start at row 1)
mymaxnumrows = 20 // get 20 rows
sql =
"""SELECT last_name
FROM employees
ORDER BY last_name
OFFSET :offset ROWS FETCH NEXT :maxnumrows ROWS ONLY"""
cur = connection.cursor()
for row in cur.execute(sql, offset=myoffset, maxnumrows=mymaxnumrows):
print(row)
In applications where the SQL query is not known in advance, this method
sometimes involves appending the ``OFFSET`` clause to the 'real' user query. Be
very careful to avoid SQL injection security issues.
For Oracle Database 11g and earlier there are several alternative ways
to limit the number of rows returned. The old, canonical paging query
is::
SELECT *
FROM (SELECT a.*, ROWNUM AS rnum
FROM (YOUR_QUERY_GOES_HERE -- including the order by) a
WHERE ROWNUM <= MAX_ROW)
WHERE rnum >= MIN_ROW
Here, ``MIN_ROW`` is the row number of first row and ``MAX_ROW`` is the row
number of the last row to return. For example::
SELECT *
FROM (SELECT a.*, ROWNUM AS rnum
FROM (SELECT last_name FROM employees ORDER BY last_name) a
WHERE ROWNUM <= 20)
WHERE rnum >= 1
This always has an 'extra' column, here called RNUM.
An alternative and preferred query syntax for Oracle Database 11g uses the
analytic ``ROW_NUMBER()`` function. For example to get the 1st to 20th names the
query is::
SELECT last_name FROM
(SELECT last_name,
ROW_NUMBER() OVER (ORDER BY last_name) AS myr
FROM employees)
WHERE myr BETWEEN 1 and 20
Make sure to use :ref:`bind variables <bind>` for the upper and lower limit
values.
.. _codecerror:
Querying Corrupt Data
---------------------
If queries fail with the error "codec can't decode byte" when you select data,
then:
* Check your :ref:`character set <globalization>` is correct. Review the
:ref:`client and database character sets <findingcharset>`. Consider using
UTF-8, if this is appropriate:
.. code-block:: python
connection = cx_Oracle.connect("hr", userpwd, "dbhost.example.com/orclpdb1",
encoding="UTF-8", nencoding="UTF-8")
* Check for corrupt data in the database.
If data really is corrupt, you can pass options to the internal `decode()
<https://docs.python.org/3/library/stdtypes.html#bytes.decode>`__ used by
cx_Oracle to allow it to be selected and prevent the whole query failing. Do
this by creating an :ref:`outputtypehandler <outputtypehandlers>` and setting
``encodingErrors``. For example to replace corrupt characters in character
columns:
.. code-block:: python
def OutputTypeHandler(cursor, name, defaultType, size, precision, scale):
if defaultType == cx_Oracle.STRING:
return cursor.var(defaultType, size, arraysize=cursor.arraysize,
encodingErrors="replace")
cursor.outputtypehandler = OutputTypeHandler
cursor.execute("select column1, column2 from SomeTableWithBadData")
Other codec behaviors can be chosen for ``encodingErrors``, see `Error Handlers
<https://docs.python.org/3/library/codecs.html#error-handlers>`__.
.. _dml:
INSERT and UPDATE Statements
============================
SQL Data Manipulation Language statements (DML) such as INSERT and UPDATE can
easily be executed with cx_Oracle. For example:
.. code-block:: python
cur = connection.cursor()
cur.execute("insert into MyTable values (:idbv, :nmbv)", [1, "Fredico"])
Do not concatenate or interpolate user data into SQL statements. See
:ref:`bind` instead.
See :ref:`txnmgmnt` for best practices on committing and rolling back data
changes.
When handling multiple data values, use :meth:`~Cursor.executemany()` for
performance. See :ref:`batchstmnt`
Inserting NULLs
---------------
Oracle requires a type, even for null values. When you pass the value None, then
cx_Oracle assumes the type is STRING. If this is not the desired type, you can
explicitly set it. For example, to insert a null :ref:`Oracle Spatial
SDO_GEOMETRY <spatial>` object:
.. code-block:: python
typeObj = connection.gettype("SDO_GEOMETRY")
cur = connection.cursor()
cur.setinputsizes(typeObj)
cur.execute("insert into sometable values (:1)", [None])
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