Tuning cx_Oracle

Some general tuning tips are:

Tuning Fetch Performance

To tune queries you can adjust cx_Oracle’s internal buffer sizes to improve the speed of fetching rows across the network from the database, and to optimize memory usage. Regardless of which cx_Oracle method is used to get query results, internally all rows are fetched in batches from the database and buffered before being returned to the application. The internal buffer sizes can have a significant performance impact. The sizes do not affect how, or when, rows are returned to your application. They do not affect the minimum or maximum number of rows returned by a query.

For best performance, tune “array fetching” with Cursor.arraysize and “row prefetching” with Cursor.prefetchrows before calling Cursor.execute(). Queries that return LOBs and similar types will never prefetch rows, so the prefetchrows value is ignored in those cases.

The common query tuning scenario is for SELECT statements that return a large number of rows over a slow network. Increasing arraysize can improve performance by reducing the number of round-trips to the database. However increasing this value increases the amount of memory required. Adjusting prefetchrows will also affect performance and memory usage.

Row prefetching and array fetching are both internal buffering techniques to reduce round-trips to the database. The difference is the code layer that is doing the buffering, and when the buffering occurs. The Oracle Client libraries used by cx_Oracle have separate “execute SQL statement” and “fetch data” calls. Prefetching allows query results to be returned to the application when the successful statement execution acknowledgment is returned from the database. This means that a subsequent internal “fetch data” operation does not always need to make a round-trip to the database because rows are already buffered in the Oracle Client libraries. Reducing round-trips helps performance and scalability. An overhead of prefetching is the need for an additional data copy from Oracle Client’s prefetch buffers.

Choosing values for arraysize and prefetchrows

The best Cursor.arraysize and Cursor.prefetchrows values can be found by experimenting with your application under the expected load of normal application use. This is because the cost of the extra memory copy from the prefetch buffers when fetching a large quantity of rows or very “wide” rows may outweigh the cost of a round-trip for a single cx_Oracle user on a fast network. However under production application load, the reduction of round-trips may help performance and overall system scalability. The documentation in round-trips shows how to measure round-trips.

Here are some suggestions for the starting point to begin your tuning:

  • To tune queries that return an unknown number of rows, estimate the number of rows returned and start with an appropriate Cursor.arraysize value. The default is 100. Then set Cursor.prefetchrows to the arraysize value. Do not make the sizes unnecessarily large. For example:

    cur = connection.cursor()
    cur.prefetchrows = 1000
    cur.arraysize = 1000
    for row in cur.execute("SELECT * FROM very_big_table"):

    Adjust the values as needed for performance, memory and round-trip usage. For a large quantity of rows or very “wide” rows on fast networks you may prefer to leave prefetchrows at its default value of 2. Keep arraysize as big, or bigger than, prefetchrows.

  • If you are fetching a fixed number of rows, start your tuning by setting arraysize to the number of expected rows, and set prefetchrows to one greater than this value. (Adding one removes the need for a round-trip to check for end-of-fetch). For example, if you are querying 20 rows, perhaps to display a page of data, set prefetchrows to 21 and arraysize to 20:

    cur = connection.cursor()
    cur.prefetchrows = 21
    cur.arraysize = 20
    for row in cur.execute("""
        SELECT last_name
           FROM employees
           ORDER BY last_name

    This will return all rows for the query in one round-trip.

  • If you know that a query returns just one row then set Cursor.arraysize to 1 to minimize memory usage. The default prefetch value of 2 allows minimal round-trips for single-row queries:

    cur = connection.cursor()
    cur.arraysize = 1
    cur.execute("select * from MyTable where id = 1"):
    row = cur.fetchone()

In cx_Oracle, the arraysize and prefetchrows values are only examined when a statement is executed the first time. To change the values, create a new cursor. For example, to change arraysize for a repeated statement:

array_sizes = (10, 100, 1000)
for size in array_sizes:
    cursor = connection.cursor()
    cursor.arraysize = size
    start = time.time()
    elapsed = time.time() - start
    print("Time for", size, elapsed, "seconds")

There are two cases that will benefit from setting Cursor.prefetchrows to 0:

  • When passing REF CURSORS into PL/SQL packages. Setting prefetchrows to 0 can stop rows being prematurely (and silently) fetched into cx_Oracle’s internal buffers, making them unavailable to the PL/SQL code that receives the REF CURSOR.
  • When querying a PL/SQL function that uses PIPE ROW to emit rows at intermittent intervals. By default, several rows needs to be emitted by the function before cx_Oracle can return them to the application. Setting prefetchrows to 0 helps give a consistent flow of data to the application.

Prefetching can also be enabled in an external oraaccess.xml file, which may be useful for tuning an application when modifying its code is not feasible. Setting the size in oraaccess.xml will affect the whole application, so it should not be the first tuning choice.

One place where increasing arraysize is particularly useful is in copying data from one database to another:

# setup cursors
sourceCursor = sourceConnection.cursor()
sourceCursor.arraysize = 1000
targetCursor = targetConnection.cursor()

# perform fetch and bulk insertion
sourceCursor.execute("select * from MyTable")
while True:
    rows = sourceCursor.fetchmany()
    if not rows:
    targetCursor.executemany("insert into MyTable values (:1, :2)", rows)

Database Round-trips

A round-trip is defined as the trip from the Oracle Client libraries (used by cx_Oracle) to the database and back. Calling each cx_Oracle function, or accessing each attribute, will require zero or more round-trips. Along with tuning an application’s architecture and tuning its SQL statements, a general performance and scalability goal is to minimize round-trips.

Some general tips for reducing round-trips are:

Finding the Number of Round-Trips

Oracle’s Automatic Workload Repository (AWR) reports show ‘SQL*Net roundtrips to/from client’ and are useful for finding the overall behavior of a system.

Sometimes you may wish to find the number of round-trips used for a specific application. Snapshots of the V$SESSTAT view taken before and after doing some work can be used for this:

SELECT ss.value, sn.display_name
FROM v$sesstat ss, v$statname sn
AND ss.statistic# = sn.statistic#
AND sn.name LIKE '%roundtrip%client%';

Statement Caching

cx_Oracle’s Cursor.execute() and Cursor.executemany() functions use the Oracle Call Interface statement cache to make re-execution of statements efficient. Each standalone or pooled connection has its own cache of statements with a default size of 20. Statement caching lets cursors be used without re-parsing the statement. Statement caching also reduces metadata transfer costs between the cx_Oracle and the database. Performance and scalability are improved.

The statement cache size can be set with Connection.stmtcachesize or SessionPool.stmtcachesize. In general, set the statement cache size to the size of the working set of statements being executed by the application. To manually tune the cache, monitor the general application load and the Automatic Workload Repository (AWR) “bytes sent via SQL*Net to client” values. The latter statistic should benefit from not shipping statement metadata to cx_Oracle. Adjust the statement cache size to your satisfaction.

Statement caching can be disabled by setting the size to 0. Disabling the cache may be beneficial when the quantity or order of statements causes cache entries to be flushed before they get a chance to be reused. For example if there are more distinct statements than cache slots, and the order of statement execution causes older statements to be flushed from the cache before the statements are re-executed.

With Oracle Database 12c, or later, the statement cache size can be automatically tuned using the oraaccess.xml file.

When it is inconvenient to pass statement text through an application, the Cursor.prepare() call can be used to avoid statement re-parsing. Subsequent execute() calls use the value None instead of the SQL text:

cur.prepare("select * from dept where deptno = :id order by deptno")

cur.execute(None, id = 20)
res = cur.fetchall()

cur.execute(None, id = 10)
res = cur.fetchall()

Statements passed to prepare() are also stored in the statement cache.

Client Result Caching

cx_Oracle applications can use Oracle Database’s Client Result Cache. The CRC enables client-side caching of SQL query (SELECT statement) results in client memory for immediate use when the same query is re-executed. This is useful for reducing the cost of queries for small, mostly static, lookup tables, such as for postal codes. CRC reduces network round-trips, and also reduces database server CPU usage.

The cache is at the application process level. Access and invalidation is managed by the Oracle Client libraries. This removes the need for extra application logic, or external utilities, to implement a cache.

CRC can be enabled by setting the database parameters CLIENT_RESULT_CACHE_SIZE and CLIENT_RESULT_CACHE_LAG, and then restarting the database, for example:


CRC can alternatively be configured in an oraaccess.xml or sqlnet.ora file on the Python host, see Client Configuration Parameters.

Tables can then be created, or altered, so repeated queries use CRC. This allows existing applications to use CRC without needing modification. For example:

SQL> CREATE TABLE cities (id number, name varchar2(40)) RESULT_CACHE (MODE FORCE);

Alternatively, hints can be used in SQL statements. For example:

SELECT /*+ result_cache */ postal_code FROM locations