Some general tuning tips are:
Tune your application architecture.
A general application goal is to reduce the number of round-trips between cx_Oracle and the database.
For multi-user applications, make use of connection pooling. Create the pool once during application initialization. Do not oversize the pool, see Connection Pooling . Use a session callback function to set session state, see Session CallBacks for Setting Pooled Connection State.
Tune your SQL statements. See the SQL Tuning Guide.
Use bind variables to avoid statement reparsing.
Do simple optimizations like limiting the number of rows and avoiding selecting columns not used in the application.
It may be faster to work with simple scalar relational values than to use Oracle Database object types.
Make good use of PL/SQL to avoid executing many individual statements from cx_Oracle.
Tune the Statement Cache.
Enable Client Result Caching for small lookup tables.
Tune your database. See the Database Performance Tuning Guide.
Tune your network. For example, when inserting or retrieving a large number of rows (or for large data), or when using a slow network, then tune the Oracle Network Session Data Unit (SDU) and socket buffer sizes, see Oracle Net Services: Best Practices for Database Performance and High Availability.
Do not commit or rollback unnecessarily. Use
Connection.autocommiton the last of a sequence of DML statements.
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
“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
prefetchrows will also affect performance and memory
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
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.arraysizevalue. The default is 100. Then set
arraysizevalue. 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"): print(row)
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
prefetchrowsat its default value of 2. Keep
arraysizeas big, or bigger than,
If you are fetching a fixed number of rows, start your tuning by setting
arraysizeto the number of expected rows, and set
prefetchrowsto 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
prefetchrowsto 21 and
cur = connection.cursor() cur.prefetchrows = 21 cur.arraysize = 20 for row in cur.execute(""" SELECT last_name FROM employees ORDER BY last_name OFFSET 0 ROWS FETCH NEXT 20 ROWS ONLY"""): print(row)
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.arraysizeto 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() print(row)
In cx_Oracle, the
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() cursor.execute(sql).fetchall() elapsed = time.time() - start print("Time for", size, elapsed, "seconds")
There are two cases that will benefit from setting
- When passing REF CURSORS into PL/SQL packages. Setting
prefetchrowsto 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
prefetchrowsto 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
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: break targetCursor.executemany("insert into MyTable values (:1, :2)", rows) targetConnection.commit()
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:
Cursor.prefetchrowsfor each query.
Cursor.executemany()for optimal DML execution.
- Only commit when necessary. Use
Connection.autocommiton the last statement of a transaction.
- For connection pools, use a callback to set connection state, see Session CallBacks for Setting Pooled Connection State.
- Make use of PL/SQL procedures which execute multiple SQL statements instead of executing them individually from cx_Oracle.
- Use scalar types instead of Oracle Database object types.
- Avoid overuse of
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 WHERE ss.sid = SYS_CONTEXT('USERENV','SID') AND ss.statistic# = sn.statistic# AND sn.name LIKE '%roundtrip%client%';
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
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
(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.
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() print(res) cur.execute(None, id = 10) res = cur.fetchall() print(res)
Statements passed to
prepare() are also stored in the statement
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_LAG, and then
restarting the database, for example:
SQL> ALTER SYSTEM SET CLIENT_RESULT_CACHE_LAG = 3000 SCOPE=SPFILE; SQL> ALTER SYSTEM SET CLIENT_RESULT_CACHE_SIZE = 64K SCOPE=SPFILE; SQL> STARTUP FORCE
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); SQL> ALTER TABLE locations RESULT_CACHE (MODE FORCE);
Alternatively, hints can be used in SQL statements. For example:
SELECT /*+ result_cache */ postal_code FROM locations