Tuning the Shared Pool

• library cache

• data dictionary cache

• session information

Tuning the Library Cache

• examining library cache activity

• reducing library cache misses

• speeding access to shared SQL and PL/SQL areas in the library cache

Examining Library Cache Activity

Parse If an application makes a parse call for a SQL statement and the parsed representation of the statement does not already exist in a shared SQL area in the library cache, Oracle parses the statement and allocates a shared SQL area. You may be able to reduce library cache misses on parse calls by ensuring that SQL statements can share a shared SQL area whenever possible.

Execute If an application makes an execute call for a SQL statement and the shared SQL area containing the parsed representation of the statement has been deallocated from the library cache to make room for another statement, Oracle implicitly reparses the statement, allocates a new shared SQL area for it, and executes it. You may be able to reduce library cache misses on execution calls by allocating more memory to the library cache.

Determine whether misses on the library cache are affecting the performance of Oracle by querying the dynamic performance table V$LIBRARYCACHE.

The V$LIBRARYCACHE Table Statistics reflecting library cache activity are kept in the dynamic performance table V$LIBRARYCACHE. These statistics reflect all library cache activity since the most recent instance startup. To monitor library cache activity, examine this table. By default, this table is only available to the user SYS and to users granted SELECT ANY TABLE system privilege, such as SYSTEM.

Each row in this table contains statistics for one type of item kept in the library cache. The item described by each row is identified by the value of the NAMESPACE column. Rows of the table with these NAMESPACE values reflect library cache activity for SQL statements and PL/SQL blocks:

• 'SQL AREA'

• 'TABLE/PROCEDURE'

• 'BODY'

• 'TRIGGER'

These columns of the V$LIBRARYCACHE table reflect library cache misses on execution calls:

PINS	This column shows the number of times an item in the library cache was executed.
RELOADS	This column shows the number of library cache misses on execution steps.

Information in the V$LIBRARYCACHE table can also be obtained through SNMP.

Querying the V$LIBRARYCACHE Table Monitor the statistics in the V$LIBRARYCACHE table over a period of time with this query:

SELECT SUM(pins) "Executions",

   SUM(reloads) "Cache Misses while Executing"

   FROM v$librarycache;

The output of this query might look like this:

Executions Cache Misses while Executing

---------- ----------------------------

    320871                          549

Interpreting the V$LIBRARYCACHE Table Examining the data returned by the sample query leads to these observations:

• The sum of the PINS column indicates that SQL statements, PL/SQL blocks, and object definitions were accessed for execution a total of 320,871 times.

• The sum of the RELOADS column indicates that 549 of those executions resulted in library cache misses causing Oracle to implicitly reparse a statement or block or reload an object definition because it had aged out of the library cache.

• The ratio of the total RELOADS to total PINS is about 0.17%. This value means that only 0.17% of executions resulted in reparsing.

Reducing Library Cache Misses

• allocating additional memory for the library cache

• writing identical SQL statements whenever possible

To take advantage of additional memory available for shared SQL areas, you may also need to increase the number of cursors permitted for a session. You can increase this limit by increasing the value of the initialization parameter OPEN_CURSORS.

Be careful not to induce paging and swapping by allocating too much memory for the library cache. The benefits of a library cache large enough to avoid cache misses can be partially offset by reading shared SQL areas into memory from disk whenever you need to access them.

Writing Identical SQL Statements You may be able to reduce library cache misses on parse calls by ensuring that SQL statements and PL/SQL blocks share a shared SQL area whenever possible. For two different occurrences of a SQL statement or PL/SQL block to share a shared SQL area, they must be identical according to these criteria:

• The text of the SQL statements or PL/SQL blocks must be identical, character for character, including spaces and case.

For example, these statements cannot use the same shared SQL area:

SELECT * FROM emp;

		SELECT *   FROM emp;

These statements cannot use the same shared SQL area:

		SELECT * FROM emp;

		SELECT * FROM Emp;

• References to schema objects in the SQL statements or PL/SQL blocks must resolve to the same object in the same schema.

For example, if the schemas of the users BOB and ED both contain an EMP table and both users issue the following statement, their statements cannot use the same shared SQL area:

		SELECT * FROM emp;

		SELECT * FROM emp;

If both statements query the same table and qualify the table with the schema, as in the following statement, then they can use the same shared SQL area:

		SELECT * FROM bob.emp;

• Bind variables in the SQL statements must match in name and datatype. For example, these statements cannot use the same shared SQL area:

		SELECT * FROM emp WHERE deptno = :department_no;

		SELECT * FROM emp WHERE deptno = :d_no;

• The SQL statements must be optimized using the same optimization approach and, in the case of the cost-based approach, the same optimization goal. For information on optimization approach and goal, see Chapter 7, "Tuning SQL Statements".

• Use bind variables rather than explicitly specified constants in your statements whenever possible.

For example, the following two statements cannot use the same shared area because they do not match character for character:

		SELECT ename, empno FROM emp WHERE deptno = 10;

		SELECT ename, empno FROM emp WHERE deptno = 20;

You can accomplish the goals of these statements by using the following statement that contains a bind variable, binding 10 for one occurrence of the statement and 20 for the other:

		SELECT ename, empno FROM emp WHERE deptno = :department_no;

The two occurrences of the statement can use the same shared SQL area.

• Be sure that individual users of the application do not change the optimization approach and goal for their individual sessions.

• Standardize naming conventions for bind variables and spacing conventions for SQL statements and PL/SQL blocks.

• Use stored procedures whenever possible. Multiple users issuing the same stored procedure automatically use the same shared PL/SQL area. Since stored procedures are stored in a parsed form, they eliminate runtime parsing altogether.

Speeding Access to Shared SQL Areas on Execution Calls

Depending on the value of CURSOR_SPACE_FOR_TIME, Oracle behaves differently when an application makes an execution call. If the value is FALSE, Oracle must take time to check that a shared SQL area containing the SQL statement is in the library cache. If the value is TRUE, Oracle need not make this check because the shared SQL area can never be deallocated while an application cursor associated with it is open. Setting the value of the parameter to TRUE saves Oracle a small amount of time and may slightly improve the performance of execution calls. This value also prevents the deallocation of private SQL areas until associated application cursors are closed.

Do not set the value of CURSOR_SPACE_FOR_TIME to TRUE if there are library cache misses on execution calls. Such library cache misses indicate that the shared pool is not large enough to hold the shared SQL areas of all concurrently open cursors. If the value is TRUE and there is no space in the shared pool for a new SQL statement, the statement cannot be parsed and Oracle returns an error saying that there is no more shared memory. If the value is FALSE and there is no space for a new statement, Oracle deallocates an existing shared SQL area. Although deallocating a shared SQL area results in a library cache miss later, it is preferable to an error halting your application because a SQL statement cannot be parsed.

Do not set the value of CURSOR_SPACE_FOR_TIME to TRUE if the amount of memory available to each user for private SQL areas is scarce. This value also prevents the deallocation of private SQL areas associated with open cursors. If the private SQL areas for all concurrently open cursors fills the user's available memory so that there is no space to allocate a private SQL area for a new SQL statement, the statement cannot be parsed and Oracle returns an error indicating that there is not enough memory.

Caching Session Cursors

Oracle uses the shared SQL area to determine if more than three parse requests have been issued on a given statement. If so, Oracle assumes the session cursor associated with the statement should be cached and moves the cursor into the session cursor cache. Subsequent requests to parse that SQL statement by the same session will then find the cursor in the session cursor cache.

To enable caching of session cursors, you must set the initialization parameter SESSION_CACHED_CURSORS. This parameter is a positive integer that specifies the maximum number of session cursors kept in the cache. A least recently used (LRU) algorithm ages out entries in the session cursor cache to make room for new entries when needed.

You can also enable the session cursor cache dynamically with the ALTER SESSION SET SESSION_CACHED_CURSORS command.

To determine whether the session cursor cache is sufficiently large for your instance, you can examine the session statistic "session cursor cache hits" in the V$SESSTAT view. This statistic counts the number of times a parse call found a cursor in the session cursor cache. If this statistic is a relatively low percentage of the total parse call count for the session, you should consider setting SESSION_CACHED_CURSORS to a larger value.

Tuning the Data Dictionary Cache

• how to monitor the activity of the data dictionary cache

• how to improve the performance of the data dictionary cache

Examining Data Dictionary Cache Activity

Misses on the data dictionary cache are to be expected in some cases. Upon instance startup, the data dictionary cache contains no data, so any SQL statement issued is likely to result in cache misses. As more data is read into the cache, the likelihood of cache misses should decrease. Eventually the database should reach a "steady state" in which the most frequently used dictionary data is in the cache. At this point, very few cache misses should occur. To tune the cache, examine its activity only after your application has been running.

The V$ROWCACHE View Statistics reflecting data dictionary activity are kept in the dynamic performance table V$ROWCACHE. By default, this table is only available to the user SYS and to users granted SELECT ANY TABLE system privilege, such as SYSTEM.

Each row in this table contains statistics for a single type of the data dictionary item. These statistics reflect all data dictionary activity since the most recent instance startup. These columns in the V$ROWCACHE table reflect the use and effectiveness of the data dictionary cache:

PARAMETER	This column identifies a particular data dictionary item. For each row, the value in this column is the item prefixed by 'dc_'.
	For example, in the row that contains statistics for file descriptions, this column has the value 'dc_files'.
GETS	This column shows the total number of requests for information on the corresponding item.
	For example, in the row that contains statistics for file descriptions, this column has the total number of requests for file descriptions data.
GETMISSES	This column shows the number of data requests resulting in cache misses.

Querying the V$ROWCACHE Table Monitor the statistics in the V$ROWCACHE table over a period of time while your application is running with this query:

SELECT SUM(gets)  "Data Dictionary Gets",

   SUM(getmisses) "Data Dictionary Cache Get Misses"

   FROM v$rowcache;

The output of this query might look like this:

Data Dictionary Gets  Data Dictionary Cache Get Misses

--------------------  --------------------------------

             1439044                              3120

Interpreting the V$ROWCACHE Table Examining the data returned by the sample query leads to these observations:

• The sum of the GETS column indicates that there were a total of 1,439,044 requests for dictionary data.

• The sum of the GETMISSES column indicates that 3120 of the requests for dictionary data resulted in cache misses.

• The ratio of the sums of GETMISSES to GETS is about 0.2%.

Reducing Data Dictionary Cache Misses

Tuning the Shared Pool with the Multi-Threaded Server

The V$SESSTAT Table

session memory	The value of this statistic is the amount of memory in bytes allocated to the session.
max session memory	The value of this statistic is the maximum amount of memory in bytes ever allocated to the session.

Querying the V$SESSTAT Table

SELECT SUM(value) || ' bytes' "Total memory for all sessions"

   FROM v$sesstat, v$statname

   WHERE name = 'session memory'

      AND v$sesstat.statistic# = v$statname.statistic#;

SELECT SUM(value) || ' bytes' "Total max mem for all sessions"

   FROM v$sesstat, v$statname

   WHERE name = 'max session memory'

      AND v$sesstat.statistic# = v$statname.statistic#;

These queries also select from the dynamic performance table V$STATNAME to obtain internal identifiers for session memory and max session memory. The results of these queries might look like this:

Total memory for all sessions

-----------------------------

157125 bytes

Total max mem for all sessions

------------------------------

417381 bytes

Interpreting the V$SESSTAT Table

You can use the result of either of these queries to determine how much larger to make the shared pool if you use the multi-threaded server. The first value is likely to be a better estimate than the second unless nearly all sessions are likely to reach their maximum allocations at the same time.