Monitoring and tuning the size of the SQL statement cache

If the size of the SQL statement cache is too small, performance problems can occur. You can monitor the effectiveness of the size of the SQL statement cache.

The following performance problems can occur:
  • Frequently executed SQL statements are not in the cache

    The statements used most often should remain in the SQL statement cache. If the SQL statement cache is not large enough, the database server might not have enough room to keep these statements when other statements come into the cache. For subsequent executions, the database server must reparse, reoptimize, and reinsert the SQL statement into the cache. Try increasing STMT_CACHE_SIZE.

  • The database server spends a lot of time cleaning the SQL statement cache

    The database server tries to prevent the SQL statement cache from allocating large amounts of memory by using a threshold (70 percent of the STMT_CACHE_SIZE parameter) to determine when to remove entries from the SQL statement cache. If the new entry causes the size of the SQL statement cache to exceed the threshold, the database server removes least recently used entries (that are not currently in use) before inserting the new entry.

    However, if a subsequent query needs the removed memory structures, the database server must reparse and reoptimize the SQL statement. The additional processing time to regenerate these memory structures adds to the total response time of the query.

You can set the size of the SQL statement cache in memory with the STMT_CACHE_SIZE configuration parameter. The value of the parameter is the size in kilobytes. If STMT_CACHE_SIZE is not set, the default value is 512 kilobytes.

The onstat -g ssc output shows the value of STMT_CACHE_SIZE in the maxsize column. In Figure 1, this maxsize column has a value of 524288, which is the default value (512 * 1024 = 524288).

Use the onstat -g ssc and onstat -g ssc all options to monitor the effectiveness of size of the SQL statement cache. If you do not see cache entries for the SQL statements that applications use most, the SQL statement cache might be too small or too many unshared SQL statement occupy the cache. The following sections describe how to determine these situations.

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