Friday, October 9, 2009

Tuning an Application

Tuning an Application / Reducing Load

If your whole application is performing suboptimally, or if you are attempting to reduce the overall CPU or I/O load on the database server, then identifying resource-intensive SQL involves the following steps:

1. Determine which period in the day you would like to examine; typically this is the
application's peak processing time.

2. Gather operating system and Oracle statistics at the beginning and end of that period. The minimum of Oracle statistics gathered should be file I/O (V$FILESTAT), system statistics (V$SYSSTAT), and SQL statistics (V$SQLAREA, V$SQL or V$SQLSTATS, V$SQLTEXT, V$SQL_PLAN, and V$SQL_PLAN_STATISTICS).

3. Using the data collected in step two, identify the SQL statements using the mostresources. A good way to identify candidate SQL statements is to query V$SQLSTATS. V$SQLSTATS contains resource usage information for all SQL statements in the shared pool. The data in V$SQLSTATS should be ordered by resource usage.

The most common resources are:

■ Buffer gets (V$SQLSTATS.BUFFER_GETS, for high CPU using statements)
■ Disk reads (V$SQLSTATS.DISK_READS, for high I/O statements)
■ Sorts (V$SQLSTATS.SORTS, for many sorts)

One method to identify which SQL statements are creating the highest load is to compare the resources used by a SQL statement to the total amount of that resource used in the period. For BUFFER_GETS, divide each SQL statement's BUFFER_GETS by the total number of buffer gets during the period. The total number of buffer gets in the system is available in the V$SYSSTAT table, for the statistic session logical reads.

Similarly, it is possible to apportion the percentage of disk reads a statement performs out of the total disk reads performed by the system by dividing V$SQL_STATS.DISK_READS by the value for the V$SYSSTAT statistic physical reads. The SQL sections of the Automatic Workload Repository report include this data, so you do not need to perform the percentage calculations manually.

After you have identified the candidate SQL statements, the next stage is to gather
information that is necessary to examine the statements and tune them.
Identifying High-Load SQL

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