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CHAPTER 21 IMPROVING DATABASE PERFORMANCE: SQL QUERY OPTIMIZATION
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The third example is a query with an index executed after collecting Optimizer statistics for the table. Oracle has the complete statistics, and it uses the CBO this time around. The CBO decides to use an index only if the cost of using the index is lower than the cost of a full table scan. The CBO decides that it won t use the index, because the query will have to read 98,304 out of a total of 98,384 rows. It rightly decides to do a full table scan instead. The results are shown in Listing 21-10. Listing 21-10. The Execution Plan with an Index After Analyzing the Table SQL> ANALYZE TABLE courses COMPUTE STATISTICS; Table analyzed. SQL> SELECT count(*) FROM courses 2 WHERE course_subject='medicine' 3 AND course_title = 'fundamentals of human anatomy'; COUNT(*) ----------98304 Execution Plan ---------------------------------------------------------0 SELECT STATEMENT Optimizer=CHOOSE (Cost=74 Card=1 Bytes=39) 1 0 SORT (AGGREGATE) 2 1 TABLE ACCESS (FULL) OF 'COURSES' (Cost=74 Card=24596 Bytes=959244) Statistics ---------------------------------------------------------290 recursive calls 0 db block gets 792 consistent gets 334 physical reads 0 redo size 381 bytes sent via SQL*Net to client 499 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 6 sorts (memory) 0 sorts (disk) 1 rows processed SQL> In this listing, the first item, recursive calls, refers to additional statements Oracle needs to make when it s processing a user s SQL statement. For example, Oracle issues recursive calls (or recursive SQL statements) to make space allocations or to query the data dictionary tables on disk. In our example, Oracle made 290 internal calls during the SQL Trace period.
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Using SQL Trace and TKPROF
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SQL Trace is an Oracle utility that helps you trace the execution of SQL statements. TKPROF is another Oracle utility that helps you format the trace files output by SQL Trace into a readable form. Although the EXPLAIN PLAN facility gives you the expected execution plan, the SQL Trace tool gives you the actual execution results of a SQL query. Sometimes, you may not be able to identify the exact code, say, for dynamically generated SQL. SQL Trace files can capture the SQL for dynamic SQL. Among other things, SQL Trace enables you to track the following variables: CPU and elapsed times Parsed and executed counts for each SQL statement Number of physical and logical reads
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CHAPTER 21 IMPROVING DATABASE PERFORMANCE: SQL QUERY OPTIMIZATION
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Execution plan for all the SQL statements Library cache hit ratios
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If your application has a lot of dynamically generated SQL, the SQL Trace utility is ideal for tuning the SQL statements.
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Although the EXPLAIN PLAN tool is important for determining the access path that the Optimizer will use, SQL Trace gives you a lot of hard information on resource use and the efficacy of the statements. You ll get a good idea of whether your statement is being parsed excessively. The statement s execute and fetch counts illustrate its efficiency. You get a good sense of how much CPU time is consumed by your queries and how much I/O is being performed during the execution phase. This helps you identify the resource-guzzling SQL statements in your application and tune them. The EXPLAIN PLAN, which is an optional part of SQL Trace, gives the row counts for the individual steps of the EXPLAIN PLAN, helping you pinpoint at what step the most work is being done. By comparing resource use with the number of rows fetched, you can easily determine how productive a particular statement is. In the next sections you ll use SQL Trace to trace a simple SQL statement and interpret it with the TKPROF utility. You start by setting a few initialization parameters to ensure tracing.
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Setting the Trace Initialization Parameters
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Collecting trace statistics imposes a performance penalty, and consequently the database doesn t automatically trace all sessions. Tracing is purely an optional process that you turn on for a limited duration to capture metrics about the performance of critical SQL statements. You need to look at four initialization parameters to set up Oracle correctly for SQL tracing, and you have to restart the database after checking that the following parameters are correctly configured. Three of these parameters are dynamic session parameters, and you can change them at the session level.
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