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CHAPTER 19 IMPROV IN G DA TA BAS E PERFORMAN CE: SQL QUERY OPTIMIZA TION
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SQL> SELECT sql_text, rows_processed, 2 buffer_gets, disk_reads, parse_calls 3 FROM V$SQL 4 WHERE buffer_gets > 100000 5 OR disk_reads > 100000 6*ORDER BY buffer_gets + 100*disk_reads DESC; SQL_TEXT ROWS_PROCESSED ---------------------------- ----------BEGIN dbms_job.run(1009133); 9659 BEGIN label_sc_pkg.launch_sc; 3928 SELECT COUNT(*) AV_YOUTHS... 70660 SELECT UC.CHART_ID... 37848 SELECT MAX(REC_NUM) FROM... 5163236 SQL> BUFFER_GETS DISK_READS PARSE_CALLS ----------- --------- -------------1615322749 125830 2078 1214405479 3680515 4 152737737 7186125 3863 96590083 5547319 5476 33272842 6034715 606
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The V$SQL view helps you find out which of your queries have high logical I/O (LIO) and high physical I/O (PIO). By also providing the number of rows processed by each statement, it tells you whether a statement is efficient or not. By providing the disk reads and the number of executions per statement, the view helps you determine whether the number of disk reads per execution is reasonable. If CPU usage is your concern, look at the statements that have a high number of buffer gets. If I/O is your primary concern, focus on the statements that perform the most disk reads. Once you settle on the statement you want to investigate further, examine the complete SQL statement and see whether you (or the developers) can improve it. One of the best ways to find poorly performing SQL queries is by using the Oracle wait interface, which I explain in detail in 20. Here s a query that uses the V$SQL view to sort the top five queries that are taking the most CPU time and the most elapsed time to complete: SQL> SELECT sql_text, executions, 2 ROUND(elapsed_time/1000000, 2) elapsed_seconds, 3 ROUND(cpu_time/1000000, 2) cpu_secs from 4 (select * from v$sql order by elapsed_time desc) 5* WHERE rownum <6; SQL_TEXT EXECUTIONS ELAPSED_SECONDS --------------------------------------------------DELETE MS_DASH_TRANLOGS... 2283 44.57 UPDATE PERSONS SET... 14132 19.74 SELECT /*+ INDEX(ud)... 9132 9.95 SELECT PROG_ID FROM UNITS ... 14132 5.26 SELECT NVL(SUM(RECHART),0)... 2284 4.13 SQL>
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Using Other Dictionary Views for SQL Tuning
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The V$SQL_PLAN and V$SQL_PLAN_STATISTICS views are highly useful for tracking the efficiency of execution plans. You should be wary of quick changes in code to fix even the worst-performing query in the system. Let s say you create an extra index on a table or change the order of columns in a composite key to fix this problem query. How do you know these aren t going to impact other queries in the application adversely This happens more often than you think, and therefore you must do your due diligence to rule out unintended consequences of your fixes.
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CHAPTER 19 IM PR OVING DA TA BAS E PERFORM ANC E: S QL QUE RY OPTIMIZA TION
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The SQL Tuning Advisor
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You can use the SQL Tuning Advisor to improve poorly performing SQL statements. The SQL Tuning Advisor provides the following to help you tune bad SQL statements: Advice on improving the execution plan Reasons for the SQL improvement recommendations Benefits you can expect by following the Advisor s advice Details of the commands to tune the misbehaving SQL statements
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The SQL Tuning Advisor can use the following sources: New SQL statements. When working with a development database, this may be your best source of SQL statements. High-load SQL statements. SQL statements from the AWR. SQL statements from the database cursor cache. The Advisor can tune sets of SQL statements called SQL Tuning Sets. An STS is a set of SQL statements combined with execution information, which includes the average elapsed time. An STS has the advantage of capturing the information about a database s workload as well as allowing you to tune several large SQL statements at once.
How the SQL Tuning Advisor Works
As mentioned previously, the optimizer will try to find the optimal execution plan for each statement you provide. However, this process happens under production conditions, so the optimizer can only devote a short amount of time to working out a solution. The optimizer uses heuristics to generate an estimate of the best solution. This is called the normal mode of the optimizer. You can also run the optimizer in tuning mode, which means that the optimizer carries out indepth analysis to come up with ways to optimize execution plans. While in this mode, the optimizer can take several minutes and produces recommendations instead of the best SQL execution plan. You, in turn, use these recommendations to optimize the SQL statements execution plans. You get the added advantage of advice that details the rationale behind the recommendations and what you will gain from implementing them. The Oracle optimizer running in tuning mode is called the Automatic Tuning Optimizer (ATO). The ATO does the following tasks: Statistics analysis SQL profiling Access path analysis SQL structure analysis I describe each of these tasks in the following sections, along with the types of recommendations that the SQL Tuning Advisor makes.
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