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CHAPTER 20 PERFOR MAN CE TUNING: TUNING THE INSTA NCE
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2. Use the following query to create a trace in the user dump directory: SQL> SELECT * FROM comp_orgs WHERE created_date > SYSDATE-5; The SQL Trace output shows the following in the output file: PARSING IN CURSOR #1 len=63 dep=0 uid=21 oct=3 lid=21 tim=1326831345 hv=71548308 SELECT * FROM comp_orgs WHERE created_date > SYSDATE-:"SYS_B_0" END OF STMT PARSE #1:c=4,e=4,p=0,cr=57,cu=3,mis=1,r=0,dep=0,og=0,tim=1326831345 Note that mis=1 indicates a hard parse because this SQL isn t present in the library cache. 3. Use a slightly different version of the previous query next. The output is the same, but Oracle won t use the previously parsed version, because the statements in steps 2 and 3 aren t identical. SQL> SELECT * FROM comp_orgs WHERE created_date > (SYSDATE -5); Here s the associated SQL Trace output: PARSING IN CURSOR #1 len=77 dep=0 uid=21 oct=3 lid=21 tim=1326833972 SELECT /* A Hint */ * FROM comp_orgs WHERE created_date > SYSDATE-:"SYS_B_0" END OF STMT PARSE #1:c=1,e=1,p=0,cr=0,cu=0,mis=1,r=0,dep=0,og=0,tim=1326833972 Again, a hard parse, indicated by mis=1, shows a library cache miss. This isn t a surprise, as this statement isn t identical to the one before, so it has to be parsed from scratch. 4. Use the original query again. Now Oracle performs only a soft parse, because the statements here and in the first step are the same. Here s the SQL Trace output: PARSING IN CURSOR #1 len=63 dep=0 uid=21 oct=3 lid=21 tim=1326834357 SELECT * FROM comp_orgs WHERE created_date > SYSDATE-:"SYS_B_0" END OF STMT PARSE #1:c=0,e=0,p=0,cr=0,cu=0,mis=0,r=0,dep=0,og=4,tim=1326834357 The statement in step 4 is identical in all respects to the statement in step 1, so Oracle reuses the parsed version. Hence mis=0 indicates there wasn t a hard parse but merely a soft parse, which is a lot cheaper in terms of resource usage. If you now look at the TKPROF output, you ll see the following section for the SQL statements in step 2 and step 4 (identical statements): ******************************************************************** SELECT * FROM comp_orgs WHERE created_date > SYSDATE - 5 call count cpu elapsed disk query current rows ------- ------ -------- ---------- ---------- ---------- ---------Parse 2 0.03 0.01 0 1 3 0 Execute 2 0.00 0.00 0 0 0 0 Fetch 4 0.07 0.10 156 166 24 10 total 8 0.10 0.11 156 167 27 10 Misses in library cache during parse: 1 ******************************************************************** As you can see, there was one miss in the library cache when you first executed the statement. The second time around, there was no hard parse and hence no library cache miss.
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CH A PT ER 2 0 PERF O RMAN CE TUNI NG: TUN ING TH E I NS TA NCE
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Measuring Library Cache Efficiency
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You can use simple ratios to see if your library cache is sized correctly. The V$LIBRARYCACHE data dictionary view provides you with all the information you need to see whether the library cache is efficiently sized. Listing 20-1 shows the structure of the V$LIBRARYCACHE view. Listing 20-1. The V$LIBRARYCACHE View SQL> DESC V$LIBRARYCACHE Name Null ----------------------------------------- -------NAMESPACE GETS GETHITS GETHITRATIO PINS PINHITS PINHITRATIO RELOADS INVALIDATIONS DLM_LOCK_REQUESTS DLM_PIN_REQUESTS DLM_PIN_RELEASES DLM_INVALIDATION_REQUESTS DLM_INVALIDATIONS SQL> Type -----------VARCHAR2(15) NUMBER NUMBER NUMBER NUMBER NUMBER NUMBER NUMBER NUMBER NUMBER NUMBER NUMBER NUMBER NUMBER
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The following formula provides you with the library cache hit ratio: SQL> SELECT SUM(pinhits)/sum(pins) 2 FROM V$LIBRARYCACHE; LIBRARY_CACHE_HIT_RATIO -----------------------.993928013 SQL> The formula indicates that the library cache currently has a higher than 99 percent hit ratio, which is considered good. However, be cautious about relying exclusively on high hit ratios for the library cache and the buffer caches, such as the one shown here. You may have a hit ratio such as 99.99 percent, but if significant waits are caused by events such as excessive parsing, you re going to have a slow database. Always keep an eye on the wait events in your system, and don t rely blindly on high hit ratios such as these. Listing 20-2 shows how to determine the number of reloads and pinhits of various statements in your library cache. Listing 20-2. Determining the Efficiency of the Library Cache SQL> SELECT namespace, pins, pinhits, reloads 2 FROM V$LIBRARYCACHE 3 ORDER BY namespace; Library_cache_hit_ratio
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