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Transaction/second CPU time (seconds) Buffer Busy Waits number/seconds Elapsed Time (minutes) Log File Sync number/seconds 39.4 55 1,807/0.01 1.06 4/0.01
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As you can see from this two-user test, PL/SQL still outperforms Pro*C, but the use of the reverse key index is showing some positive benefits on the PL/SQL side and not so much on the Pro*C side. That, too, is a trend that will continue. The reverse key index is solving the buffer busy wait problem we have due to the contention for the rightmost block in the index structure; however, it does nothing for the log file sync waits that affect the Pro*C program. This was the main reason for performing both a PL/SQL and a Pro*C test: to see the differences between these two environments. This begs the question, why
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CHAPTER 11 INDEXES
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would a reverse key index apparently benefit PL/SQL but not Pro*C in this case It comes down to the log file sync wait event. PL/SQL was able to continuously insert and rarely had to wait for the log file sync wait event upon commit, whereas Pro*C was waiting every 100 rows. Therefore, PL/SQL in this case was impacted more heavily by buffer busy waits than Pro*C was. Alleviating the buffer busy waits in the PL/SQL case allowed it to process more transactions, and so the reverse key index positively benefited PL/SQL. But in the Pro*C case, the buffer busy waits were not the issue they were not the major performance bottleneck, so removing the waits had no impact on overall performance. Let s move on to the five-user test, shown in Table 11-3. Table 11-3. Performance Test for Use of Reverse Key Indexes with PL/SQL and Pro*C: 5 Users
Reverse PL/SQL
Transaction/second CPU time (seconds) Buffer Busy Waits number/seconds Elapsed Time (minutes) Log File Sync number/seconds 22.4 466 10,921/28 2.08 9/0.01
No Reverse PL/SQL
20.6 498 72,953/66 2.27 8/0.01
Reverse Pro*C
16.8 583 11,563/17 3.58 3,635/71
No Reverse Pro*C
15.9 613 100,599/84 3.78 3,772/52
We see more of the same. PL/SQL, running full steam ahead with few log file sync waits, was very much impacted by the buffer busy waits. With a conventional index and all five users attempting to insert into the right-hand side of the index structure, PL/SQL suffered the most from the buffer busy waits and therefore benefited the most when they were reduced. Taking a look at the ten-user test in Table 11-4, we can see the trend continues. Table 11-4. Performance Test for Use of Reverse Key Indexes with PL/SQL and Pro*C: 10 Users
Reverse PL/SQL
Transaction/second CPU time (seconds) Buffer Busy Waits number/seconds Elapsed Time (minutes) Log File Sync number/seconds 22.1 965 13,457/150 4.23 16/0.01
No Reverse PL/SQL
19.9 1,064 165,229/929 4.73 17/0.01
Reverse Pro*C
16.6 1,206 15,251/117 7.33 7,207/245
No Reverse Pro*C
15.3 1,328 212,155/1,210 7.98 7,778/134
PL/SQL, in the absence of the log file sync wait, is very much helped by removing the buffer busy wait events. Pro*C is experiencing more buffer busy wait contention now but, due to the fact it is
CHAPTER 11 INDEXES
waiting on log file sync events frequently, is not benefiting. One way to improve the performance of the PL/SQL implementation with a regular index would be to introduce a small wait. That would reduce the contention on the right-hand side of the index and increase overall performance. For space reasons, I will not include the 15- and 20-user tests here, but I will confirm that the trend observed in this section continued. We can take away two things from this demonstration. A reverse key index can help alleviate a buffer busy wait situation, but depending on other factors you will get varying returns on investment. In looking at Table 11-4 for the ten-user test, the removal of buffer busy waits (the most waited for wait event in that case) affected transaction throughput marginally, but it did show increased scalability with higher concurrency levels. Doing the same thing for PL/SQL had a markedly different impact on performance: we achieved a measurable increase in throughput by removing that bottleneck.
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