Apress.Expert.Oracle.Database.Architecture.9i.and.10g.Programming.Techniques.and.Solutions.Sep.2005

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CHAPTER 11 ■ INDEXES 433 if ( (++cnt%100) == 0 ) { exec sql commit; } } exec sql whenever notfound continue; exec sql commit; exec sql close c; The Pro*C was precompiled with a PREFETCH of 100, making this C code analogous to the PL/SQL code in Oracle 10g. ■Note In Oracle 10g Release 1 and above, a simple FOR X IN ( SELECT * FROM T ) in PL/SQL will silently array fetch 100 rows at a time, whereas in Oracle9i and before, it fetches just a single row at a time. Therefore, if you want to reproduce this example on Oracle9i and before, you will need to modify the PL/SQL code to also array fetch with the BULK COLLECT syntax. Both would fetch 100 rows at a time and then single row insert the data into another table. The following tables summarize the differences between the various runs, starting with the single user test in Table 11-1. Table 11-1. Performance Test for Use of Reverse Key Indexes with PL/SQL and Pro*C: Single User Reverse No Reverse Reverse No Reverse PL/SQL PL/SQL Pro*C Pro*C Transaction/second 38.24 43.45 17.35 19.08 CPU time (seconds) 25 22 33 31 Buffer busy waits number/time 0/0 0/0 0/0 0/0 Elapsed time (minutes) 0.42 0.37 0.92 0.83 Log file sync number/time 6/0 1,940/7 1,940/7 From the first single-user test, we can see that PL/SQL was measurably more efficient than Pro*C in performing this operation, a trend we’ll continue to see as we scale up the user load. Part of the reason Pro*C won’t scale as well as PL/SQL will be the log file sync waits that Pro*C must wait for, but which PL/SQL has an optimization to avoid. It would appear from this single-user test that reverse key indexes consume more CPU. This makes sense because the database must perform extra work as it carefully reverses the bytes in the key. But, we’ll see that this logic won’t hold true as we scale up the users. As we introduce contention, the overhead of the reverse key index will completely disappear. In fact, even by the time we get the two-user test, the overhead is mostly offset by the contention on the right-hand side of the index, as shown in Table 11-2.
434 CHAPTER 11 ■ INDEXES Table 11-2. Performance Test for Use of Reverse Key Indexes with PL/SQL and Pro*C: Two Users Reverse No Reverse Reverse No Reverse PL/SQL PL/SQL Pro*C Pro*C Transaction/second 46.59 49.03 20.07 20.29 CPU time (seconds) 77 73 104 101 Buffer busy waits number/time 4,267/2 133,644/2 3,286/0 23,688/1 Elapsed time (minutes) 0.68 0.65 1.58 1.57 Log file sync number/time 19/0 18/0 3,273/29 2,132/29 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 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: Five Users Reverse No Reverse Reverse No Reverse PL/SQL PL/SQL Pro*C Pro*C Transaction/second 43.84 39.78 19.22 18.15 CPU time (seconds) 389 395 561 588 Buffer busy waits number/time 19,259/45 221,353/153 19,118/9 157,967/56 Elapsed time (minutes) 1.82 2.00 4.13 4.38 Log file sync number/time 691/14 6,655/73 5,391/82 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. Now, taking a look at the ten-user test in Table 11-4, we can see the trend continues.
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Expert Oracle Database Architecture
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Contents Foreword . . . . . . . . .
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■CONTENTS v Block Buffer Cache .
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■CONTENTS vii ■CHAPTER 9 Redo a
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■CONTENTS ix ■CHAPTER 12 Dataty
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Foreword “THINK.” In 1914, Thom
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■FOREWORD xiii Tom is an aficiona
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About the Technical Reviewers ■JO
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Introduction The inspiration for th
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■INTRODUCTION xix • Exposure to
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■INTRODUCTION xxi Chapter 3: File
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■INTRODUCTION xxiii Next up are t
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Setting Up Your Environment In this
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CHAPTER 1 ■ DEVELOPING SUCCESSFUL
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CHAPTER 4 ■ MEMORY STRUCTURES 117
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CHAPTER 5 ■ ORACLE PROCESSES 157
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CHAPTER 8 ■ ■ ■ Transactions
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284 CHAPTER 9 ■ REDO AND UNDO cri
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338 CHAPTER 10 ■ DATABASE TABLES
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CHAPTER 13 ■ ■ ■ Partitioning
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Apress.Expert.Oracle.Database.Architecture.9i.and.10g.Programming.Techniques.and.Solutions.Sep.2005

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