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

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CHAPTER 11 ■ INDEXES 427 We then create an IDX_STATS table in which to save INDEX_STATS information, and we label the rows in the table as “noncompressed”: ops$tkyte@ORA10G> create table idx_stats 2 as 3 select 'noncompressed' what, a.* 4 from index_stats a; Table created. Now, we could realize that the OWNER component is repeated many times, meaning that a single index block in this index will have dozens of entries, as shown in Figure 11-2. Figure 11-2. Index block with OWNER column repeated We could factor the repeated OWNER column out of this, resulting in a block that looks more like Figure 11-3. Figure 11-3. Index block with OWNER column factored out In Figure 11-3, the owner name appears once on the leaf block—not once per repeated entry. We run the following script, passing in the number 1, to re-create the scenario whereby the index is using compression on just the leading column: drop index t_idx; create index t_idx on t(owner,object_type,object_name) compress &1;
428 CHAPTER 11 ■ INDEXES analyze index t_idx validate structure; insert into idx_stats select 'compress &1', a.* from index_stats a; For comparison reasons, we run this script not only with one column, but also two and three compressed columns, to see what happens. At the end, we query IDX_STATS and should observe this: ops$tkyte@ORA10G> select what, height, lf_blks, br_blks, 2 btree_space, opt_cmpr_count, opt_cmpr_pctsave 3 from idx_stats 4 / WHAT HEIGHT LF_BLKS BR_BLKS BTREE_SPACE OPT_CMPR_COUNT OPT_CMPR_PCTSAVE ------------- ------- -------- ------- ----------- -------------- ---------------- noncompressed 3 337 3 2718736 2 28 compress 1 3 300 3 2421684 2 19 compress 2 2 240 1 1926108 2 0 compress 3 3 375 3 3021084 2 35 We see that the COMPRESS 1 index is about 89 percent the size of the noncompressed index (comparing BTREE_SPACE). The number of leaf blocks has decreased measurably. Further, when we use COMPRESS 2, the savings are even more impressive. The resulting index is about 70 percent the size of the original, and so much data is able to be placed on individual blocks that the height of the index actually decreased from 3 to 2. In fact, using the column OPT_CMPR_ PCTSAVE, which stands for optimum compression percent saved or the expected savings from compression, we could have guessed the size of the COMPRESS 2 index: ops$tkyte@ORA10G> select 2718736*(1-0.28) from dual; 2718736*(1-0.28) ---------------- 1957489.92 ■Note The ANALYZE command against the noncompressed index populated the OPT_CMPR_ PCTSAVE/OPT_CMPR_COUNT columns and estimated a 28 percent savings with COMPRESS 2, and we achieved just about exactly that. But notice what happens with COMPRESS 3. The resulting index is actually larger: 110 percent the size of the original index. This is due to the fact that each repeated prefix we remove saves the space of N copies, but adds 4 bytes of overhead on the leaf block as part of the compression scheme. By adding in the OBJECT_NAME column to the compressed key, we made that key unique—in this case, meaning there were no duplicate copies to factor out. Therefore,
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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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■SETTING UP YOUR ENVIRONMENT xlii
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CHAPTER 1 ■ ■ ■ Developing Su
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CHAPTER 1 ■ DEVELOPING SUCCESSFUL
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50 CHAPTER 2 ■ ARCHITECTURE OVERV
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CHAPTER 3 ■ ■ ■ Files In this
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CHAPTER 3 ■ FILES 69 and even dat
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CHAPTER 3 ■ FILES 71 all operatio
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CHAPTER 3 ■ FILES 73 *.cluster_da
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CHAPTER 3 ■ FILES 75 ops$tkyte@OR
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CHAPTER 3 ■ FILES 77 • To maint
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CHAPTER 3 ■ FILES 79 • Resource
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CHAPTER 3 ■ FILES 83 Trace Files
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CHAPTER 3 ■ FILES 87 6 ( 7 TYPE O
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CHAPTER 3 ■ FILES 89 A Brief Revi
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CHAPTER 3 ■ FILES 91 Redundant Ar
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CHAPTER 3 ■ FILES 93 (data from m
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CHAPTER 3 ■ FILES 95 dictionary t
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CHAPTER 3 ■ FILES 99 “accidenta
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CHAPTER 4 ■ ■ ■ Memory Struct
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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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292 CHAPTER 9 ■ REDO AND UNDO Wha
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294 CHAPTER 9 ■ REDO AND UNDO row
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298 CHAPTER 9 ■ REDO AND UNDO ops
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306 CHAPTER 9 ■ REDO AND UNDO ins
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318 CHAPTER 9 ■ REDO AND UNDO •
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332 CHAPTER 9 ■ REDO AND UNDO Whe
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336 CHAPTER 9 ■ REDO AND UNDO tou
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338 CHAPTER 10 ■ DATABASE TABLES
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CHAPTER 13 ■ ■ ■ Partitioning
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CHAPTER 13 ■ PARTITIONING 561 els
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CHAPTER 13 ■ PARTITIONING 565 Enh
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CHAPTER 14 ■ PARALLEL EXECUTION 6
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706 ■INDEX autonomous transaction
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