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

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CHAPTER 11 ■ INDEXES 483 ops$tkyte@ORA10GR1> analyze index t_pk validate structure; Index analyzed. ops$tkyte@ORA10GR1> select lf_blks, br_blks, btree_space 2 from index_stats; LF_BLKS BR_BLKS BTREE_SPACE ---------- ---------- ----------- 1 0 7996 This shows the space in the index was reused. As with most myths, however, there is a nugget of truth in there. The truth is that the space used by that initial number 2 (in between 1 and 9,999,999,999) would remain on that index block forever. The index will not “coalesce” itself. This means if I load a table with values 1 to 500,000 and then delete every other row (all of the even numbers), there will be 250,000 “holes” in the index on that column. Only if I reinsert data that will fit onto a block where there is a hole will the space be reused. Oracle will make no attempt to “shrink” or compact the index. This can be done via an ALTER INDEX REBUILD or COALESCE command. On the other hand, if I load a table with values 1 to 500,000 and then delete from the table every row where the value was 250,000 or less, I would find the blocks that were cleaned out of the index were put back onto the freelist for the index. This space can be totally reused. If you recall, this was the second myth: index space is never “reclaimed.” It states that once an index block is used, it will be stuck in that place in the index structure forever and will only be reused if you insert data that would go into that place in the index anyway. We can show that this is false as well. First, we need to build a table with about 500,000 rows in it. For that, we’ll use the big_table script. After we have that table with its corresponding primary key index, we’ll measure how many leaf blocks are in the index and how many blocks are on the freelist for the index. Remember, with an index, a block will only be on the freelist if the block is entirely empty, unlike a table. So any blocks we see on the freelist are completely empty and available for reuse: ops$tkyte@ORA10GR1> select count(*) from big_table; COUNT(*) ---------- 500000 ops$tkyte@ORA10GR1> declare 2 l_freelist_blocks number; 3 begin 4 dbms_space.free_blocks 5 ( segment_owner => user, 6 segment_name => 'BIG_TABLE_PK', 7 segment_type => 'INDEX', 8 freelist_group_id => 0, 9 free_blks => l_freelist_blocks ); 10 dbms_output.put_line( 'blocks on freelist = ' || l_freelist_blocks ); 11 end;
484 CHAPTER 11 ■ INDEXES 12 / blocks on freelist = 0 PL/SQL procedure successfully completed. ops$tkyte@ORA10GR1> select leaf_blocks from user_indexes 2 where index_name = 'BIG_TABLE_PK'; LEAF_BLOCKS ----------- 1043 Before we perform this mass deletion, we have no blocks on the freelist and there are 1,043 blocks in the “leafs” of the index, holding data. Now, we’ll perform the delete and measure the space utilization again: ops$tkyte@ORA10GR1> delete from big_table where id commit; Commit complete. ops$tkyte@ORA10GR1> declare 2 l_freelist_blocks number; 3 begin 4 dbms_space.free_blocks 5 ( segment_owner => user, 6 segment_name => 'BIG_TABLE_PK', 7 segment_type => 'INDEX', 8 freelist_group_id => 0, 9 free_blks => l_freelist_blocks ); 10 dbms_output.put_line( 'blocks on freelist = ' || l_freelist_blocks ); 11 dbms_stats.gather_index_stats 12 ( user, 'BIG_TABLE_PK' ); 13 end; 14 / blocks on freelist = 520 PL/SQL procedure successfully completed. ops$tkyte@ORA10GR1> select leaf_blocks from user_indexes 2 where index_name = 'BIG_TABLE_PK'; LEAF_BLOCKS ----------- 523 As we can see, over half of the index is on the freelist now (520 blocks) and there are only 523 leaf blocks. If we add 523 and 520, we get the original 1043. This means the blocks are totally empty and ready to be reused (blocks on the freelist for an index must be empty, unlike
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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 xxvi
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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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CHAPTER 3 ■ FILES 71 all operatio
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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 111 IMPDP, howe
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CHAPTER 3 ■ FILES 113 tkyte@ORA10
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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 6 ■ ■ ■ Locking and L
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CHAPTER 6 ■ LOCKING AND LATCHING
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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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296 CHAPTER 9 ■ REDO AND UNDO If
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298 CHAPTER 9 ■ REDO AND UNDO ops
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300 CHAPTER 9 ■ REDO AND UNDO Inv
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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 11 ■ ■ ■ Indexes Inde
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CHAPTER 11 ■ INDEXES 423 value of
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CHAPTER 11 ■ INDEXES 431 The data
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CHAPTER 13 ■ ■ ■ Partitioning
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706 ■INDEX autonomous transaction
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