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

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CHAPTER 11 ■ INDEXES 481 ops$tkyte@ORA10GR1> set autotrace on explain ops$tkyte@ORA10GR1> select count(y) from t where x < 50; COUNT(Y) ---------- 49 Execution Plan ---------------------------------------------------------- 0 SELECT STATEMENT Optimizer=ALL_ROWS (Cost=3 Card=1 Bytes=28) 1 0 SORT (AGGREGATE) 2 1 TABLE ACCESS (BY INDEX ROWID) OF 'T' (TABLE) (Cost=3 Card=41 Bytes=1148) 3 2 INDEX (RANGE SCAN) OF 'SYS_C009167' (INDEX (UNIQUE)) (Cost=2 Card=41) it will happily use the index; however, we’ll find that when the estimated number of rows to be retrieved via the index crosses a threshold (which varies depending on various optimizer settings, physical statistics, and so on), we’ll start to observe a full table scan: ops$tkyte@ORA10GR1> select count(y) from t where x < 15000; COUNT(Y) ---------- 14999 Execution Plan ---------------------------------------------------------- 0 SELECT STATEMENT Optimizer=ALL_ROWS (Cost=57 Card=1 Bytes=28) 1 0 SORT (AGGREGATE) 2 1 TABLE ACCESS (FULL) OF 'T' (TABLE) (Cost=57 Card=14994 Bytes=419832) This example shows the optimizer won’t always use an index and, in fact, it makes the right choice in skipping indexes. While tuning your queries, if you discover that an index isn’t used when you think it “ought to be,” don’t just force it to be used—test and prove first that the index is indeed faster (via elapsed and I/O counts) before overruling the CBO. Reason it out. Case 6 We haven’t analyzed our tables in a while. They used to be small, but now when we look at them, they have grown quite large. An index will now make sense, whereas it didn’t originally. If we analyze the table, it will use the index. Without correct statistics, the CBO cannot make the correct decisions. Index Case Summary In my experience, these six cases are the main reasons I find that indexes are not being used. It usually boils down to a case of “They cannot be used—using them would return incorrect results” or “They should not be used—if they were used, performance would be terrible.”
482 CHAPTER 11 ■ INDEXES Myth: Space Is Never Reused in an Index This is a myth that I would like to dispel once and for all: space is reused in an index. The myth goes like this: you have a table, T, in which there is a column, X. At some point, you put the value X=5in the table. Later you delete it. The myth is that the space used by X=5will not be reused unless you put X=5back into the index later. The myth states that once an index slot is used, it will be there forever and can be reused only by the same value. A corollary to this is the myth that free space is never returned to the index structure, and a block will never be reused. Again, this is simply not true. The first part of the myth is trivial to disprove. All we need to do is to create a table like this: ops$tkyte@ORA10GR1> create table t ( x int, constraint t_pk primary key(x) ); Table created. ops$tkyte@ORA10GR1> insert into t values (1); 1 row created. ops$tkyte@ORA10GR1> insert into t values (2); 1 row created. ops$tkyte@ORA10GR1> insert into t values (9999999999); 1 row created. ops$tkyte@ORA10GR1> analyze index t_pk validate structure; Index analyzed. ops$tkyte@ORA10GR1> select lf_blks, br_blks, btree_space 2 from index_stats; LF_BLKS BR_BLKS BTREE_SPACE ---------- ---------- ----------- 1 0 7996 So, according to the myth, if I delete from T where X=2, that space will never be reused unless I reinsert the number 2. Currently, this index is using one leaf block of space. If the index key entries are never reused upon deletion, and I keep inserting and deleting and never reuse a value, this index should grow like crazy. Let’s see: ops$tkyte@ORA10GR1> begin 2 for i in 2 .. 999999 3 loop 4 delete from t where x = i; 5 commit; 6 insert into t values (i+1); 7 commit; 8 end loop; 9 end; 10 /
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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 xxiii Next up are t
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Setting Up Your Environment In this
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Apress.Expert.Oracle.Database.Architecture.9i.and.10g.Programming.Techniques.and.Solutions.Sep.2005

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