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

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CHAPTER 1 ■ DEVELOPING SUCCESSFUL ORACLE APPLICATIONS 43 ops$tkyte@ORA10G> select /* TAG */ substr( username, 1, 1 ) 2 from all_users au1 3 where rownum = 1; S - B ops$tkyte@ORA10G> alter session set cursor_sharing=force; Session altered. ops$tkyte@ORA10G> select /* TAG */ substr( username, 1, 1 ) 2 from all_users au2 3 where rownum = 1; SUBSTR(USERNAME,1,1) ------------------------------ B What happened there? Why is the column reported by SQL*Plus suddenly so large for the second query, which is arguably the same query? If we look at what the cursor sharing setting did for us, it (and something else) will become obvious: ops$tkyte@ORA10G> select sql_text from v$sql 2 where sql_text like 'select /* TAG */ %'; SQL_TEXT ------------------------------------------------------------------------------- select /* TAG */ substr( username, 1, 1 ) from all_users au1 where rownum = 1 select /* TAG */ substr( username, :"SYS_B_0", :"SYS_B_1" ) 2 where rownum = :"SYS_B_2" from all_users au The cursor sharing removed information from the query. It found every literal, including the parameters to the built-in substring function, which were constants we were using. It removed them from the query and replaced them with bind variables. The SQL engine no longer knows that the column is a substring of length 1—it is of indeterminate length. Also, you can see that where rownum = 1 is now bound as well. That seems like a good idea; however, the optimizer has just had some important information removed. It no longer knows that “this query will retrieve a single row”; it now believes “this query will return the first N rows and N could be any number at all.” In fact, if you run these queries with SQL_TRACE=TRUE, you will find the query plans used by each query and the amount of work they perform to be very different. Consider the following: select /* TAG */ substr( username, 1, 1 ) from all_users au1 where rownum = 1
44 CHAPTER 1 ■ DEVELOPING SUCCESSFUL ORACLE APPLICATIONS call count cpu elapsed disk query current rows ------- ------ -------- ---------- ---------- ---------- ---------- ---------- Parse 1 0.00 0.00 0 0 0 0 Execute 1 0.00 0.00 0 0 0 0 Fetch 2 0.00 0.00 0 77 0 1 ------- ------ -------- ---------- ---------- ---------- ---------- ---------- total 4 0.00 0.00 0 77 0 1 Misses in library cache during parse: 0 Optimizer mode: ALL_ROWS Parsing user id: 412 Rows Row Source Operation ------- --------------------------------------------------- 1 COUNT STOPKEY (cr=77 pr=0 pw=0 time=5767 us) 1 HASH JOIN (cr=77 pr=0 pw=0 time=5756 us) 1028 HASH JOIN (cr=70 pr=0 pw=0 time=8692 us) 9 TABLE ACCESS FULL TS$ (cr=15 pr=0 pw=0 time=335 us) 1028 TABLE ACCESS FULL USER$ (cr=55 pr=0 pw=0 time=2140 us) 4 TABLE ACCESS FULL TS$ (cr=7 pr=0 pw=0 time=56 us) ******************************************************************************** select /* TAG */ substr( username, :"SYS_B_0", :"SYS_B_1" ) from all_users au2 where rownum = :"SYS_B_2" call count cpu elapsed disk query current rows ------- ------ -------- ---------- ---------- ---------- ---------- ---------- Parse 1 0.00 0.00 0 0 0 0 Execute 1 0.00 0.00 0 0 0 0 Fetch 2 0.00 0.00 0 85 0 1 ------- ------ -------- ---------- ---------- ---------- ---------- ---------- total 4 0.00 0.00 0 85 0 1 Misses in library cache during parse: 0 Optimizer mode: ALL_ROWS Parsing user id: 412 Rows Row Source Operation ------- --------------------------------------------------- 1 COUNT (cr=85 pr=0 pw=0 time=3309 us) 1 FILTER (cr=85 pr=0 pw=0 time=3301 us) 1028 HASH JOIN (cr=85 pr=0 pw=0 time=5343 us) 1028 HASH JOIN (cr=70 pr=0 pw=0 time=7398 us) 9 TABLE ACCESS FULL TS$ (cr=15 pr=0 pw=0 time=148 us) 1028 TABLE ACCESS FULL USER$ (cr=55 pr=0 pw=0 time=1079 us) 9 TABLE ACCESS FULL TS$ (cr=15 pr=0 pw=0 time=90 us)
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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 4 ■ MEMORY STRUCTURES 117
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CHAPTER 5 ■ ORACLE PROCESSES 157
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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 11 ■ INDEXES 435 Table 11
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CHAPTER 11 ■ INDEXES 439 an 8KB b
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CHAPTER 11 ■ INDEXES 445 Indicate
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CHAPTER 11 ■ INDEXES 447 an index
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CHAPTER 11 ■ INDEXES 449 Table 11
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CHAPTER 13 ■ ■ ■ Partitioning
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CHAPTER 13 ■ PARTITIONING 563 BIG
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Apress.Expert.Oracle.Database.Architecture.9i.and.10g.Programming.Techniques.and.Solutions.Sep.2005

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