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

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CHAPTER 11 ■ INDEXES 441 select * from colocated where x between 20000 and 40000 call count cpu elapsed disk query current rows ------- ------ -------- ---------- ---------- ---------- ---------- ---------- Parse 5 0.00 0.00 0 0 0 0 Execute 5 0.00 0.00 0 0 0 0 Fetch 6675 0.59 0.60 0 14495 0 100005 ------- ------ -------- ---------- ---------- ---------- ---------- ---------- total 6685 0.59 0.60 0 14495 0 100005 Rows Row Source Operation ------- --------------------------------------------------- 20001 TABLE ACCESS BY INDEX ROWID COLOCATED (cr=2899 pr=0 pw=0 time=120134 us) 20001 INDEX RANGE SCAN COLOCATED_PK (cr=1374 pr=0 pw=0 time=40081 us)(object id... ******************************************************************************** select /*+ index( disorganized disorganized_pk ) */* from disorganized where x between 20000 and 40000 call count cpu elapsed disk query current rows ------- ------ -------- ---------- ---------- ---------- ---------- ---------- Parse 5 0.00 0.00 0 0 0 0 Execute 5 0.00 0.00 0 0 0 0 Fetch 6675 0.85 0.87 0 106815 0 100005 ------- ------ -------- ---------- ---------- ---------- ---------- ---------- total 6685 0.85 0.87 0 106815 0 100005 Rows Row Source Operation ------- --------------------------------------------------- 20001 TABLE ACCESS BY INDEX ROWID DISORGANIZED (cr=21363 pr=0 pw=0 time=220144 ... 20001 INDEX RANGE SCAN DISORGANIZED_PK (cr=1374 pr=0 pw=0 time=40311 us)(... ■Note I ran each query five times in order to get a good “average” runtime for each. I think this is pretty incredible. What a difference physical data layout can make! Table 11-5 summarizes the results. Table 11-5. Investigating the Effect of Physical Data Layout on the Cost of Index Access Table CPU Time Logical I/O Co-located 0.59 seconds 14,495 Disorganized 0.85 seconds 106,815 Co-located % 70% 13%
442 CHAPTER 11 ■ INDEXES In my database using an 8KB blocksize, these tables had the following number of total blocks apiece: ops$tkyte@ORA10G> select table_name, blocks 2 from user_tables 3 where table_name in ( 'COLOCATED', 'DISORGANIZED' ); TABLE_NAME BLOCKS ------------------------------ ---------- COLOCATED 1252 DISORGANIZED 1219 The query against the disorganized table bears out the simple math we did earlier: we did 20,000+ logical I/Os (100,000 total blocks queried and five runs of the query). We processed each and every block 20 times! On the other hand, the physically COLOCATED data took the logical I/Os way down. Here is the perfect illustration of why rules of thumb are so hard to provide—in one case, using the index works great, and in the other case it doesn’t. Consider this the next time you dump data from your production system and load it into development, as it may very well provide at least part of the answer to the question, “Why is it running differently on this machine—aren’t they identical?” They are not identical. ■Note Recall from Chapter 6 that increased logical I/O is the tip of the iceberg here. Each logical I/O involves one or more latches into the buffer cache. In a multiuser/CPU situation, the CPU used by the second query would have undoubtedly gone up many times faster than the first as we spin and wait for latches. The second example query not only performs more work, but also will not scale as well as the first. THE EFFECT OF ARRAYSIZE ON LOGICAL I/O It is interesting to note the effect of the ARRAYSIZE on logical I/O performed. ARRAYSIZE is the number of rows Oracle returns to a client when they ask for the next row. The client will then buffer these rows and use them before asking the database for the next set of rows. The ARRAYSIZE may have a very material affect on the logical I/O performed by a query, resulting from the fact that if you have to access the same block over and over again across calls (across fetch calls specifically in this case) to the database, Oracle must retrieve that block again from the buffer cache. Therefore, if you ask for 100 rows from the database in a single call, Oracle might be able to fully process a database block and not need to retrieve that block again. If you ask for 15 rows at a time, Oracle might well have to get the same block over and over again to retrieve the same set of rows. In the example earlier in this section, we were using SQL*Plus’s default array fetch size of 15 rows (if you divide the total rows fetched by the number of fetch calls, the result is very close to 15). If we were to compare the execution of the previous queries using 15 rows per fetch versus 100 rows per fetch, we would observe the following for the COLOCATED table:
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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 3 ■ FILES 89 A Brief Revi
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CHAPTER 4 ■ MEMORY STRUCTURES 117
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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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