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

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CHAPTER 14 ■ PARALLEL EXECUTION 619 6,000,000 12,000,000 Figure 14-1. Parallel select count (status) depiction We can see this in an explain plan. Using a BIG_TABLE with 10 million rows in it, we’ll walk through enabling a parallel query for that table and discover how we can “see” parallel query in action. This example was performed on a four-CPU machine with default values for all parallel parameters; that is, this is an out-of-the-box installation where only necessary parameters were set, including SGA_TARGET (set to 1GB), CONTROL_FILES, DB_BLOCK_SIZE (set to 8KB), and PGA_AGGREGATE_TARGET (set to 512MB). Initially, we would expect to see the following plan: big_table@ORA10GR1> explain plan for 2 select count(status) from big_table; Explained. big_table@ORA10GR1> select * from table(dbms_xplan.display); PLAN_TABLE_OUTPUT ---------------------------------------- Plan hash value: 1287793122 ---------------------------------------------------------------------------- | Id | Operation | Name | Rows|Bytes | Cost (%CPU)|Time | ---------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | 17 | 32390 (2)|00:06:29 | | 1 | SORT AGGREGATE | | 1 | 17 | | | | 2 | TABLE ACCESS FULL| BIG_TABLE | 10M| 162M| 32390 (2)|00:06:29 | ----------------------------------------------------------------------------
620 CHAPTER 14 ■ PARALLEL EXECUTION That is a typical serial plan. No parallelism is involved because we did not request parallel query to be enabled, and by default it will not be. We may enable parallel query in a variety of ways, including use of a hint directly in the query or by altering the table to enable parallel execution paths to be considered (which is the option we use here). We can specifically dictate the degree of parallelism to be considered in execution paths against this table. We can tell Oracle, “We would like you to use parallel degree 4 when creating execution plans against this table,” for example: big_table@ORA10GR1> alter table big_table parallel 4; Table altered. I prefer to just tell Oracle, “Please consider parallel execution, but you figure out the appropriate degree of parallelism based on the current system workload and the query itself.” That is, let the degree of parallelism vary over time as the workload on the system increases and decreases. If we have plenty of free resources, the degree of parallelism will go up; in times of limited available resources, the degree of parallelism will go down. Rather than overload the machine with a fixed degree of parallelism, this approach allows Oracle to dynamically increase or decrease the amount of concurrent resources required by the query. Therefore, we simply enable parallel query against this table via the following ALTER TABLE command: big_table@ORA10GR1> alter table big_table parallel; Table altered. That is all there is to it—parallel query will now be considered for operations against this table. When we rerun the explain plan, this time we see the following: big_table@ORA10GR1> explain plan for 2 select count(status) from big_table; Explained. big_table@ORA10GR1> select * from table(dbms_xplan.display); PLAN_TABLE_OUTPUT ----------------------------------------- Plan hash value: 1651916128 ---------------------------------------------------------------------------- |Id | Operation | Name |Cost(%CPU)| TQ |IN-OUT|PQ Distrib | ---------------------------------------------------------------------------- | 0| SELECT STATEMENT | | 4465 (1)| | | | | 1| SORT AGGREGATE | | | | | | | 2| PX COORDINATOR | | | | | | | 3| PX SEND QC (RANDOM) | :TQ10000 | |Q1,00| P->S |QC (RAND) | | 4| SORT AGGREGATE | | |Q1,00| PCWP | | | 5| PX BLOCK ITERATOR | | 4465 (1)|Q1,00| PCWC | | | 6| TABLE ACCESS FULL| BIG_TABLE| 4465 (1)|Q1,00| PCWP | | ----------------------------------------------------------------------------
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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 89 A Brief Revi
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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 LATCHING
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CHAPTER 8 ■ ■ ■ Transactions
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284 CHAPTER 9 ■ REDO AND UNDO cri
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338 CHAPTER 10 ■ DATABASE TABLES
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CHAPTER 11 ■ ■ ■ Indexes Inde
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CHAPTER 13 ■ ■ ■ Partitioning
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706 ■INDEX autonomous transaction
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Apress.Expert.Oracle.Database.Architecture.9i.and.10g.Programming.Techniques.and.Solutions.Sep.2005

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