Analyze Statement and dbms_stats

參考 http://www.itpub.net/viewthread.php?tid=845777&extra=&page=1

Analyze Statement
The ANALYZE statement can be used to gather statistics for a specific table, index or cluster. The statistics can be computed exactly, or estimated based on a specific number of rows, or a percentage of rows:

ANALYZE TABLE employees COMPUTE STATISTICS;
ANALYZE INDEX employees_pk COMPUTE STATISTICS;

ANALYZE TABLE employees ESTIMATE STATISTICS SAMPLE 100 ROWS;
ANALYZE TABLE employees ESTIMATE STATISTICS SAMPLE 15 PERCENT;
DBMS_UTILITY
The DBMS_UTILITY package can be used to gather statistics for a whole schema or database. Both methods follow the same format as the analyze statement:

EXEC DBMS_UTILITY.analyze_schema('SCOTT','COMPUTE');
EXEC DBMS_UTILITY.analyze_schema('SCOTT','ESTIMATE', estimate_rows => 100);
EXEC DBMS_UTILITY.analyze_schema('SCOTT','ESTIMATE', estimate_percent => 15);

EXEC DBMS_UTILITY.analyze_database('COMPUTE');
EXEC DBMS_UTILITY.analyze_database('ESTIMATE', estimate_rows => 100);
EXEC DBMS_UTILITY.analyze_database('ESTIMATE', estimate_percent => 15);
DBMS_STATS
The DBMS_STATS package was introduced in Oracle 8i and is Oracles preferred method of gathering object statistics. Oracle list a number of benefits to using it including parallel execution, long term storage of statistics and transfer of statistics between servers. Once again, it follows a similar format to the other methods:

EXEC DBMS_STATS.gather_database_stats;
EXEC DBMS_STATS.gather_database_stats(estimate_percent => 15);

EXEC DBMS_STATS.gather_schema_stats('SCOTT');
EXEC DBMS_STATS.gather_schema_stats('SCOTT', estimate_percent => 15);

EXEC DBMS_STATS.gather_table_stats('SCOTT', 'EMPLOYEES');
EXEC DBMS_STATS.gather_table_stats('SCOTT', 'EMPLOYEES', estimate_percent => 15);

EXEC DBMS_STATS.gather_index_stats('SCOTT', 'EMPLOYEES_PK');
EXEC DBMS_STATS.gather_index_stats('SCOTT', 'EMPLOYEES_PK', estimate_percent => 15);
This package also gives you the ability to delete statistics:

EXEC DBMS_STATS.delete_database_stats;
EXEC DBMS_STATS.delete_schema_stats('SCOTT');
EXEC DBMS_STATS.delete_table_stats('SCOTT', 'EMPLOYEES');
EXEC DBMS_STATS.delete_index_stats('SCOTT', 'EMPLOYEES_PK');

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自從Oracle8.1.5引入dbms_stats包，Experts們便推薦使用dbms_stats取代analyze。 理由如下

dbms_stats可以並行分析
dbms_stats有自動分析的功能(alter table monitor )
analyze 分析統計信息的不準確some times


1,2好理解，且第2點實際上在VLDB中是最吸引人的；3以前比較模糊，看了metalink236935.1 解釋，analyze在分析Partition表的時候，有時候會計算出不準確的Global statistics .

原因是，dbms_stats會實在的去分析表全局統計信息（當指定參數）；而analyze是將表分區（局部）的statistics 彙總計算成表全局statistics ,可能導致誤差。

如果想分析整個用戶或數據庫，還可以採用工具包，可以並行分析
Dbms_utility(8i以前的工具包)
Dbms_stats(8i以後提供的工具包)
如
dbms_stats.gather_schema_stats(User,estimate_percent=>100,cascade=> TRUE);
dbms_stats.gather_table_stats(User,TableName,degree => 4,cascade => true);

這是對命令與工具包的一些總結

1、對於分區表，建議使用DBMS_STATS，而不是使用Analyze語句。
a) 可以並行進行，對多個用戶，多個Table
b) 可以得到整個分區表的數據和單個分區的數據。
c) 可以在不同級別上Compute Statistics：單個分區，子分區，全表，所有分區
d) 可以倒出統計信息
e) 可以用戶自動收集統計信息

2、DBMS_STATS的缺點
a) 不能Validate Structure
b) 不能收集CHAINED ROWS, 不能收集CLUSTER TABLE的信息，這兩個仍舊需要使用Analyze語句。
c) DBMS_STATS 默認不對索引進行Analyze，因爲默認Cascade是False，需要手工指定爲True

3、對於oracle 9裏面的External Table，Analyze不能使用，只能使用DBMS_STATS來收集信息。

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10G的文檔是這麼說的：
Do not use the COMPUTE and ESTIMATE clauses of ANALYZE to collect optimizer statistics. These clauses are supported for backward compatibility. Instead, use the DBMS_STATS package, which lets you collect statistics in parallel, collect global statistics for partitioned objects, and fine tune your statistics collection in other ways. The cost-based optimizer, which depends upon statistics, will eventually use only statistics that have been collected by DBMS_STATS

analyze的功能已經明確：
Use the ANALYZE statement (rather than DBMS_STATS) for statistics collection not related to the cost-based optimizer :

To use the VALIDATE or LIST CHAINED ROWS clauses

To collect information on freelist blocks

在收集與CBO優化器不相關的統計信息的時候ANALYZE語句要優於DBMS_STATS包

-----------------------------------
EX:
begin
        for owner in (select username from dba_users where username not in ('SYS','SYSTEM'))
        loop
         dbms_output.disable;
         dbms_output.enable(1000000);
         dbms_output.put_line('Schema: '||owner.username);
         select sysdate into start_time from dual;
         dbms_output.put_line('Analyze start from : '||start_time);
         dbms_stats.gather_schema_stats(ownname => owner.username, estimate_percent => 20, block_sample=> true, cascade=>true);
         select sysdate into end_time from dual;
         dbms_output.put_line('Analyze complete at : '||end_time);
         dbms_output.put_line('---------------------------');
        end loop;


dbms_stats.gather_table_stats(ownname          =>,
                                tabname          =>,
                                partname         =>,
                                estimate_percent =>,
                                block_sample     =>,
                                method_opt       =>,
                                degree           =>,parallel degree(並行收集維度)　看CPU個數
                                granularity      =>,
                                cascade          =>,true is also gather columns and index’s statistics;
                                no_invalidate    =>);

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