dbms_stats.gather_table_stats与analyze table 的区别

转自https://www.cnblogs.com/laoyangHJ/articles/gather_table_stats-analyze-table.html

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 inOracle8i 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');

--------------------------------------------------------------------------------------

自从Oracle8.1.5引入dbms_stats包，Experts们便推荐使用dbms_stats取代analyze。理由如下

1. dbms_stats可以并行分析
2. dbms_stats有自动分析的功能(alter table monitor )
3. analyze 分析统计信息的不准确some times

1,2好理解，且第2点实际上在VLDB中是最吸引人的；3以前比较模糊，看了metalink236935.1 解释，analyze在分析Partition表的时候，有时候会计算出不准确的Global statistics .
原因是，dbms_stats会实在的去分析表全局统计信息（当指定参数）；而analyze是将表分区（局部）的statistics 汇总计算成表全局statistics ,可能导致误差。

如果想分析整个用户或数据库，还可以采用工具包，可以并行分析
Dbms_utility(8i以前的工具包)
Dbms_stats(8i以后提供的工具包)
如

dbms_stats.gather_schema_stats(User,estimate_percent=>100,cascade=> TRUE);
dbms_stats.gather_table_stats(User,TableName,degree => 4,cascade => true);

这是对命令与工具包的一些总结

1、对于分区表，建议使用DBMS_STATS，而不是使用Analyze语句。
a) 可以并行进行，对多个用户，多个Table
b) 可以得到整个分区表的数据和单个分区的数据。
c) 可以在不同级别上Compute Statistics：单个分区，子分区，全表，所有分区
d) 可以倒出统计信息
e) 可以用户自动收集统计信息

2、DBMS_STATS的缺点
a) 不能Validate Structure
b) 不能收集CHAINED ROWS, 不能收集CLUSTER TABLE的信息，这两个仍旧需要使用Analyze语句。
c) DBMS_STATS 默认不对索引进行Analyze，因为默认Cascade是False，需要手工指定为True

3、对于oracle 9里面的External Table，Analyze不能使用，只能使用DBMS_STATS来收集信息。

-----------------------------------------------------------------
10G的文档是这么说的：
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的功能已经明确：
Use the ANALYZE statement (rather than DBMS_STATS) forstatistics collection not related to the cost-based optimizer:

To use the VALIDATE or LIST CHAINED ROWS clauses

To collect information on freelist blocks

在收集与CBO优化器不相关的统计信息的时候ANALYZE语句要优于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(并行收集维度)　看CPU个数
granularity =>,
cascade =>,true is also gather columns and index’s statistics;
no_invalidate =>);

///

当索引整个块成为空块的时候，会被放置于freelist上，可以被重用。但是重用前结构上还是临时放置于B-Tree中。

这时analyze & dbms_stats在分析时会有不同，analyze会将这些空的block计算为leaf block，而dbms_stats不将其计算在内。

请看例子

1 create table test
2 as
3 select rownum x
4* from dba_objects
SQL> /

Table created.

SQL> select count(*) from test;

COUNT(*)
———-
6114

1 create index test_idx on test(x)
2* pctfree 95
SQL> /

Index created.

SQL> analyze table test compute statistics;

Table analyzed.

SQL> select index_name,blevel,leaf_blocks,distinct_keys,num_rows,last_analyzed from dba_indexes where index_name=’TEST_IDX’ and wner=’SYS’;

INDEX_NAME BLEVEL LEAF_BLOCKS DISTINCT_KEYS NUM_ROWS LAST_ANALYZED
—————————— ———- ———– ————- ———- —————–
TEST_IDX 1 408 6114 6114 20060513 01:00:25

SQL> delete from test where x<3000;

2999 rows deleted.

SQL> commit;

Commit complete.

SQL> analyze index test_idx compute statistics;

Index analyzed.

SQL> select index_name,blevel,leaf_blocks,distinct_keys,num_rows,last_analyzed
2 from dba_indexes where index_name=’TEST_IDX’ and wner=’SYS’;

INDEX_NAME BLEVEL LEAF_BLOCKS DISTINCT_KEYS NUM_ROWS LAST_ANALYZED
—————————— ———- ———– ————- ———- —————–
TEST_IDX 1 408 3115 3115 20060513 01:03:31

SQL> exec DBMS_STATS.GATHER_INDEX_STATS(’SYS’,'TEST_IDX’);

PL/SQL procedure successfully completed.

SQL> select index_name,blevel,leaf_blocks,distinct_keys,num_rows,last_analyzed
2 from dba_indexes where index_name=’TEST_IDX’ and wner=’SYS’;

INDEX_NAME BLEVEL LEAF_BLOCKS DISTINCT_KEYS NUM_ROWS LAST_ANALYZED
—————————— ———- ———– ————- ———- —————–
TEST_IDX 1 209 3115 3115 20060513 01:04:28

FROM:http://www.dbafan.com/blog/?p=21

我们知道从oracle8i开始﹐analyze语句和dbms_stats包都可以收集相关对象(Tables﹑Indexes﹑Clusters and Materialized Views)的statistics。那哪些statistics的收集应该使用analyze语句﹐哪些statistics的收集应该使用dbms_stats包呢﹖

对于使用哪个去收集statistics﹐应该把握一个原则: 凡是与cost-based optimizer相关的statistics﹐都应通过dbms_stats包收集; 与cost-based optimizer无关的statistics(如empty blocks﹐average space等)都应通过analyze语句去收集。

之所以要用dbms_stats包去替代analyze收集优化器statistics﹐是因为dbms_stats包能收集并行statistics和分区对象的全局statistics。

当然analyze语句在其它方面的statistics收集﹐是dbms_stats所无法取代的﹐如﹕

1.收集在freelist上的blocks信息

2.检验存储格式的合法性

analyze table bk_test_t validate structure cascade online;

3.识别表或cluster的行迁移与行链接

为了能使用analyze….list chained rows语句识别行迁移与行链接﹐必须先在执行analyze语句所在的schema内执行$ORACLE_HOME/rdbms/admin/utlchain.sql(或utlchn1.sql)脚本建立chained_rows表。在chained_rows建立之后﹐就可以执行下面的语句﹕

analyze table bk_test list chained rows into chained_rows;

dbms_stats.gather_table_stats与analyze table 的区别

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