如何收集统计信息.doc

如何收集统计信息dbms_stats.gather_table_stats与analyze table 的区别转贴 - 参考 /viewthread.php?tid=845777&extra=&page=1Analyze StatementThe 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_UTILITYThe 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_STATSThe 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);-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dbms_stats可以并行分析dbms_stats有自动分析的功能(alter table monitor )analyze 分析统计信息的不准确some times1,2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Tableb) 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 Structureb) 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True3、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_STATSanalyze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 clausesTo 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:beginfor owner in (select username from dba_users where username not in (SYS,SYSTEM)loopdbms_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_st