emp表有如下數據。SQL>?select?ename,deptno?from?emp; ENAME???DEPTNO ------------------------------?---------- SMITH???????20 ALLEN???????30 WARD???????30 JONES???????20 MARTIN???????30 BLAKE???????30 CLARK???????10 SCOTT???????20 KING???????10 TURNER???????30 ADAMS???????20 JAMES???????30 FORD???????20 MILLER???????10 14?rows?selected.

假設我們有如下簡單的查詢select?ename,deptno?from?emp?where?ename='RICH'?and?deptno=10;

那么Oracle在執行查詢的時候，是先比較ename字段呢？還是先比較deptno字段呢？

顯然先比較deptno再比較ename字段的效率明顯低于先比較ename，再比較deptno。 那Oracle究竟如何去判斷呢？

我們先查詢一張表SQL>?COL?COLUMN_NAME?FOR?A30 SQL>?SELECT?column_name,?num_distinct,?density ??FROM?dba_tab_columns ?WHERE?owner?=?'SCOTT'?AND?table_name?=?'EMP'; COLUMN_NAME???????NUM_DISTINCT????DENSITY ------------------------------?------------?---------- EMPNO?14?.071428571 ENAME?14?.071428571 JOB??5????.2 MGR??6?.166666667 HIREDATE?13?.076923077 SAL?12?.083333333 COMM??4???.25 DEPTNO??3?.333333333 8?rows?selected.

Oracle其實知道，你的表中存放數據的一些特征，上面語句顯示的只是鳳毛麟角。通過這些特征，Oracle優化器就能知道如何去查詢，使得執行的效率最高。

以上這些信息，我們稱之為對象的統計信息。那么如何收集統計信息呢？

一、 analyze 命令

使用analyze命令可以收集統計信息，如：收集或刪除對象的統計信息

驗證對象的結構

確定table 或cluster的migrated 和chained rows。

示例：SQL>?create?user?anal?identified?by?anal?; User?created. SQL>?grant?resource,connect?to?anal; Grant?succeeded. SQL>?grant?select?any?dictionary?to?anal; Grant?succeeded. SQL>?conn?anal/anal Connected. SQL>?create?table?t1?as?select?*?from?dba_objects; SQL>?create?table?t2?as?select?*?from?dba_objects; SQL>?create?table?t3?as?select?*?from?dba_objects; SQL>?create?table?t4?as?select?*?from??dba_objects; SQL>?create?table?t5?as?select?*?from?dba_objects; SQL>?create?table?t6?as?select?*?from?dba_objects; SQL>??create?unique?index?pk_t1_idx?on?t1(object_id); SQL>??create?unique?index?pk_t2_idx?on?t2(object_id); SQL>??create?unique?index?pk_t3_idx?on?t3(object_id); SQL>??create?unique?index?pk_t4_idx?on?t4(object_id); SQL>??create?unique?index?pk_t5_idx?on?t5(object_id); SQL>??create?unique?index?pk_t6_idx?on?t6(object_id);

我們先查看一下統計信息是否存在

查看表的統計信息SQL>?select?table_name,?num_rows,?blocks,?empty_blocks ??????from?user_tables ?????where?table_name?in?('T1',?'T2',?'T3',?'T4',?'T5','T6');

查看字段統計信息select?table_name, ???????column_name, ???????num_distinct, ???????low_value, ???????high_value, ???????density ??from?user_tab_columns ?where?table_name?in?('T1',?'T2',?'T3',?'T4','T5','T6');

查看索引統計信息SQL>?col?table_name?for?a30 SQL>?col?index_name?for?a30 SELECT?table_name, ???????index_name, ???????blevel, ???????leaf_blocks, ???????distinct_keys, ???????avg_leaf_blocks_per_key?avg_leaf_blocks, ???????avg_data_blocks_per_key?avg_data_blocks, ???????clustering_factor, ???????num_rows ??FROM?user_indexes TABLE_NAME???????INDEX_NAME??BLEVEL?LEAF_BLOCKS?DISTINCT_KEYS?AVG_LEAF_BLOCKS?AVG_DATA_BLOCKS?CLUSTERING_FACTOR???NUM_ROWS ------------------------------?------------------------------?----------?-----------?-------------?---------------?---------------?-----------------?---------- T6???????PK_T6_IDX???????1?155?????74564?1?11174??74564 T5???????PK_T5_IDX???????1?155?????74563?1?11174??74563 T4???????PK_T4_IDX???????1?155?????74562?1?11174??74562 T3???????PK_T3_IDX???????1?155?????74561?1?11174??74561 T2???????PK_T2_IDX???????1?155?????74560?1?11174??74560 T1???????PK_T1_IDX???????1?155?????74559?1?11174??74559 6?rows?selected.

表沒有任何統計數據，但是索引已經有統計信息，可見在建立表的時候會默認收集統計信息。

先將索引的統計信息刪除SQL>?analyze?table?t1?delete?statistics; analyze?table?t2?delete?statistics; analyze?table?t3?delete?statistics; analyze?table?t4?delete?statistics; analyze?table?t5?delete?statistics; analyze?table?t6?delete?statistics;

驗證索引上是否還存在統計信息SELECT?table_name, ???????index_name, ???????blevel, ???????leaf_blocks, ???????distinct_keys, ???????avg_leaf_blocks_per_key?avg_leaf_blocks, ???????avg_data_blocks_per_key?avg_data_blocks, ???????clustering_factor, ???????num_rows ??FROM?user_indexes

執行統計信息命令，并查看統計信息有無變化

analyze table t1 compute statistics for table;

–針對表收集信息，查看user_tables

analyze table t2 compute statistics for all columns;

–針對表字段收集信息，查看user_tab_columns

analyze table t3 compute statistics for all indexed columns;

–收集索引字段信息

analyze table t4 compute statistics;

–收集表，表字段，索引信息

analyze table t5 compute statistics for all indexes;

–收集索引信息

analyze table t6 compute statistics for table for all indexes for all columns;

–收集表，表字段，索引信息

二、DBMS_STATS包

Oracle推薦使用DBMS_STATS這個包來收集統計信息。這個包的功能非常多。可以收集數據庫級別、schema級別及表級別的統計信息。還可以對統計信息刪除、鎖定、導出、導入等。我們以最常用的表級別統計為例說明DBMS_STATS該如何使用。

收集的統計信存儲在dba_tab_statistics、dba_ind_statistics和dba_tab_col_statistics表中。DBMS_STATS.GATHER_TABLE_STATS?( ???ownname??????????VARCHAR2,? ???tabname??????????VARCHAR2,? ???partname?????????VARCHAR2?DEFAULT?NULL, ???estimate_percent?NUMBER???DEFAULT?to_estimate_percent_type? ????????????????????????????????????????????????(get_param('ESTIMATE_PERCENT')),? ???block_sample?????BOOLEAN??DEFAULT?FALSE, ???method_opt???????VARCHAR2?DEFAULT?get_param('METHOD_OPT'), ???degree???????????NUMBER???DEFAULT?to_degree_type(get_param('DEGREE')), ???granularity??????VARCHAR2?DEFAULT?GET_PARAM('GRANULARITY'),? ???cascade??????????BOOLEAN??DEFAULT?to_cascade_type(get_param('CASCADE')), ???stattab??????????VARCHAR2?DEFAULT?NULL,? ???statid???????????VARCHAR2?DEFAULT?NULL, ???statown??????????VARCHAR2?DEFAULT?NULL, ???no_invalidate????BOOLEAN??DEFAULT??to_no_invalidate_type?( ?????????????????????????????????????get_param('NO_INVALIDATE')), ???stattype?????????VARCHAR2?DEFAULT?'DATA', ???force????????????BOOLEAN??DEFAULT?FALSE);

參數說明如下：

導出、導入統計信息要導出統計信息首先要建立一個統計表

語法：DBMS_STATS.CREATE_STAT_TABLE?( ???ownname??VARCHAR2,? ???stattab??VARCHAR2, ???tblspace?VARCHAR2?DEFAULT?NULL);SQL>?exec?DBMS_STATS.CREATE_STAT_TABLE?(user,'STAT_TMP','SYSAUX'); PL/SQL?procedure?successfully?completed.

2. 將表t1統計信息導出DBMS_STATS.EXPORT_TABLE_STATS?( ???ownname?????????VARCHAR2,? ???tabname?????????VARCHAR2,? ???partname????????VARCHAR2?DEFAULT?NULL, ???stattab?????????VARCHAR2,? ???statid??????????VARCHAR2?DEFAULT?NULL, ???cascade?????????BOOLEAN??DEFAULT?TRUE, ???statown?????????VARCHAR2?DEFAULT?NULL, ???stat_category???VARCHAR2?DEFAULT?DEFAULT_STAT_CATEGORY);SQL>?EXEC?DBMS_STATS.EXPORT_TABLE_STATS?(ownname=>USER,tabname=>'T1',stattab=>'STAT_TMP'); PL/SQL?procedure?successfully?completed.

3. 導入統計信息

語法：DBMS_STATS.IMPORT_TABLE_STATS?( ???ownname?????????VARCHAR2,? ???tabname?????????VARCHAR2, ???partname????????VARCHAR2?DEFAULT?NULL, ???stattab?????????VARCHAR2,? ???statid??????????VARCHAR2?DEFAULT?NULL, ???cascade?????????BOOLEAN??DEFAULT?TRUE, ???statown?????????VARCHAR2?DEFAULT?NULL, ???no_invalidate???BOOLEAN?DEFAULT?to_no_invalidate_type( ????????????????????????????????????get_param('NO_INVALIDATE')), ???force???????????BOOLEAN?DEFAULT?FALSE, ???stat_category???VARCHAR2?DEFAULT?DEFAULT_STAT_CATEGORY);SQL>?exec?dbms_stats.UNlock_table_stats(user,'T1'); PL/SQL?procedure?successfully?completed. SQL>?exec?dbms_stats.delete_table_stats(user,'T1'); PL/SQL?procedure?successfully?completed. SQL>?EXEC?DBMS_STATS.IMPORT_TABLE_STATS?(ownname=>USER,tabname=>'T1',stattab=>'STAT_TMP'); PL/SQL?procedure?successfully?completed. SQL>?SELECT?table_name, ???????num_rows, ???????blocks, ???????empty_blocks, ???????avg_row_len ??FROM?user_tab_statistics;??2????3????4????5????6?? TABLE_NAME?NUM_ROWS?????BLOCKS?EMPTY_BLOCKS?AVG_ROW_LEN ------------------------------?----------?----------?------------?----------- T1????7455910880???98 T2 T3 T4 T5 T6 STAT_TMP 7?rows?selected.

如果是分區表，新的分區來不及收集統計系統，可以使用其它的分區統計信息來生成新分區的統計信息DBMS_STATS.COPY_TABLE_STATS?( ???ownname??????????VARCHAR2,? ???tabname??????????VARCHAR2,? ???srcpartname??????VARCHAR2, ???dstpartname??????VARCHAR2,? ???scale_factor?????VARCHAR2?DEFAULT?1, ???force????????????BOOLEAN?DEFAULT?FALSE);

如果表還沒有統計信息，那么在執行sql語句時，Oracle會動態的采樣表中的一部分數據，生成統計信息。

本文轉自51CTO博客，版權歸原作者所有！如果侵犯到您的版權，請及時聯系我們刪除！SQL>?show?parameter?optimizer_dynamic_sampling?; NAME?????TYPE???????VALUE ------------------------------------?---------------------------------?------------------------------ optimizer_dynamic_sampling?????integer???????2

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