dbms_stats.gather_table_stats和analyze table的一点细小差别

SQL> select * from v$version;

BANNER
----------------------------------------------------------------
Oracle9i Enterprise Edition Release 9.2.0.4.0 - 64bit Production
PL/SQL Release 9.2.0.4.0 - Production
CORE 9.2.0.3.0 Production
TNS for Solaris: Version 9.2.0.4.0 - Production
NLSRTL Version 9.2.0.4.0 - Production

SQL> create table t_test3 as select * from dba_tables;

Table created.

SQL> analyze table t_test3 compute statistics;

Table analyzed.

SQL> select avg_row_len,num_rows,blocks from user_tables where table_name = 'T_TEST3';

AVG_ROW_LEN NUM_ROWS BLOCKS
----------- ---------- ----------
188 5502 146

SQL> analyze table t_test3 delete statistics;

Table analyzed.

SQL> exec dbms_stats.gather_table_stats(user,'T_TEST3');

PL/SQL procedure successfully completed.

SQL> select avg_row_len,num_rows,blocks from user_tables where table_name = 'T_TEST3';

AVG_ROW_LEN NUM_ROWS BLOCKS
----------- ---------- ----------
183 5502 146

SQL>

默认dbms_stats.gather_table_stats分析表得到平均行长貌视少一些。

SQL> create table t_test4 as select * from t_test3;

Table created.

SQL> exec dbms_stats.gather_table_stats(user,'T_TEST4',estimate_percent=>100);

PL/SQL procedure successfully completed.

SQL> select avg_row_len,num_rows,blocks from user_tables where table_name = 'T_TEST4';

AVG_ROW_LEN NUM_ROWS BLOCKS
----------- ---------- ----------
183 5502 146

SQL>

dbms_stats.gather_table_stats的输入参数estimate_percent为100（即100%评估），分析表得到平均行长就是准确了。

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from:http://hi.baidu.com/edeed/blog/item/b8ad3901cabd5b0a7bec2ca8.html