dbms_stats和analyze 评估AVG_ROW_LEN

随着Oracle版本的不断更新，我们推荐使用dbms_stats来代替analyze获得对象的统计信息。本文主要讨论dbms_stats和analyze对AVG_ROW_LEN取值的异同
创建测试表格

引用

SQL> create table testt1 as select * from dba_objects;
SQL> insert into testt1 select * from testt1;
SQL> commit;
SQL> insert into testt1 select * from testt1;
SQL> commit;
SQL> select count(*) from testt1;
    181860

用dbms_stats收集表格信息，可以发现AVG_ROW_LEN*NUM_ROWS小于表格实际物理大小

引用

SQL> exec dbms_stats.gather_table_stats('ZHOU','TESTT1');


SQL> select AVG_ROW_LEN,NUM_ROWS from dba_tables where table_name='TESTT1';
         94     181860
SQL> select bytes from dba_segments where segment_name='TESTT1';
  20971520
SQL> select 94*181860 from dual;
  17094840

用analyze命令收集表格信息，我们发现AVG_ROW_LEN*NUM_ROWS比dbms_stats更接近于实际物理大小， 不过这差别并不会对CBO构成致命影响

引用

SQL> analyze table TESTT1 compute statistics;
SQL> select AVG_ROW_LEN,NUM_ROWS from dba_tables where table_name='TESTT1';
         97     181860
SQL> select 97*181860 from dual;
  17640420