http://www.itpub.net/thread-1340290-1-1.html
“回表”一般就是指执行计划里显示的“TABLE ACCESS BY INDEX ROWID”。 例如select的字段里有索引不包含的列， SQL> select ename from emp where empno =1; Execution Plan ---------------------------------------------------------- Plan hash value: 4066871323 ---------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost | ---------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | 8 | 1 | | 1 | TABLE ACCESS BY INDEX ROWID | EMP | 1 | 8 | 1 | |* 2 | INDEX UNIQUE SCAN | PK_EMP | 1 | | 1 | ---------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 2 - access("EMPNO"=1) 再例如，虽然只查询索引里的列，但是需要回表过滤掉其他行。 SQL> select empno from emp where empno =1 and ename='hao'; Execution Plan ---------------------------------------------------------- Plan hash value: 4066871323 ---------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost | ---------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | 8 | 1 | |* 1 | TABLE ACCESS BY INDEX ROWID| EMP | 1 | 8 | 1 | |* 2 | INDEX UNIQUE SCAN | PK_EMP | 1 | | 1 | ---------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 1 - filter("ENAME"='hao') 2 - access("EMPNO"=1)
