createman是非空字段吧。 还是count(createman)快,因为他只要找index,不用访问表既可以完成工作
SQL> explain plan for select count(*) from customer; Explained SQL> select * from table(dbms_xplan.display); PLAN_TABLE_OUTPUT -------------------------------------------------------------------------------- Plan hash value: 2982274804 ------------------------------------------------------------------------------ | Id | Operation | Name | Rows | Cost (%CPU)| Time | ------------------------------------------------------------------------------ | 0 | SELECT STATEMENT | | 1 | 10724 (2)| 00:02:09 | | 1 | SORT AGGREGATE | | 1 | | | | 2 | INDEX FAST FULL SCAN| SYS_C0011627 | 20M| 10724 (2)| 00:02:09 | ------------------------------------------------------------------------------ SQL> explain plan for select count(1) from customer; Explained SQL> select * from table(dbms_display); select * from table(dbms_display) ORA-00904: "DBMS_DISPLAY": 标识符无效 SQL> select * from table(dbms_xplan.display); PLAN_TABLE_OUTPUT -------------------------------------------------------------------------------- Plan hash value: 2982274804 ------------------------------------------------------------------------------ | Id | Operation | Name | Rows | Cost (%CPU)| Time | ------------------------------------------------------------------------------ | 0 | SELECT STATEMENT | | 1 | 10724 (2)| 00:02:09 | | 1 | SORT AGGREGATE | | 1 | | | | 2 | INDEX FAST FULL SCAN| SYS_C0011627 | 20M| 10724 (2)| 00:02:09 | ------------------------------------------------------------------------------ 9 rows selected SQL> explain plan for select count(1) from customer; Explained SQL> select * from table(dbms_display); select * from table(dbms_display) ORA-00904: "DBMS_DISPLAY": 标识符无效 SQL> select * from table(dbms_xplan.display); PLAN_TABLE_OUTPUT -------------------------------------------------------------------------------- Plan hash value: 2982274804 ------------------------------------------------------------------------------ | Id | Operation | Name | Rows | Cost (%CPU)| Time | ------------------------------------------------------------------------------ | 0 | SELECT STATEMENT | | 1 | 10724 (2)| 00:02:09 | | 1 | SORT AGGREGATE | | 1 | | | | 2 | INDEX FAST FULL SCAN| SYS_C0011627 | 20M| 10724 (2)| 00:02:09 | ------------------------------------------------------------------------------ 9 rows selected 为什么count(*) 、ount(createman)和count(createTime)的执行计划是一样的
17,086
社区成员
55,238
社区内容
加载中
试试用AI创作助手写篇文章吧