sql查询的最右边的rownum和第二个右边的rownum有什么区别？或者您可以说如果我们交换rownum
select * from EMPLOYEE where FIRSTNAME = 'X' and rownum <5;
// Taking Less Time compare to next one in oracle
select * from EMPLOYEE where rownum <5 and FIRSTNAME = 'X';
结果相同，但从技术上讲有什么区别。
PS：同一个人正在接受我的采访，但他似乎不满意。我只能回答说，执行极权是第一位。
答案 0 :(得分：7)
[TL; DR]这两个查询的EXPLAIN PLAN没有区别；它们的评估结果相同。
测试数据：
CREATE TABLE employee ( firstname ) AS
SELECT CHR( 64 + level ) FROM DUAL CONNECT BY LEVEL <= 26;
说明计划：
EXPLAIN PLAN FOR
select * from EMPLOYEE where FIRSTNAME = 'X' and rownum <5;
SELECT PLAN_TABLE_OUTPUT FROM TABLE(DBMS_XPLAN.DISPLAY());
输出：
| PLAN_TABLE_OUTPUT |
| :------------------------------------------------------------------------------ |
| Plan hash value: 3463100252 |
| |
| ------------------------------------------------------------------------------- |
| | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | |
| ------------------------------------------------------------------------------- |
| | 0 | SELECT STATEMENT | | 1 | 4 | 3 (0)| 00:00:01 | |
| |* 1 | COUNT STOPKEY | | | | | | |
| |* 2 | TABLE ACCESS FULL| EMPLOYEE | 1 | 4 | 3 (0)| 00:00:01 | |
| ------------------------------------------------------------------------------- |
| |
| Predicate Information (identified by operation id): |
| --------------------------------------------------- |
| |
| 1 - filter(ROWNUM<5) |
| 2 - filter("FIRSTNAME"='X') |
| |
| Note |
| ----- |
| - dynamic sampling used for this statement (level=2) |
和
EXPLAIN PLAN FOR
select * from EMPLOYEE where rownum <5 and FIRSTNAME = 'X';
SELECT PLAN_TABLE_OUTPUT FROM TABLE(DBMS_XPLAN.DISPLAY());
输出：
| PLAN_TABLE_OUTPUT |
| :------------------------------------------------------------------------------ |
| Plan hash value: 3463100252 |
| |
| ------------------------------------------------------------------------------- |
| | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | |
| ------------------------------------------------------------------------------- |
| | 0 | SELECT STATEMENT | | 1 | 4 | 3 (0)| 00:00:01 | |
| |* 1 | COUNT STOPKEY | | | | | | |
| |* 2 | TABLE ACCESS FULL| EMPLOYEE | 1 | 4 | 3 (0)| 00:00:01 | |
| ------------------------------------------------------------------------------- |
| |
| Predicate Information (identified by operation id): |
| --------------------------------------------------- |
| |
| 1 - filter(ROWNUM<5) |
| 2 - filter("FIRSTNAME"='X') |
| |
| Note |
| ----- |
| - dynamic sampling used for this statement (level=2) |
db <>提琴here
如您所见，Oracle的SQL引擎为两个查询生成相同的计划，并将对它们进行相同的评估；与WHERE子句中的表达式顺序没有什么不同。