<pre name="code" class="html">SQL> select * from t1 where id>=1 and id<=20; ID A1 A2 A3 ---------- ---------- ---------- ---------- 1 1 1 a1 2 2 2 a2 3 3 3 a3 4 4 4 a4 5 5 5 a5 6 6 6 a6 7 7 7 a7 8 8 8 a8 9 9 9 a9 10 10 10 a10 11 11 11 a11 12 12 12 a12 13 13 13 a13 14 14 14 a14 15 15 15 a15 16 16 16 a16 17 17 17 a17 18 18 18 a18 19 19 19 a19 20 20 20 a20 已选择20行。 SQL> explain plan for select id from t1 where rownum<20; 已解释。 SQL> select * from table(dbms_xplan.display()); PLAN_TABLE_OUTPUT -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- Plan hash value: 3581814200 --------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | --------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 19 | 209 | 1 (0)| 00:00:01 | |* 1 | COUNT STOPKEY | | | | | | | 2 | INDEX FULL SCAN| SYS_C0022200 | 19 | 209 | 1 (0)| 00:00:01 | --------------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 1 - filter(ROWNUM<20) 已选择14行。 INDEX FULL SCAN 返回数据有序: SQL> select id from t1 where rownum<20; ID ---------- 1 10 100 1000 10000 1001 1002 1003 1004 1005 1006 1007 1008 1009 101 1010 1011 1012 1013 已选择19行。 不走索引,随机读: SQL> select * from table(dbms_xplan.display()); PLAN_TABLE_OUTPUT -------------------------------------------------------------------------------- Plan hash value: 3836375644 --------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | --------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 19 | 836 | 2 (0)| 00:00:01 | |* 1 | COUNT STOPKEY | | | | | | | 2 | TABLE ACCESS FULL| T1 | 19 | 836 | 2 (0)| 00:00:01 | --------------------------------------------------------------------------- Predicate Information (identified by operation id): PLAN_TABLE_OUTPUT -------------------------------------------------------------------------------- --------------------------------------------------- 1 - filter(ROWNUM<20) 已选择14行。 SQL> select * from t1 where rownum<20; ID A1 A2 A3 ---------- ---------- ---------- ---------- 495 495 495 a495 496 496 496 a496 497 497 497 a497 498 498 498 a498 499 499 499 a499 500 500 500 a500 501 501 501 a501 502 502 502 a502 503 503 503 a503 504 504 504 a504 505 505 505 a505 ID A1 A2 A3 ---------- ---------- ---------- ---------- 506 506 506 a506 507 507 507 a507 508 508 508 a508 509 509 509 a509 510 510 510 a510 511 511 511 a511 512 512 512 a512 513 513 513 a513 已选择19行 排序规则: SQL> select id from t1 where rownum<20; ID ---------- 1 10 100 1000 10000 1001 1002 1003 1004 1005 1006 1007 1008 1009 101 1010 1011 1012 1013 已选择19行。 SQL> select * from ( select id from t1 where rownum<20) order by id; ID ---------- 1 10 100 1000 10000 1001 1002 1003 1004 1005 1006 1007 1008 1009 101 1010 1011 1012 1013 已选择19行。 SQL> select * from (select * from t1 order by id ) where rownum<20; ID A1 A2 A3 ---------- ---------- ---------- ---------- 1 1 1 a1 10 10 10 a10 100 100 100 a100 1000 1000 1000 a1000 10000 10000 10000 a10000 1001 1001 1001 a1001 1002 1002 1002 a1002 1003 1003 1003 a1003 1004 1004 1004 a1004 1005 1005 1005 a1005 1006 1006 1006 a1006 1007 1007 1007 a1007 1008 1008 1008 a1008 1009 1009 1009 a1009 101 101 101 a101 1010 1010 1010 a1010 1011 1011 1011 a1011 1012 1012 1012 a1012 1013 1013 1013 a1013 已选择19行。 SQL> select * from (select * from t1 where id<100 order by id) where rownum<20; ID A1 A2 A3 ---------- ---------- ---------- ---------- 1 1 1 a1 10 10 10 a10 11 11 11 a11 12 12 12 a12 13 13 13 a13 14 14 14 a14 15 15 15 a15 16 16 16 a16 17 17 17 a17 18 18 18 a18 19 19 19 a19 2 2 2 a2 20 20 20 a20 21 21 21 a21 22 22 22 a22 23 23 23 a23 24 24 24 a24 25 25 25 a25 26 26 26 a26 已选择19行。