20条记录一页,扫描第2页就需要访问40条记录。 SQL> select * from ( select * from ( select /*+ index_desc(a idx_page_3) */ a.*,rownum rn from page a where object_id >1000 and owner='SYS' order by object_id desc ) where rownum<=40 ) where rn>=21 2 3 4 ; SQL> select * from table(dbms_xplan.display_cursor(null,null,'ALLSTATS LAST')); PLAN_TABLE_OUTPUT ------------------------------------------------------------------------------------------------------------------------------------------------------------ -------------------------------------------- SQL_ID bwcbf54a6h4th, child number 1 ------------------------------------- select * from ( select * from ( select /*+ index_desc(a idx_page_3) */ a.*,rownum rn from page a where object_id >1000 and owner='SYS' order by object_id desc ) where rownum<=40 ) where rn>=21 Plan hash value: 3526010999 --------------------------------------------------------------------------------------------------------- | Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | --------------------------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | | 20 |00:00:00.01 | 10 | |* 1 | VIEW | | 1 | 40 | 20 |00:00:00.01 | 10 | |* 2 | COUNT STOPKEY | | 1 | | 40 |00:00:00.01 | 10 | | 3 | VIEW | | 1 | 30556 | 40 |00:00:00.01 | 10 | | 4 | COUNT | | 1 | | 40 |00:00:00.01 | 10 | | 5 | TABLE ACCESS BY INDEX ROWID | PAGE | 1 | 30556 | 40 |00:00:00.01 | 10 | |* 6 | INDEX RANGE SCAN DESCENDING| IDX_PAGE_3 | 1 | 30556 | 40 |00:00:00.01 | 4 | --------------------------------------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 1 - filter("RN">=21) 2 - filter(ROWNUM<=40) 6 - access("OBJECT_ID" IS NOT NULL AND "OBJECT_ID">1000 AND "OWNER"='SYS') filter("OWNER"='SYS') 28 rows selected. SQL> select * from table(dbms_xplan.display_cursor(null,null,'ALLSTATS LAST')); PLAN_TABLE_OUTPUT ------------------------------------------------------------------------------------------------------------------------------------------------------------ -------------------------------------------- SQL_ID 6gay6kkxxsqgw, child number 0 ------------------------------------- select * from ( select * from ( select /*+ index_desc(a idx_page_3) */ a.*,rownum rn from page a where object_id >1000 and owner='SYS' order by object_id desc ) where rownum<=60 ) where rn>=41 Plan hash value: 3526010999 --------------------------------------------------------------------------------------------------------- | Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | --------------------------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | | 20 |00:00:00.01 | 11 | |* 1 | VIEW | | 1 | 60 | 20 |00:00:00.01 | 11 | |* 2 | COUNT STOPKEY | | 1 | | 60 |00:00:00.01 | 11 | | 3 | VIEW | | 1 | 30556 | 60 |00:00:00.01 | 11 | | 4 | COUNT | | 1 | | 60 |00:00:00.01 | 11 | | 5 | TABLE ACCESS BY INDEX ROWID | PAGE | 1 | 30556 | 60 |00:00:00.01 | 11 | |* 6 | INDEX RANGE SCAN DESCENDING| IDX_PAGE_3 | 1 | 30556 | 60 |00:00:00.01 | 4 | --------------------------------------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 1 - filter("RN">=41) 2 - filter(ROWNUM<=60) 6 - access("OBJECT_ID" IS NOT NULL AND "OBJECT_ID">1000 AND "OWNER"='SYS') filter("OWNER"='SYS') 28 rows selected.