我们经常会对select结果进行对比，常用的函数如minus， 那么当两张表数据量较大时，有什么方式可以提高效率呢？ #### minus 方式 minus 执行计划如下所示， 通过排序后再进行对比 ``` sql select object_id from t2 minus select object_id from t1; .. 2255 rows selected. Elapsed: 00:00:00.93 Execution Plan ---------------------------------------------------------- Plan hash value: 1578327006 ----------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes |TempSpc| Cost (%CPU)| Time | ----------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 102K| 2357K| | 1318 (1)| 00:00:16 | | 1 | MINUS | | | | | | | | 2 | SORT UNIQUE | | 102K| 1304K| 2024K| 861 (1)| 00:00:11 | | 3 | TABLE ACCESS FULL | T2 | 102K| 1304K| | 371 (1)| 00:00:05 | | 4 | SORT UNIQUE | | 82926 | 1052K| 1640K| 457 (1)| 00:00:06 | | 5 | INDEX FAST FULL SCAN| T1_IDX | 82926 | 1052K| | 60 (0)| 00:00:01 | ----------------------------------------------------------------------------------------- Note ----- - dynamic sampling used for this statement (level=2) Statistics ---------------------------------------------------------- 9 recursive calls --递归 0 db block gets 1681 consistent gets 1798 physical reads 0 redo size 41734 bytes sent via SQL*Net to client 2173 bytes received via SQL*Net from client 152 SQL*Net roundtrips to/from client 2 sorts (memory) 0 sorts (disk) 2255 rows processed ``` #### 使用join方式替代 尤其数据量越大的情况下，join方式，不排序，走的hash算法，执行效率会更好 ``` sql alter system flush buffer cache; select t2.object_id t2_id from t1 right join t2 on t1.object_id=t2.object_id where t1.object_id is null; 2255 rows selected. Elapsed: 00:00:00.56 Execution Plan ---------------------------------------------------------- Plan hash value: 4276371593 ---------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes |TempSpc| Cost (%CPU)| Time | ---------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 102K| 2609K| | 653 (1)| 00:00:08 | |* 1 | HASH JOIN RIGHT ANTI | | 102K| 2609K| 2032K| 653 (1)| 00:00:08 | | 2 | INDEX FAST FULL SCAN| T1_IDX | 82926 | 1052K| | 60 (0)| 00:00:01 | | 3 | TABLE ACCESS FULL | T2 | 102K| 1304K| | 371 (1)| 00:00:05 | ---------------------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 1 - access("T1"."OBJECT_ID"="T2"."OBJECT_ID") Note ----- - dynamic sampling used for this statement (level=2) Statistics ---------------------------------------------------------- 0 recursive calls --没有递归调用 0 db block gets 1692 consistent gets 1534 physical reads 0 redo size 41730 bytes sent via SQL*Net to client 2173 bytes received via SQL*Net from client 152 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 2255 rows processed ``` 上述语句可以使用右连接，也可以使用左连接