今天看到一条SQL大量使用了with，开发人员为了逻辑清晰，把一些结果集先用with缓存起来，后面还有很多地方用到这个结果集，原始的SQL需要执行2个多小时。优化方法是将把最先缓存的SQL放到用的地方,优化后12s。
下面来模拟这个场景，不用纠结SQL的意义，把当时的SQL抽象就是这样的。可以看到SQL1中先把语句a中的结果缓存起来，当语句b要用的时候，object_id上的索引是用不到的，它只有在结果集中过滤。简单点说，SQL1相对于SQL2来说，谓词没有推进。所以在SQL的写法上，追求书面上的清晰和性能要做一个平衡。
--制造数据
SQL> drop table test purge;
SQL> create table test as select * from dba_objects;
SQL> create index ind_t_object_id on test(object_id) nologging;
SQL> exec dbms_stats.gather_table_stats(user,'test',cascade => true);
SQL> set autotrace traceonly
--SQL1，优化前没有谓词推进
SQL> with a as(select * from test where object_type='TABLE'),
? ? b as (select count(1) from a where object_id<10),
? ? c as (select count(1) from a where object_id>=10 and object_id<20)
? ? select (select * from b) bc,
? ? ? ? ? (select * from c) cc
? ? from dual;
执行计划
----------------------------------------------------------
Plan hash value: 2659770981
----------------------------------------------------------------------------------------------
| Id? | Operation? ? ? ? ? ? ? ? ? | Name? ? ? ? ? ? | Rows? | Bytes | Cost (%CPU)| Time? |
----------------------------------------------------------------------------------------------
|? 0 | SELECT STATEMENT? ? ? ? ? |? ? ? ? ? ? ? ? |? ? 1 |? ? ? |? 198? (1)| 00:00:03 |
|? 1 |? VIEW? ? ? ? ? ? ? ? ? ? ? |? ? ? ? ? ? ? ? |? ? 1 |? ? 13 |? ? 6? (0)| 00:00:01 |
|? 2 |? SORT AGGREGATE? ? ? ? ? |? ? ? ? ? ? ? ? |? ? 1 |? ? 13 |? ? ? ? ? ? |? ? ? |
|*? 3 |? ? VIEW? ? ? ? ? ? ? ? ? ? |? ? ? ? ? ? ? ? |? 1600 | 20800 |? ? 6? (0)| 00:00:01 |
|? 4 |? ? TABLE ACCESS FULL? ? ? | SYS_TEMP_0FD9D66|? 1600 |? 154K|? ? 6? (0)| 00:00:01 |
|? 5 |? VIEW? ? ? ? ? ? ? ? ? ? ? |? ? ? ? ? ? ? ? |? ? 1 |? ? 13 |? ? 6? (0)| 00:00:01 |
|? 6 |? SORT AGGREGATE? ? ? ? ? |? ? ? ? ? ? ? ? |? ? 1 |? ? 13 |? ? ? ? ? ? |? ? ? |
|*? 7 |? ? VIEW? ? ? ? ? ? ? ? ? ? |? ? ? ? ? ? ? ? |? 1600 | 20800 |? ? 6? (0)| 00:00:01 |
|? 8 |? ? TABLE ACCESS FULL? ? ? | SYS_TEMP_0FD9D66|? 1600 |? 154K|? ? 6? (0)| 00:00:01 |
|? 9 |? TEMP TABLE TRANSFORMATION |? ? ? ? ? ? ? ? |? ? ? |? ? ? |? ? ? ? ? ? |? ? ? |
|? 10 |? LOAD AS SELECT? ? ? ? ? | SYS_TEMP_0FD9D66|? ? ? |? ? ? |? ? ? ? ? ? |? ? ? |
|* 11 |? ? TABLE ACCESS FULL? ? ? | TEST? ? ? ? ? ? |? 1600 |? 154K|? 196? (1)| 00:00:03 |
|? 12 |? FAST DUAL? ? ? ? ? ? ? ? |? ? ? ? ? ? ? ? |? ? 1 |? ? ? |? ? 2? (0)| 00:00:01 |
----------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
? 3 - filter("OBJECT_ID"<10)
? 7 - filter("OBJECT_ID">=10 AND "OBJECT_ID"<20)
? 11 - filter("OBJECT_TYPE"='TABLE')
统计信息
----------------------------------------------------------
? ? ? ? 394? recursive calls
? ? ? ? 22? db block gets
? ? ? 589? consistent gets
? ? ? ? 15? physical reads
? ? ? ? 600? redo size
? ? ? ? 381? bytes sent via SQL*Net to client
? ? ? ? 337? bytes received via SQL*Net from client
? ? ? ? ? 2? SQL*Net roundtrips to/from client
? ? ? ? ? 0? sorts (memory)
? ? ? ? ? 0? sorts (disk)
? ? ? ? ? 1? rows processed
--SQL2，优化后
SQL> with b as (select count(1) from test where object_id<10),
? ? c as (select count(1) from test where object_id>=10 and object_id<20)
? ? select (select * from b) bc,
? ? ? ? ? (select * from c) cc
? ? from dual;
执行计划
----------------------------------------------------------
Plan hash value: 1155001961
--------------------------------------------------------------------------------------
| Id? | Operation? ? ? ? ? | Name? ? ? ? ? ? | Rows? | Bytes | Cost (%CPU)| Time? ? |
--------------------------------------------------------------------------------------
|? 0 | SELECT STATEMENT? |? ? ? ? ? ? ? ? |? ? 1 |? ? ? |? ? 2? (0)| 00:00:01 |
|? 1 |? VIEW? ? ? ? ? ? ? |?