result_cache
oracle在sga的shared_pool内存中专门开辟了一个result cache内存结构，当第一个会话查询时会先从磁盘取数据，同时将结果放在shared_pool中的result cache中，第二个回话相同的sql查询时会直接从result cache中读取数据，极大的提升了查询的性能
测试：
--不使用result_cache两次查询
SQL> alter system flush shared_pool; System altered. SQL> alter system flush buffer_cache; System altered. SQL> select deptno, avg(sal) from emp group by deptno; DEPTNO AVG(SAL) ---------- ---------- 30 1566.66667 20 2175 10 2916.66667 Execution Plan ---------------------------------------------------------- Plan hash value: 4067220884 --------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | --------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 3 | 21 | 4 (25)| 00:00:01 | | 1 | HASH GROUP BY | | 3 | 21 | 4 (25)| 00:00:01 | | 2 | TABLE ACCESS FULL| EMP | 14 | 98 | 3 (0)| 00:00:01 | --------------------------------------------------------------------------- Statistics ---------------------------------------------------------- 11 recursive calls 0 db block gets 14 consistent gets 3 physical reads 0 redo size 704 bytes sent via SQL*Net to client 523 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 1 sorts (memory) 0 sorts (disk) 3 rows processed SQL> select deptno, avg(sal) from emp group by deptno; DEPTNO AVG(SAL) ---------- ---------- 30 1566.66667 20 2175 10 2916.66667 Execution Plan ---------------------------------------------------------- Plan hash value: 4067220884 --------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | --------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 3 | 21 | 4 (25)| 00:00:01 | | 1 | HASH GROUP BY | | 3 | 21 | 4 (25)| 00:00:01 | | 2 | TABLE ACCESS FULL| EMP | 14 | 98 | 3 (0)| 00:00:01 | --------------------------------------------------------------------------- Statistics ---------------------------------------------------------- 0 recursive calls 0 db block gets 2 consistent gets 0 physical reads 0 redo size 704 bytes sent via SQL*Net to client 523 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 3 rows processed
--使用result_cache两次查询结果
SQL> alter system flush shared_pool; System altered. SQL> alter system flush buffer_cache; System altered. SQL> select /*+ result_cache */ deptno, avg(sal) from emp group by deptno; SQL> select /*+ result_cache */ deptno, avg(sal) from emp 2 group by deptno; DEPTNO AVG(SAL) ---------- ---------- 30 1566.66667 20 2175 10 2916.66667 Execution Plan ---------------------------------------------------------- Plan hash value: 4067220884 -------------------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | -------------------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 3 | 21 | 4 (25)| 00:00:01 | | 1 | RESULT CACHE | 69cmubna179r6gh0sd1y59kbqz | | | | | | 2 | HASH GROUP BY | | 3 | 21 | 4 (25)| 00:00:01 | | 3 | TABLE ACCESS FULL| EMP | 14 | 98 | 3 (0)| 00:00:01 | -------------------------------------------------------------------------------------------------- Result Cache Information (identified by operation id): ------------------------------------------------------ 1 - column-count=2; dependencies=(SCOTT.EMP); name="select /*+ result_cache */ deptno, avg(sal) from emp group by deptno" Statistics ---------------------------------------------------------- 64 recursive calls 0 db block gets 53 consistent gets 14 physical reads 0 redo size 704 bytes sent via SQL*Net to client 523 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 7 sorts (memory) 0 sorts (disk) 3 rows processed SQL> select /*+ result_cache */ deptno, avg(sal) from emp 2 group by deptno; DEPTNO AVG(SAL) ---------- ---------- 30 1566.66667 20 2175 10 2916.66667 Execution Plan ---------------------------------------------------------- Plan hash value: 4067220884 -------------------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | -------------------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 3 | 21 | 4 (25)| 00:00:01 | | 1 | RESULT CACHE | 69cmubna179r6gh0sd1y59kbqz | | | | | | 2 | HASH GROUP BY | | 3 | 21 | 4 (25)| 00:00:01 | | 3 | TABLE ACCESS FULL| EMP | 14 | 98 | 3 (0)| 00:00:01 | -------------------------------------------------------------------------------------------------- Result Cache Information (identified by operation id): ------------------------------------------------------ 1 - column-count=2; dependencies=(SCOTT.EMP); name="select /*+ result_cache */ deptno, avg(sal) from emp group by deptno" Statistics ---------------------------------------------------------- 0 recursive calls 0 db block gets 0 consistent gets 0 physical reads 0 redo size 704 bytes sent via SQL*Net to client 523 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 3 rows processed
可见在使用result_cache第二次查询根本没有访问buffer_cahce，执行性能非常高
适用场景：
查询记录很多，但返回的记录较少，并且重复查询的次数比较高，表的数据相对静态，变化不到
开启result_cache参数
NAME TYPE VALUE ------------------------------------ ----------- ------------------------------ client_result_cache_lag big integer 3000 client_result_cache_size big integer 0 result_cache_max_result integer 5 result_cache_max_size big integer 2624K result_cache_mode string MANUAL result_cache_remote_expiration integer 0
result_cache_mode Define using or not using result_cache
MANUAL 当sql语句有相关hint result_cache的时候使用此技术
FORCE 只要有可能，所有的语句都将使用result_cache
无论result_cache_mode如何测试，oracle将优先考虑sql语句中hint关于result_cache or no_result_cache的提示
默认情况下oracle result_cache_mode 为manual，一般不要把其设置为force，如果为force所有的查询将考虑result_cache 会增加不必要的开销。
时间： 2024-10-15 17:47:40