【IT168 技术文档】这篇文章介绍11g新增功能缓存结果集中SQL语句的缓存。SQL语句的缓存结果集功能是Oracle11g中最吸引我的一个新功能。
DB_BUFFER只能缓存访问过的BLOCK，部分解决了物理读的问题，查询仍然需要大量的逻辑读。物化视图提供的是查询重写的功能，对于大部分的情况，只是简化了SQL的复杂度，即使是完全满足查询的条件，物化视图的扫描也是不可避免的。而缓存结果集——RESULT CACHE则截然不同，它缓存的是查询的结果。不在需要大量的逻辑读，不在需要任何的复杂计算，而是直接将已经缓存的结果返回。
Oracle新增了两个HINT，RESULT_CACHE和NO_RESULT_CACHE。通过这两个提示，可以明确的指出下面的语句是否进行RESULT CACHE。Oracle还增加了几个初始化参数来管理RESULT CACHE功能，如：RESULT_CACHE_MODE、RESULT_CACHE_MAX_SIZE等。RESULT_CACHE_MAX_SIZE指明SGA中RESULT CACHE功能可以使用的最大的内存容量。如果这个参数设置为0，则关闭RESULT CACHE功能。RESULT_CACHE_MODE参数设置Oracle如何使用RESULT CACHE，该参数有三个值：MANUAL、AUTO、FORCE。后面会通过几个例子来说明三种情况的区别。
Oracle提供了DBMS_RESULT_CACHE包来管理和维护RESULT CACHE。Oracle还新增了几个关于RESULT CACHE的系统视图，用户可以看到和RESULT CACHE相关的各种信息，视图包括：V$RESULT_CACHE_DEPENDENCY、V$RESULT_CACHE_MEMORY、V$RESULT_CACHE_OBJECTS和V$RESULT_CACHE_STATISTICS等。
首先先看一下RESULT_CACHE所带来的性能提升：
SQL> create table t as select * from dba_objects; 表已创建。 SQL> set autot on SQL> set timing on SQL> select count(*) from t; COUNT(*) ---------- 68324 已用时间: 00: 00: 00.12 执行计划 ---------------------------------------------------------- Plan hash value: 2966233522 ------------------------------------------------------------------- | Id | Operation | Name | Rows | Cost (%CPU)| Time | ------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | 282 (1)| 00:00:04 | | 1 | SORT AGGREGATE | | 1 | | | | 2 | TABLE ACCESS FULL| T | 52976 | 282 (1)| 00:00:04 | ------------------------------------------------------------------- Note ----- - dynamic sampling used for this statement 统计信息 ---------------------------------------------------------- 28 recursive calls 0 db block gets 1066 consistent gets 1006 physical reads 0 redo size 338 bytes sent via SQL*Net to client 377 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 1 rows processed SQL> select /*+ result_cache */ count(*) from t; COUNT(*) ---------- 68324 已用时间: 00: 00: 00.21 执行计划 ---------------------------------------------------------- Plan hash value: 2966233522 ------------------------------------------------------------------------------------------ | Id | Operation | Name | Rows | Cost (%CPU)| Time | ------------------------------------------------------------------------------------------ | 0 | SELECT STATEMENT | | 1 | 282 (1)| 00:00:04 | | 1 | RESULT CACHE | 1t707xy2juqfq1xgx9dvy4bcqj | | | | | 2 | SORT AGGREGATE | | 1 | | | | 3 | TABLE ACCESS FULL| T | 52976 | 282 (1)| 00:00:04 | ------------------------------------------------------------------------------------------ Result Cache Information (identified by operation id): ------------------------------------------------------ 1 - column-count=1; dependencies=(YANGTK.T); attributes=(single-row); name="select /*+ result_cac t" Note ----- - dynamic sampling used for this statement 统计信息 ---------------------------------------------------------- 4 recursive calls 0 db block gets 1064 consistent gets 0 physical reads 0 redo size 338 bytes sent via SQL*Net to client 377 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 1 rows processed SQL> select /*+ result_cache */ count(*) from t; COUNT(*) ---------- 68324 已用时间: 00: 00: 00.00 执行计划 ---------------------------------------------------------- Plan hash value: 2966233522 ------------------------------------------------------------------------------------------ | Id | Operation | Name | Rows | Cost (%CPU)| Time | ------------------------------------------------------------------------------------------ | 0 | SELECT STATEMENT | | 1 | 282 (1)| 00:00:04 | | 1 | RESULT CACHE | 1t707xy2juqfq1xgx9dvy4bcqj | | | | | 2 | SORT AGGREGATE | | 1 | | | | 3 | TABLE ACCESS FULL| T | 52976 | 282 (1)| 00:00:04 | ------------------------------------------------------------------------------------------ Result Cache Information (identified by operation id): ------------------------------------------------------ 1 - column-count=1; dependencies=(YANGTK.T); attributes=(single-row); name="select /*+ result_cac t" Note ----- - dynamic sampling used for this statement 统计信息 ---------------------------------------------------------- 0 recursive calls 0 db block gets 0 consistent gets 0 physical reads 0 redo size 338 bytes sent via SQL*Net to client 377 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 1 rows processed
由于第二次执行SQL语句的时候使用了RESULT_CACHE提示，Oracle将这个查询的结果集记录在共享内存中，当第三次查询执行的时候，不需要进行任何的处理，而直接读取结果集，几乎没有任何的消耗，逻辑读为0，运行时间也几乎为0。
下面简单说明一下RESULT_CACHE_MODE的三种模式下，缓存结果集是如何工作的。
当参数值设置为MANUAL时，只有通过HINT明确提示的SQL才会读取缓存结果集。如果不加提示，那么Oracle不会利用已经缓存的结果。
SQL> alter session set result_cache_mode = manual; 会话已更改。 SQL> select /*+ result_cache */ object_name from t where object_name = 'DUAL'; OBJECT_NAME ----------------------------------------------------------- DUAL DUAL 执行计划 ---------------------------------------------------------- Plan hash value: 1601196873 ------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes |Cost (%CPU)| Time | -------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 11 | 726 | 282 (1)| 00:00:04 | | 1 | RESULT CACHE |fhp10hrt50095fufmw3s75txcx| | | | | |* 2 | TABLE ACCESS FULL| T | 11 | 726 | 282 (1)| 00:00:04 | --------------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 2 - filter("OBJECT_NAME"='DUAL') Result Cache Information (identified by operation id): ------------------------------------------------------ 1 - column-count=1; dependencies=(YANGTK.T); parameters=(nls); name="select /*+ result_cache */ o ere object_name = 'DUAL'" Note ----- - dynamic sampling used for this statement 统计信息 ---------------------------------------------------------- 28 recursive calls 0 db block gets 1067 consistent gets 0 physical reads 0 redo size 366 bytes sent via SQL*Net to client 377 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 2 rows processed SQL> select /*+ result_cache */ object_name from t where object_name = 'DUAL'; OBJECT_NAME ------------------------------------------------------------ DUAL DUAL 执行计划 ---------------------------------------------------------- Plan hash value: 1601196873 ------------------------------------------------------------------------------------------ | Id | Operation | Name | Rows | Bytes |Cost (%CPU)| Time | ------------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 11 | 726 | 282 (1)| 00:00:04 | | 1 | RESULT CACHE | fhp10hrt50095fufmw3s75txcx| | | | | |* 2 | TABLE ACCESS FULL| T | 11 | 726 | 282 (1)| 00:00:04 | ------------------------------------------------------------------------------------------- redicate Information (identified by operation id): --------------------------------------------------- 2 - filter("OBJECT_NAME"='DUAL') Result Cache Information (identified by operation id): ------------------------------------------------------ 1 - column-count=1; dependencies=(YANGTK.T); parameters=(nls); name="select /*+ result_cache */ o ere object_name = 'DUAL'" Note ----- - dynamic sampling used for this statement 统计信息 ---------------------------------------------------------- 0 recursive calls 0 db block gets 0 consistent gets 0 physical reads 0 redo size 366 bytes sent via SQL*Net to client 377 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 2 rows processed SQL> select object_name from t where object_name = 'DUAL'; OBJECT_NAME ----------------------------------------------------------- DUAL DUAL 执行计划 ---------------------------------------------------------- Plan hash value: 1601196873 -------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | -------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 11 | 726 | 282 (1)| 00:00:04 | |* 1 | TABLE ACCESS FULL| T | 11 | 726 | 282 (1)| 00:00:04 | -------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 1 - filter("OBJECT_NAME"='DUAL') Note ----- - dynamic sampling used for this statement 统计信息 ---------------------------------------------------------- 4 recursive calls 0 db block gets 1065 consistent gets 0 physical reads 0 redo size 366 bytes sent via SQL*Net to client 377 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 2 rows processed
当设置为MANUAL时，只有使用HINT的情况下，Oracle才会利用缓存结果集。而对于AUTO模式，Oracle如果发现缓冲结果集已经存在，那么就会使用。但是如果缓冲结果集不存在，Oracle并不会自动进行缓冲。只有使用HINT的情况下，Oracle才会将执行的结果集缓存。
SQL> alter session set result_cache_mode = auto; 会话已更改。 SQL> select object_name from t where object_name = 'DUAL'; OBJECT_NAME ------------------------------ DUAL DUAL 执行计划 ---------------------------------------------------------- Plan hash value: 1601196873 ------------------------------------------------------------------------------------------ | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | ----------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 11 | 726 | 282 (1)| 00:00:04 | | 1 | RESULT CACHE |fhp10hrt50095fufmw3s75txcx| | | | | |* 2 | TABLE ACCESS FULL| T | 11 | 726 | 282 (1)| 00:00:04 | ------------------------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 2 - filter("OBJECT_NAME"='DUAL') Result Cache Information (identified by operation id): ------------------------------------------------------ 1 - column-count=1; type=AUTO; dependencies=(YANGTK.T); parameters=(nls); name="select object_nam t_name = 'DUAL'" Note ----- - dynamic sampling used for this statement 统计信息 ---------------------------------------------------------- 5 recursive calls 0 db block gets 53 consistent gets 0 physical reads 0 redo size 366 bytes sent via SQL*Net to client 377 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 2 rows processed SQL> select object_name from t where object_name = 'DUAL'; OBJECT_NAME ------------------------------ DUAL DUAL 执行计划 ---------------------------------------------------------- Plan hash value: 1601196873 ------------------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | -----------------------------------------------------------------------------------| 0 | SELECT STATEMENT | | 11 | 726 | 282 (1)| 00:00:04 | | 1 | RESULT CACHE | fhp10hrt50095fufmw3s75txcx | | | | | |* 2 | TABLE ACCESS FULL| T | 11 | 726 | 282 (1)| 00:00:04 | ----------------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 2 - filter("OBJECT_NAME"='DUAL') Result Cache Information (identified by operation id): ------------------------------------------------------ 1 - column-count=1; type=AUTO; dependencies=(YANGTK.T); parameters=(nls); name="select object_nam t_name = 'DUAL'" Note ----- - dynamic sampling used for this statement 统计信息 ---------------------------------------------------------- 0 recursive calls 0 db block gets 0 consistent gets 0 physical reads 0 redo size 366 bytes sent via SQL*Net to client 377 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 2 rows processed SQL> select object_name from t where object_name = 'DBA_TABLES'; OBJECT_NAME ------------------------------ DBA_TABLES DBA_TABLES 执行计划 ---------------------------------------------------------- Plan hash value: 1601196873 ----------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | ----------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 11 | 726 | 282 (1)| 00:00:04 | | 1 | RESULT CACHE | 14buvcmfum6h9622vpkxm7sj2w| | | | | |* 2 | TABLE ACCESS FULL| T | 11 | 726 | 282 (1)| 00:00:04 | ----------------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 2 - filter("OBJECT_NAME"='DBA_TABLES') Result Cache Information (identified by operation id): ------------------------------------------------------ 1 - column-count=1; type=AUTO; dependencies=(YANGTK.T); parameters=(nls); name="select object_nam t_name = 'DBA_TABLES'" Note ----- - dynamic sampling used for this statement 统计信息 ---------------------------------------------------------- 4 recursive calls 0 db block gets 1065 consistent gets 0 physical reads 0 redo size 372 bytes sent via SQL*Net to client 377 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 2 rows processed SQL> select object_name from t where object_name = 'DBA_TABLES'; OBJECT_NAME ------------------------------ DBA_TABLES DBA_TABLES 执行计划 ---------------------------------------------------------- Plan hash value: 1601196873 ------------------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | ------------------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 11 | 726 | 282 (1)| 00:00:04 | | 1 | RESULT CACHE | 14buvcmfum6h9622vpkxm7sj2w | | | | | |* 2 | TABLE ACCESS FULL| T | 11 | 726 | 282 (1)| 00:00:04 | ------------------------------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 2 - filter("OBJECT_NAME"='DBA_TABLES') Result Cache Information (identified by operation id): ------------------------------------------------------ 1 - column-count=1; type=AUTO; dependencies=(YANGTK.T); parameters=(nls); name="select object_nam t_name = 'DBA_TABLES'" Note ----- - dynamic sampling used for this statement 统计信息 ---------------------------------------------------------- 0 recursive calls 0 db block gets 1012 consistent gets 0 physical reads 0 redo size 372 bytes sent via SQL*Net to client 377 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 2 rows processed
对比上面两种情况，由于第一个SQL的缓存结果集已经存在，Oracle采用了缓存结果集，而对于第二个SQL，Oracle是不会自动使用缓存结果集的。
SQL> select /*+ result_cache */ object_name from t where object_name = 'DBA_TABLES'; OBJECT_NAME ------------------------------ DBA_TABLES DBA_TABLES 执行计划 ---------------------------------------------------------- Plan hash value: 1601196873 ------------------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | ------------------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 11 | 726 | 282 (1)| 00:00:04 | | 1 | RESULT CACHE | 14buvcmfum6h9622vpkxm7sj2w | | | | | |* 2 | TABLE ACCESS FULL| T | 11 | 726 | 282 (1)| 00:00:04 | ------------------------------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 2 - filter("OBJECT_NAME"='DBA_TABLES') Result Cache Information (identified by operation id): ------------------------------------------------------ 1 - column-count=1; dependencies=(YANGTK.T); parameters=(nls); name="select /*+ result_cache */ o ere object_name = 'DBA_TABLES'" Note ----- - dynamic sampling used for this statement 统计信息 ---------------------------------------------------------- 4 recursive calls 0 db block gets 1065 consistent gets 0 physical reads 0 redo size 372 bytes sent via SQL*Net to client 377 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 2 rows processed SQL> select object_name from t where object_name = 'DBA_TABLES'; OBJECT_NAME ------------------------------ DBA_TABLES DBA_TABLES 执行计划 ---------------------------------------------------------- Plan hash value: 1601196873 ------------------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | ------------------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 11 | 726 | 282 (1)| 00:00:04 | | 1 | RESULT CACHE | 14buvcmfum6h9622vpkxm7sj2w | | | | | |* 2 | TABLE ACCESS FULL| T | 11 | 726 | 282 (1)| 00:00:04 | ------------------------------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 2 - filter("OBJECT_NAME"='DBA_TABLES') Result Cache Information (identified by operation id): ------------------------------------------------------ 1 - column-count=1; type=AUTO; dependencies=(YANGTK.T); parameters=(nls); name="select object_nam t_name = 'DBA_TABLES'" Note ----- - dynamic sampling used for this statement 统计信息 ---------------------------------------------------------- 0 recursive calls 0 db block gets 0 consistent gets 0 physical reads 0 redo size 372 bytes sent via SQL*Net to client 377 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 2 rows processed
当使用HINT后，结果集被缓存，随后的相同的查询就会使用缓存的结果集。
对于FORCE参数，顾名思义，就是会对所有的SQL进行缓存，除非明确使用NO_RESULT_CACHE提示：
SQL> alter session set result_cache_mode = force; 会话已更改。 SQL> select object_name from t where object_name = 'DBA_VIEWS'; OBJECT_NAME ------------------------------ DBA_VIEWS DBA_VIEWS 执行计划 ---------------------------------------------------------- Plan hash value: 1601196873 ------------------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | ------------------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 11 | 726 | 282 (1)| 00:00:04 | | 1 | RESULT CACHE | bdkgz2jxpb7tc8pkwn478qt3p0 | | | | | |* 2 | TABLE ACCESS FULL| T | 11 | 726 | 282 (1)| 00:00:04 | ------------------------------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 2 - filter("OBJECT_NAME"='DBA_VIEWS') Result Cache Information (identified by operation id): ------------------------------------------------------ 1 - column-count=1; dependencies=(YANGTK.T); parameters=(nls); name="select object_name from t wh BA_VIEWS'" Note ----- - dynamic sampling used for this statement 统计信息 ---------------------------------------------------------- 5 recursive calls 0 db block gets 1065 consistent gets 0 physical reads 0 redo size 371 bytes sent via SQL*Net to client 377 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 2 rows processed SQL> select object_name from t where object_name = 'DBA_VIEWS'; OBJECT_NAME ------------------------------ DBA_VIEWS DBA_VIEWS 执行计划 ---------------------------------------------------------- Plan hash value: 1601196873 ------------------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | ------------------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 11 | 726 | 282 (1)| 00:00:04 | | 1 | RESULT CACHE | bdkgz2jxpb7tc8pkwn478qt3p0 | | | | | |* 2 | TABLE ACCESS FULL| T | 11 | 726 | 282 (1)| 00:00:04 | ------------------------------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 2 - filter("OBJECT_NAME"='DBA_VIEWS') Result Cache Information (identified by operation id): ------------------------------------------------------ 1 - column-count=1; dependencies=(YANGTK.T); parameters=(nls); name="select object_name from t wh BA_VIEWS'" Note ----- - dynamic sampling used for this statement 统计信息 ---------------------------------------------------------- 0 recursive calls 0 db block gets 0 consistent gets 0 physical reads 0 redo size 371 bytes sent via SQL*Net to client 377 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 2 rows processed SQL> select /*+ no_result_cache */ object_name from t where object_name = 'DBA_VIEWS'; OBJECT_NAME ------------------------------ DBA_VIEWS DBA_VIEWS 执行计划 ---------------------------------------------------------- Plan hash value: 1601196873 -------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | -------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 11 | 726 | 282 (1)| 00:00:04 | |* 1 | TABLE ACCESS FULL| T | 11 | 726 | 282 (1)| 00:00:04 | -------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 1 - filter("OBJECT_NAME"='DBA_VIEWS') Note ----- - dynamic sampling used for this statement 统计信息 ---------------------------------------------------------- 4 recursive calls 0 db block gets 1065 consistent gets 0 physical reads 0 redo size 371 bytes sent via SQL*Net to client 377 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 2 rows processed
为了测试方便，将RESULT_CACHE_MODE设置为AUTO模式。
设置参数为AUTO后，第二次执行SQL已经使用了RESULT CACHE。下面来看RESULT CACHE的第一个特点，由于RESULT CACHE所使用的内存来自SGA，因此RESULT CACHE的结果对于所有的SESSION都是可见的。SQL> ALTER SESSION SET RESULT_CACHE_MODE = AUTO; 会话已更改。 SQL> SET AUTOT ON SQL> SELECT /*+ RESULT_CACHE */ COUNT(*) FROM T; COUNT(*) ---------- 68324 执行计划 ---------------------------------------------------------- Plan hash value: 2966233522 ------------------------------------------------------------------------------------------ | Id | Operation | Name | Rows | Cost (%CPU)| Time | ------------------------------------------------------------------------------------------ | 0 | SELECT STATEMENT | | 1 | 282 (1)| 00:00:04 | | 1 | RESULT CACHE | 1t707xy2juqfq1xgx9dvy4bcqj | | | | | 2 | SORT AGGREGATE | | 1 | | | | 3 | TABLE ACCESS FULL| T | 68324 | 282 (1)| 00:00:04 | ------------------------------------------------------------------------------------------ Result Cache Information (identified by operation id): ------------------------------------------------------ 1 - column-count=1; dependencies=(YANGTK.T); attributes=(single-row); name="SELECT /*+ RESULT_CAC T" 统计信息 ---------------------------------------------------------- 56 recursive calls 0 db block gets 1019 consistent gets 1007 physical reads 0 redo size 338 bytes sent via SQL*Net to client 377 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 2 sorts (memory) 0 sorts (disk) 1 rows processed SQL> SELECT COUNT(*) FROM T; COUNT(*) ---------- 68324 执行计划 ---------------------------------------------------------- Plan hash value: 2966233522 ------------------------------------------------------------------------------------------ | Id | Operation | Name | Rows | Cost (%CPU)| Time | ------------------------------------------------------------------------------------------ | 0 | SELECT STATEMENT | | 1 | 282 (1)| 00:00:04 | | 1 | RESULT CACHE | 1t707xy2juqfq1xgx9dvy4bcqj | | | | | 2 | SORT AGGREGATE | | 1 | | | | 3 | TABLE ACCESS FULL| T | 68324 | 282 (1)| 00:00:04 | ------------------------------------------------------------------------------------------ Result Cache Information (identified by operation id): ------------------------------------------------------ 1 - column-count=1; type=AUTO; dependencies=(YANGTK.T); attributes=(single-row); name="SELECT COU 统计信息 ---------------------------------------------------------- 1 recursive calls 0 db block gets 0 consistent gets 0 physical reads 0 redo size 338 bytes sent via SQL*Net to client 377 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 1 rows processed
下面启动第二个SQLPLUS SESSION：
由于SELECT COUNT(*) FROM T的结果已经保存在SGA中，其他的SESSION也是可以使用这个缓存结果集的。RESULT CACHE是满足一致性读的，而且当缓存表数据发生变化的时候，Oracle会自动INVALIDATE缓存结果集：SQL> SET SQLPROMPT 'SQL2> ' SQL2> SET AUTOT ON SQL2> ALTER SESSION SET RESULT_CACHE_MODE = AUTO; 会话已更改。 SQL2> SELECT COUNT(*) FROM T; COUNT(*) ---------- 68324 执行计划 ---------------------------------------------------------- Plan hash value: 2966233522 ------------------------------------------------------------------------------------------ | Id | Operation | Name | Rows | Cost (%CPU)| Time | ------------------------------------------------------------------------------------------ | 0 | SELECT STATEMENT | | 1 | 282 (1)| 00:00:04 | | 1 | RESULT CACHE | 1t707xy2juqfq1xgx9dvy4bcqj | | | | | 2 | SORT AGGREGATE | | 1 | | | | 3 | TABLE ACCESS FULL| T | 68324 | 282 (1)| 00:00:04 | ------------------------------------------------------------------------------------------ Result Cache Information (identified by operation id): ------------------------------------------------------ 1 - column-count=1; type=AUTO; dependencies=(YANGTK.T); attributes=(single-row); name="SELECT COU 统计信息 ---------------------------------------------------------- 0 recursive calls 0 db block gets 0 consistent gets 0 physical reads 0 redo size 338 bytes sent via SQL*Net to client 377 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 1 rows processed
由于进行了修改，当前会话查询将无法在利用RESULT CACHE，但是这个时候如果从第二个会话进行查询：SQL> DELETE T WHERE ROWNUM = 1; 已删除 1 行。 SQL> SELECT COUNT(*) FROM T; COUNT(*) ---------- 68323 执行计划 ---------------------------------------------------------- Plan hash value: 2966233522 ------------------------------------------------------------------------------------------ | Id | Operation | Name | Rows | Cost (%CPU)| Time | ------------------------------------------------------------------------------------------ | 0 | SELECT STATEMENT | | 1 | 282 (1)| 00:00:04 | | 1 | RESULT CACHE | 1t707xy2juqfq1xgx9dvy4bcqj | | | | | 2 | SORT AGGREGATE | | 1 | | | | 3 | TABLE ACCESS FULL| T | 68324 | 282 (1)| 00:00:04 | ------------------------------------------------------------------------------------------ Result Cache Information (identified by operation id): ------------------------------------------------------ 1 - column-count=1; type=AUTO; dependencies=(YANGTK.T); attributes=(single-row); name="SELECT COU 统计信息 ---------------------------------------------------------- 0 recursive calls 0 db block gets 1011 consistent gets 0 physical reads 0 redo size 338 bytes sent via SQL*Net to client 377 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 1 rows processed
由于会话1没有提交，修改对于会话2来说是不可见的。因此会话2仍然可以使用缓存结果集。SQL2> SELECT COUNT(*) FROM T; COUNT(*) ---------- 68324 执行计划 ---------------------------------------------------------- Plan hash value: 2966233522 ------------------------------------------------------------------------------------------ | Id | Operation | Name | Rows | Cost (%CPU)| Time | ------------------------------------------------------------------------------------------ | 0 | SELECT STATEMENT | | 1 | 282 (1)| 00:00:04 | | 1 | RESULT CACHE | 1t707xy2juqfq1xgx9dvy4bcqj | | | | | 2 | SORT AGGREGATE | | 1 | | | | 3 | TABLE ACCESS FULL| T | 68324 | 282 (1)| 00:00:04 | ------------------------------------------------------------------------------------------ Result Cache Information (identified by operation id): ------------------------------------------------------ 1 - column-count=1; type=AUTO; dependencies=(YANGTK.T); attributes=(single-row); name="SELECT COU 统计信息 ---------------------------------------------------------- 0 recursive calls 0 db block gets 0 consistent gets 0 physical reads 0 redo size 338 bytes sent via SQL*Net to client 377 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 1 rows processed
将会话1的修改回滚，则会话1又可以利用RESULT CACHE。SQL> ROLLBACK; 回退已完成。 SQL> SELECT COUNT(*) FROM T; COUNT(*) ---------- 68324 执行计划 ---------------------------------------------------------- Plan hash value: 2966233522 ------------------------------------------------------------------------------------------ | Id | Operation | Name | Rows | Cost (%CPU)| Time | ------------------------------------------------------------------------------------------ | 0 | SELECT STATEMENT | | 1 | 282 (1)| 00:00:04 | | 1 | RESULT CACHE | 1t707xy2juqfq1xgx9dvy4bcqj | | | | | 2 | SORT AGGREGATE | | 1 | | | | 3 | TABLE ACCESS FULL| T | 68324 | 282 (1)| 00:00:04 | ------------------------------------------------------------------------------------------ Result Cache Information (identified by operation id): ------------------------------------------------------ 1 - column-count=1; type=AUTO; dependencies=(YANGTK.T); attributes=(single-row); name="SELECT COU 统计信息 ---------------------------------------------------------- 0 recursive calls 0 db block gets 0 consistent gets 0 physical reads 0 redo size 338 bytes sent via SQL*Net to client 377 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 1 rows processed
由于进行DDL操作，Oracle自动INVALIDATE了RESULT CACHE结果集，可以通过V$RESULT_CACHE_OBJECTS观察到：SQL> CREATE INDEX IND_T_OBJECT_ID ON T(OBJECT_ID); 索引已创建。 SQL> SELECT COUNT(*) FROM T; COUNT(*) ---------- 68324 执行计划 ---------------------------------------------------------- Plan hash value: 2966233522 ------------------------------------------------------------------------------------------ | Id | Operation | Name | Rows | Cost (%CPU)| Time | ------------------------------------------------------------------------------------------ | 0 | SELECT STATEMENT | | 1 | 282 (1)| 00:00:04 | | 1 | RESULT CACHE | 1t707xy2juqfq1xgx9dvy4bcqj | | | | | 2 | SORT AGGREGATE | | 1 | | | | 3 | TABLE ACCESS FULL| T | 68324 | 282 (1)| 00:00:04 | ------------------------------------------------------------------------------------------ Result Cache Information (identified by operation id): ------------------------------------------------------ 1 - column-count=1; type=AUTO; dependencies=(YANGTK.T); attributes=(single-row); name="SELECT COU 统计信息 ---------------------------------------------------------- 1 recursive calls 0 db block gets 1011 consistent gets 0 physical reads 0 redo size 338 bytes sent via SQL*Net to client 377 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 1 rows processed SQL> ALTER TABLE T MODIFY OBJECT_ID NOT NULL; 表已更改。 SQL> SELECT COUNT(*) FROM T; COUNT(*) ---------- 68324 执行计划 ---------------------------------------------------------- Plan hash value: 3841213438 --------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Cost (%CPU)| Time | --------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | 44 (3)| 00:00:01 | | 1 | SORT AGGREGATE | | 1 | | | | 2 | INDEX FAST FULL SCAN| IND_T_OBJECT_ID | 68324 | 44 (3)| 00:00:01 | --------------------------------------------------------------------------------- 统计信息 ---------------------------------------------------------- 204 recursive calls 0 db block gets 187 consistent gets 151 physical reads 0 redo size 338 bytes sent via SQL*Net to client 377 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 6 sorts (memory) 0 sorts (disk) 1 rows processed
从最后几个查询可以看出，RESULT CACHE忽略SQL语句的执行计划，即使执行计划发生了变化，只有最终获取的结果是一样的，Oracle仍然会使用RESULT CACHE。SQL> SELECT ID, TYPE, STATUS FROM V$RESULT_CACHE_OBJECTS 2 WHERE NAME = 'SELECT /*+ RESULT_CACHE */ COUNT(*) FROM T'; ID TYPE STATUS ---------- ---------- --------- 1 Result Invalid 最后看一下Oracle RESULT CACHE的另外一个特点，是否使用RESULT_CACHE与Oracle的执行计划无关： SQL> SELECT /*+ RESULT_CACHE */ COUNT(*) FROM T; COUNT(*) ---------- 68324 执行计划 ---------------------------------------------------------- Plan hash value: 3841213438 --------------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Cost (%CPU)| Time | --------------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | 44 (3)| 00:00:01 | | 1 | RESULT CACHE | 1t707xy2juqfq1xgx9dvy4bcqj | | | | | 2 | SORT AGGREGATE | | 1 | | | | 3 | INDEX FAST FULL SCAN| IND_T_OBJECT_ID | 68324 | 44 (3)| 00:00:01 | --------------------------------------------------------------------------------------------- Result Cache Information (identified by operation id): ------------------------------------------------------ 1 - column-count=1; dependencies=(YANGTK.T); attributes=(single-row); name="SELECT /*+ RESULT_CAC T" 统计信息 ---------------------------------------------------------- 1 recursive calls 0 db block gets 158 consistent gets 0 physical reads 0 redo size 338 bytes sent via SQL*Net to client 377 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 1 rows processed SQL> SELECT COUNT(*) FROM T; COUNT(*) ---------- 68324 执行计划 ---------------------------------------------------------- Plan hash value: 3841213438 --------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Cost (%CPU)| Time | --------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | 44 (3)| 00:00:01 | | 1 | SORT AGGREGATE | | 1 | | | | 2 | INDEX FAST FULL SCAN| IND_T_OBJECT_ID | 68324 | 44 (3)| 00:00:01 | --------------------------------------------------------------------------------- 统计信息 ---------------------------------------------------------- 0 recursive calls 0 db block gets 158 consistent gets 0 physical reads 0 redo size 338 bytes sent via SQL*Net to client 377 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 1 rows processed SQL> SELECT /*+ FULL(T) */ COUNT(*) FROM T; COUNT(*) ---------- 68324 执行计划 ---------------------------------------------------------- Plan hash value: 2966233522 ------------------------------------------------------------------------------------------ | Id | Operation | Name | Rows | Cost (%CPU)| Time | ------------------------------------------------------------------------------------------ | 0 | SELECT STATEMENT | | 1 | 282 (1)| 00:00:04 | | 1 | RESULT CACHE | 1t707xy2juqfq1xgx9dvy4bcqj | | | | | 2 | SORT AGGREGATE | | 1 | | | | 3 | TABLE ACCESS FULL| T | 68324 | 282 (1)| 00:00:04 | ------------------------------------------------------------------------------------------ Result Cache Information (identified by operation id): ------------------------------------------------------ 1 - column-count=1; type=AUTO; dependencies=(YANGTK.T); attributes=(single-row); name="SELECT /*+ FROM T" 统计信息 ---------------------------------------------------------- 1 recursive calls 0 db block gets 0 consistent gets 0 physical reads 0 redo size 338 bytes sent via SQL*Net to client 377 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 1 rows processed SQL> SELECT /*+ INDEX(T) */ COUNT(*) FROM T; COUNT(*) ---------- 68324 执行计划 ---------------------------------------------------------- Plan hash value: 2391220499 ---------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Cost (%CPU)| Time | ---------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | 153 (1)| 00:00:02 | | 1 | RESULT CACHE | 1t707xy2juqfq1xgx9dvy4bcqj | | | | | 2 | SORT AGGREGATE | | 1 | | | | 3 | INDEX FULL SCAN| IND_T_OBJECT_ID | 68324 | 153 (1)| 00:00:02 | ---------------------------------------------------------------------------------------- Result Cache Information (identified by operation id): ------------------------------------------------------ 1 - column-count=1; type=AUTO; dependencies=(YANGTK.T); attributes=(single-row); name="SELECT /*+ ) FROM T" 统计信息 ---------------------------------------------------------- 1 recursive calls 0 db block gets 0 consistent gets 0 physical reads 0 redo size 338 bytes sent via SQL*Net to client 377 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 1 rows processed SQL> SELECT COUNT(*) FROM T A; COUNT(*) ---------- 68324 执行计划 ---------------------------------------------------------- Plan hash value: 3841213438 --------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Cost (%CPU)| Time | --------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | 44 (3)| 00:00:01 | | 1 | SORT AGGREGATE | | 1 | | | | 2 | INDEX FAST FULL SCAN| IND_T_OBJECT_ID | 68324 | 44 (3)| 00:00:01 | --------------------------------------------------------------------------------- 统计信息 ---------------------------------------------------------- 1 recursive calls 0 db block gets 158 consistent gets 0 physical reads 0 redo size 338 bytes sent via SQL*Net to client 377 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 1 rows processed
注意当执行计划不同导致结果不同时，Oracle使用RESULT CACHE可能会得到不正确的结果。
前文介绍了RESULT CACHE的一些特点，其中最后提到了使用RESULT CACHE可能会导致查询结果不正确。下面看一个简单的例子：
先将RESULT_CACHE_MODE设置为MANUAL，观察全表扫描和索引全扫描两种查询的结果。由于执行计划的不同，虽然语句本身是一样的，但是返回结果中记录的顺序是不同的。在这个例子中，如果使用全表扫描会返回ICOL$，而采用索引扫描，返回结果为C_OBJ#。SQL> SET AUTOT ON SQL> ALTER SESSION SET RESULT_CACHE_MODE = MANUAL; 会话已更改。 SQL> SELECT /*+ FULL(T) */ OBJECT_NAME FROM T WHERE ROWNUM = 1; OBJECT_NAME ------------------------------ ICOL$ 执行计划 ---------------------------------------------------------- Plan hash value: 508354683 --------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | --------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | 25 | 2 (0)| 00:00:01 | |* 1 | COUNT STOPKEY | | | | | | | 2 | TABLE ACCESS FULL| T | 1 | 25 | 2 (0)| 00:00:01 | --------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 1 - filter(ROWNUM=1) 统计信息 ---------------------------------------------------------- 1 recursive calls 0 db block gets 4 consistent gets 0 physical reads 0 redo size 342 bytes sent via SQL*Net to client 377 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 1 rows processed SQL> SELECT /*+ INDEX(T) */ OBJECT_NAME FROM T WHERE ROWNUM = 1; OBJECT_NAME ------------------------------ C_OBJ# 执行计划 ---------------------------------------------------------- Plan hash value: 370698254 ------------------------------------------------------------------------------------------------ | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | ------------------------------------------------------------------------------------------------ | 0 | SELECT STATEMENT | | 1 | 25 | 3 (0)| 00:00:01 | |* 1 | COUNT STOPKEY | | | | | | | 2 | TABLE ACCESS BY INDEX ROWID| T | 1 | 25 | 3 (0)| 00:00:01 | | 3 | INDEX FULL SCAN | IND_T_OBJECT_ID | 68324 | | 2 (0)| 00:00:01 | ------------------------------------------------------------------------------------------------ Predicate Information (identified by operation id): --------------------------------------------------- 1 - filter(ROWNUM=1) 统计信息 ---------------------------------------------------------- 1 recursive calls 0 db block gets 3 consistent gets 0 physical reads 0 redo size 343 bytes sent via SQL*Net to client 377 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 1 rows processed
下面看看启用缓存结果集的情况：
由于第二个查询指定使用了RESULT CACHE，虽然Oracle显示的执行计划是索引扫描，但是利用的缓存结果集是全表扫描生成的，所以第二个查询得到的结果也和第一个查询一样。SQL> SELECT /*+ RESULT_CACHE FULL(T) */ OBJECT_NAME FROM T WHERE ROWNUM = 1; OBJECT_NAME ------------------------------ ICOL$ 执行计划 ---------------------------------------------------------- Plan hash value: 508354683 -------------------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | -------------------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | 25 | 2 (0)| 00:00:01 | | 1 | RESULT CACHE | 668yp49bw4mfnak70m2pww541d | | | | | |* 2 | COUNT STOPKEY | | | | | | | 3 | TABLE ACCESS FULL| T | 1 | 25 | 2 (0)| 00:00:01 | -------------------------------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 2 - filter(ROWNUM=1) Result Cache Information (identified by operation id): ------------------------------------------------------ 1 - column-count=1; dependencies=(YANGTK.T); name="SELECT /*+ RESULT_CACHE FULL(T) */ OBJECT_NAME = 1" 统计信息 ---------------------------------------------------------- 1 recursive calls 0 db block gets 4 consistent gets 0 physical reads 0 redo size 342 bytes sent via SQL*Net to client 377 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 1 rows processed SQL> SELECT /*+ RESULT_CACHE INDEX(T) */ OBJECT_NAME FROM T WHERE ROWNUM = 1; OBJECT_NAME ------------------------------ ICOL$ 执行计划 ---------------------------------------------------------- Plan hash value: 370698254 ---------------------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | ---------------------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | 25 | 3 (0)| 00:00:01 | | 1 | RESULT CACHE | 668yp49bw4mfnak70m2pww541d | | | | | |* 2 | COUNT STOPKEY | | | | | | | 3 | TABLE ACCESS BY INDEX ROWID| T | 1 | 25 | 3 (0)| 00:00:01 | | 4 | INDEX FULL SCAN | IND_T_OBJECT_ID | 68324 | | 2 (0)| 00:00:01 | ---------------------------------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 2 - filter(ROWNUM=1) Result Cache Information (identified by operation id): ------------------------------------------------------ 1 - column-count=1; dependencies=(YANGTK.T); name="SELECT /*+ RESULT_CACHE INDEX(T) */ OBJECT_NAM M = 1" 统计信息 ---------------------------------------------------------- 1 recursive calls 0 db block gets 0 consistent gets 0 physical reads 0 redo size 342 bytes sent via SQL*Net to client 377 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 1 rows processed
对于以前的系统中，试图利用索引按照顺序返回结果集的特点来避免排序的写法，在利用RESULT CACHE时，很可能会出现问题。不知道Oracle是否会认为这是一个bug，是否以后会有所改进。RESULT CACHE不仅对查询的最终结果生效，查询的中间结果也是可以使用RESULT CACHE的。
在进行测试之前，先将前面的RESULT CACHE清除掉。Oracle提供了一个DBMS_RESULT_CACHE包来管理RESULT CACHE：
这个包还包括其他一些功能，比如BYPASS可以设置当前实例绕过RESULT CACHE机制。INVALIDATE可以设置某个对象关联的所有RESULT CACHE失效。STATUS返回结果集状态。SQL> SET AUTOT OFF SQL> SELECT COUNT(*) FROM V$RESULT_CACHE_OBJECTS; COUNT(*) ---------- 5 SQL> EXEC DBMS_RESULT_CACHE.FLUSH PL/SQL 过程已成功完成。 SQL> SELECT COUNT(*) FROM V$RESULT_CACHE_OBJECTS; COUNT(*) ---------- 0
下面看看Oracle如何对查询的中间结果使用RESULT CACHE：
需要注意，对于SQL中间结果使用RESULT CACHE必须使用RESULT_CACHE提示进行强制。SQL> SET AUTOT ON SQL> CREATE TABLE T1 AS SELECT * FROM DBA_SEGMENTS; 表已创建。 SQL> SELECT /*+ RESULT_CACHE */ OWNER, COUNT(*) NUM FROM T GROUP BY OWNER; OWNER NUM ------------------------------ ---------- WKSYS 371 MDSYS 1281 YANGTK 13 . . . WMSYS 315 SI_INFORMTN_SCHEMA 8 已选择22行。 执行计划 ---------------------------------------------------------- Plan hash value: 47235625 -------------------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | -------------------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 22 | 132 | 287 (3)| 00:00:04 | | 1 | RESULT CACHE | g02167xk9g68s52fnfssvx00tr | | | | | | 2 | HASH GROUP BY | | 22 | 132 | 287 (3)| 00:00:04 | | 3 | TABLE ACCESS FULL| T | 68324 | 400K| 282 (1)| 00:00:04 | -------------------------------------------------------------------------------------------------- Result Cache Information (identified by operation id): ------------------------------------------------------ 1 - column-count=2; dependencies=(YANGTK.T); parameters=(nls); name="SELECT /*+ RESULT_CACHE */ O ROM T GROUP BY OWNER" 统计信息 ---------------------------------------------------------- 1 recursive calls 0 db block gets 1011 consistent gets 0 physical reads 0 redo size 826 bytes sent via SQL*Net to client 388 bytes received via SQL*Net from client 3 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 22 rows processed SQL> SELECT /*+ RESULT_CACHE */ OWNER, SUM(BYTES)/1024 K FROM T1 GROUP BY OWNER; OWNER K ------------------------------ ---------- WKSYS 7616 MDSYS 47744 YANGTK 11392 . . . SYS 854656 WMSYS 7296 已选择18行。 执行计划 ---------------------------------------------------------- Plan hash value: 136660032 -------------------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | -------------------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 7146 | 209K| 36 (6)| 00:00:01 | | 1 | RESULT CACHE | f072b82hjsvsm9yvr91n766xwp | | | | | | 2 | HASH GROUP BY | | 7146 | 209K| 36 (6)| 00:00:01 | | 3 | TABLE ACCESS FULL| T1 | 7146 | 209K| 34 (0)| 00:00:01 | -------------------------------------------------------------------------------------------------- Result Cache Information (identified by operation id): ------------------------------------------------------ 1 - column-count=2; dependencies=(YANGTK.T1); parameters=(nls); name="SELECT /*+ RESULT_CACHE */ 24 K FROM T1 GROUP BY OWNER" Note ----- - dynamic sampling used for this statement 统计信息 ---------------------------------------------------------- 68 recursive calls 0 db block gets 183 consistent gets 109 physical reads 0 redo size 773 bytes sent via SQL*Net to client 388 bytes received via SQL*Net from client 3 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 18 rows processed SQL> SELECT A.OWNER, NUM, K FROM 2 (SELECT /*+ RESULT_CACHE */ OWNER, COUNT(*) NUM FROM T GROUP BY OWNER) A, 3 (SELECT /*+ RESULT_CACHE */ OWNER, SUM(BYTES)/1024 K FROM T1 GROUP BY OWNER) B 4 WHERE A.OWNER = B.OWNER; OWNER NUM K ------------------------------ ---------- ---------- WKSYS 371 7616 MDSYS 1281 47744 YANGTK 13 11392 . . . SYS 29743 854656 WMSYS 315 7296 已选择18行。 执行计划 ---------------------------------------------------------- Plan hash value: 3810504953 ---------------------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | ---------------------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 7146 | 418K| 323 (3)| 00:00:04 | |* 1 | HASH JOIN | | 7146 | 418K| 323 (3)| 00:00:04 | | 2 | VIEW | | 22 | 660 | 287 (3)| 00:00:04 | | 3 | RESULT CACHE | g02167xk9g68s52fnfssvx00tr | | | | | | 4 | HASH GROUP BY | | 22 | 132 | 287 (3)| 00:00:04 | | 5 | TABLE ACCESS FULL| T | 68324 | 400K| 282 (1)| 00:00:04 | | 6 | VIEW | | 7146 | 209K| 36 (6)| 00:00:01 | | 7 | RESULT CACHE | f072b82hjsvsm9yvr91n766xwp | | | | | | 8 | HASH GROUP BY | | 7146 | 209K| 36 (6)| 00:00:01 | | 9 | TABLE ACCESS FULL| T1 | 7146 | 209K| 34 (0)| 00:00:01 | ---------------------------------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 1 - access("A"."OWNER"="B"."OWNER") Result Cache Information (identified by operation id): ------------------------------------------------------ 3 - column-count=2; dependencies=(YANGTK.T); parameters=(nls); name="SELECT /*+ RESULT_CACHE */ O ROM T GROUP BY OWNER" 7 - column-count=2; dependencies=(YANGTK.T1); parameters=(nls); name="SELECT /*+ RESULT_CACHE */ 24 K FROM T1 GROUP BY OWNER" Note ----- - dynamic sampling used for this statement 统计信息 ---------------------------------------------------------- 127 recursive calls 8 db block gets 194 consistent gets 0 physical reads 0 redo size 889 bytes sent via SQL*Net to client 388 bytes received via SQL*Net from client 3 SQL*Net roundtrips to/from client 10 sorts (memory) 0 sorts (disk) 18 rows processed
RESULT CACHE功能对于下列情况是无效的：系统表和临时表；序列的NEXTVAL和CURRVAL伪列；SYSDATE、SYSTIMESTAMP等函数；所有非确定性PL/SQL函数。
