【DB笔试面试611】在Oracle中，什么是结果集缓存？
2019-08-06 10:00
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在Oracle中，什么是结果集缓存？
[strong]♣[/strong]答案部分
结果集缓存（Result Cache）是Oracle 11g的新特性，用于存储经常使用的SQL语句和函数的查询结果。当相同语句再次执行的时候，Oracle就不用再次重复执行（包括扫描索引、回表、计算、逻辑读、物理读等操作），而是直接访问内存得到结果。结果集缓存可以将SQL语句查询的结果缓存在内存（SGA的Shared Pool）中，从而显著地改进需要多次执行和查询相同结果的SQL语句的性能。结果集缓存的优点是可以重用相同的结果集，减少逻辑I/O，从而提高系统性能。结果集缓存最适合的是静态表（例如只读表），即结果集缓存最适合返回同样结果的查询。若SQL语句中包含的对象（例如表）做了UPDATE，INSERT，DELETE或是DDL操作，则相关的所有SQL的缓存结果集就自动失效了。所以，Result Cache只对那些在平时几乎没有任何DML操作的只读表比较有用，可以减轻I/O的压力。在实际情况中，结果集缓存仅在少数的情况下是有效的。在以下情况中，结果集不会被缓存：① 查询使用非确定性的函数、序列和临时表的结果集不会被缓存。② 查询违反了读一致性时结果集将不会被缓存。③ 引用数据字典视图的查询的结果集不会被缓存。④ 查询结果集大于可用缓存结果集可用空间的不会被缓存。⑤ 对依赖对象的任何改变（DML、DDL）都会使整个缓存的结果集变为无效，结果集缓存最适合那些只读或接近只读的表。⑥ ADG的备库不能使用结果集缓存。Oracle数据库引擎提供了3种结果集缓存，包括：服务器查询结果集缓存、PL/SQL函数结果集缓存和客户端结果集缓存。（一）服务器查询结果集缓存服务器查询结果集缓存由以下一些参数控制：l RESULT_CACHE_MODE：该参数用来控制结果集缓存的操作模式。AUTO表示优化程序将根据重复的执行操作确定将哪些结果存储在高速缓存中。MANUAL表示只有使用了RESULT_CAHCE提示的查询或对带有RESULT_CACHE属性的表访问的查询才会被缓存，MANUAL为该参数的默认值。FORCE表示所有合适的查询都会被缓存。对于AUTO和FORCE设置，如果语句中包含[NO_]RESULT_CACHE提示，那么该提示优先于参数设置。l RESULT_CACHE_MAX_SIZE：控制结果集缓存的大小，默认值取决于其它内存设置（MEMORY_TARGET的0.25%或SGA_TARGET的0.5%或SHARED_POOL_SIZE的1%）。当RESULT_CACHE_MAX_SIZE为0的时候，代表不启用结果集缓存。需要注意的是，若将该参数的值从0修改为非0值后，则需要重启数据库才能启用服务器查询结果集缓存特性，反之亦然。l RESULT_CACHE_MAX_RESULT：单个结果集能够消耗的缓存的最大百分比，比这个值大的结果集将不能被缓存，默认大小为RESULT_CACHE_MAX_SIZE的5%。l RESULT_CACHE_REMOTE_EXPIRATION：设置远程数据库结果集缓存过期的时间，以分钟为单位，默认值为0，表示不缓存远程数据库结果集。与结果集缓存相关的一些视图：l V$RESULT_CACHE_STATISTICS：列出各种缓存设置和内存使用统计数据。l V$RESULT_CACHE_MEMORY：列出所有的内存块和相应的统计信息。l V$RESULT_CACHE_OBJECTS：列出所有的对象（缓存的结果和依赖的对象）和它们的属性。l V$RESULT_CACHE_DEPENDENCY：列出缓存的结果和依赖对象间的依赖详情。与结果集缓存相关的包是DBMS_RESULT_CACHE。l STATUS函数：返回值若为DISABLED则表示没有开启结果集缓存；若为ENABLED则表示已经开启并且可以使用结果集缓存；若为BYPASS则表示已经开启结果集缓存但不可以使用结果集缓存，此时可以通过执行“EXEC DBMS_RESULT_CACHE.BYPASS(FALSE);”来使用结果集缓存。执行后如果返回值仍然是BYPASS，那么可能是参数RESULT_CACHE_MAX_SIZE的值为0的原因。STATUS函数返回值若为SYNC则表示结果缓存是可用的，但是目前正与其它RAC节点重新同步。可以使用SQL语句“SELECT DBMS_RESULT_CACHE.STATUS FROM DUAL;”来检查是否开启了结果集缓存机制。l MEMORY_REPORT存储过程：列出结果缓存内存利用的一个概要（默认）或详细的报表。如下所示：
1SYS@orclasm > set serveroutput on 2SYS@orclasm > exec dbms_result_cache.Memory_Report; 3R e s u l t C a c h e M e m o r y R e p o r t 4[Parameters] 5Block Size = 1K bytes 6Maximum Cache Size = 2816K bytes (2816 blocks) 7Maximum Result Size = 140K bytes (140 blocks) 8[Memory] 9Total Memory = 169328 bytes [0.034% of the Shared Pool]10... Fixed Memory = 5352 bytes [0.001% of the Shared Pool]11... Dynamic Memory = 163976 bytes [0.033% of the Shared Pool]12....... Overhead = 131208 bytes13....... Cache Memory = 32K bytes (32 blocks)14........... Unused Memory = 25 blocks15........... Used Memory = 7 blocks16............... Dependencies = 2 blocks (2 count)17............... Results = 5 blocks18................... SQL = 5 blocks (5 count)1920PL/SQL procedure successfully completed.set serveroutput onl FLUSH函数：清空整个结果缓存的内容。l INVALIDATE函数：使结果缓存中某个特定对象的缓存结果无效。l INVALIDATE_OBJECT函数：根据缓存ID使某个特定结果缓存无效。可以在表级别设置RESULT_CACHE，如下所示：
2SYS@orclasm > exec dbms_result_cache.Memory_Report;
3R e s u l t C a c h e M e m o r y R e p o r t
4[Parameters]
5Block Size = 1K bytes
6Maximum Cache Size = 2816K bytes (2816 blocks)
7Maximum Result Size = 140K bytes (140 blocks)
8[Memory]
9Total Memory = 169328 bytes [0.034% of the Shared Pool]
10... Fixed Memory = 5352 bytes [0.001% of the Shared Pool]
11... Dynamic Memory = 163976 bytes [0.033% of the Shared Pool]
12....... Overhead = 131208 bytes
13....... Cache Memory = 32K bytes (32 blocks)
14........... Unused Memory = 25 blocks
15........... Used Memory = 7 blocks
16............... Dependencies = 2 blocks (2 count)
17............... Results = 5 blocks
18................... SQL = 5 blocks (5 count)
19
20PL/SQL procedure successfully completed.
1ALTER TABLE HR.EMPLOYEES RESULT_CACHE(MODE FORCE);ALTER TABLE HR.EMPLOYEES RESULT_CACHE(MODE FORCE);下面给出一个服务器查询结果集缓存示例：
1LHR@orclasm > show parameter result_cache 2 3NAME TYPE VALUE 4------------------------------------ -------------------------------- ------------------------------ 5client_result_cache_lag big integer 3000 6client_result_cache_size big integer 0 7result_cache_max_result integer 5 8result_cache_max_size big integer 2816K 9result_cache_mode string MANUAL 10result_cache_remote_expiration integer 0 11 12LHR@orclasm > create table t_rc_20170611_lhr as select * from dba_objects; 13 14Table created. 15 16 17LHR@orclasm > set autotrace traceonly 18LHR@orclasm > select /*+result_cache*/ * from t_rc_20170611_lhr where object_id=10; 19 20 21Execution Plan 22---------------------------------------------------------- 23Plan hash value: 3007689452 24 25------------------------------------------------------------------------------------------------- 26| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | 27------------------------------------------------------------------------------------------------- 28| 0 | SELECT STATEMENT | | 13 | 2691 | 312 (1)| 00:00:04 | 29| 1 | RESULT CACHE | 4ns8fbzpm3a10agkanndg1mqbn | | | | | 30|* 2 | TABLE ACCESS FULL| T_RC_20170611_LHR | 13 | 2691 | 312 (1)| 00:00:04 | 31------------------------------------------------------------------------------------------------- 32 33Predicate Information (identified by operation id): 34--------------------------------------------------- 35 36 2 - filter("OBJECT_ID"=10) 37 38Result Cache Information (identified by operation id): 39------------------------------------------------------ 40 41 1 - column-count=15; dependencies=(LHR.T_RC_20170611_LHR); name="select /*+result_cache*/ * from t_rc_20170611_lhr where object_id=10" 42 43Note 44----- 45 - dynamic sampling used for this statement (level=2) 46 47 48Statistics 49---------------------------------------------------------- 50 24 recursive calls 51 0 db block gets 52 1226 consistent gets 53 1440 physical reads 54 0 redo size 55 1609 bytes sent via SQL*Net to client 56 519 bytes received via SQL*Net from client 57 2 SQL*Net roundtrips to/from client 58 0 sorts (memory) 59 0 sorts (disk) 60 1 rows processed 61 62LHR@orclasm > select /*+result_cache*/ * from t_rc_20170611_lhr where object_id=10; 63 64 65Execution Plan 66---------------------------------------------------------- 67Plan hash value: 3007689452 68 69------------------------------------------------------------------------------------------------- 70| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | 71------------------------------------------------------------------------------------------------- 72| 0 | SELECT STATEMENT | | 13 | 2691 | 312 (1)| 00:00:04 | 73| 1 | RESULT CACHE | 4ns8fbzpm3a10agkanndg1mqbn | | | | | 74|* 2 | TABLE ACCESS FULL| T_RC_20170611_LHR | 13 | 2691 | 312 (1)| 00:00:04 | 75------------------------------------------------------------------------------------------------- 76 77Predicate Information (identified by operation id): 78--------------------------------------------------- 79 80 2 - filter("OBJECT_ID"=10) 81 82Result Cache Information (identified by operation id): 83------------------------------------------------------ 84 85 1 - column-count=15; dependencies=(LHR.T_RC_20170611_LHR); name="select /*+result_cache*/ * from t_rc_20170611_lhr where object_id=10" 86 87Note 88----- 89 - dynamic sampling used for this statement (level=2) 90 91 92Statistics 93---------------------------------------------------------- 94 0 recursive calls 95 0 db block gets 96 0 consistent gets 97 0 physical reads 98 0 redo size 99 1609 bytes sent via SQL*Net to client100 519 bytes received via SQL*Net from client101 2 SQL*Net roundtrips to/from client102 0 sorts (memory)103 0 sorts (disk)104 1 rows processed105SYS@orclasm > SELECT d.NAMESPACE,d.DEPEND_COUNT,d.SCN,d.INVALIDATIONS FROM V$RESULT_CACHE_OBJECTS d WHERE d.CACHE_ID='4ns8fbzpm3a10agkanndg1mqbn';106107NAMES DEPEND_COUNT SCN INVALIDATIONS108----- ------------ ---------- -------------109SQL 1 66062134 0110SYS@orclasm > select * from v$result_cache_statistics;111112 ID NAME VALUE113---------- ---------------------------------------- -------------------114 1 Block Size (Bytes) 1024115 2 Block Count Maximum 2816116 3 Block Count Current 32117 4 Result Size Maximum (Blocks) 140118 5 Create Count Success 5119 6 Create Count Failure 0120 7 Find Count 8121 8 Invalidation Count 0122 9 Delete Count Invalid 0123 10 Delete Count Valid 0124 11 Hash Chain Length 1125 12 Find Copy Count 8126show parameter result_cache
2
3NAME TYPE VALUE
4------------------------------------ -------------------------------- ------------------------------
5client_result_cache_lag big integer 3000
6client_result_cache_size big integer 0
7result_cache_max_result integer 5
8result_cache_max_size big integer 2816K
9result_cache_mode string MANUAL
10result_cache_remote_expiration integer 0
11
12LHR@orclasm > create table t_rc_20170611_lhr as select * from dba_objects;
13
14Table created.
15
16
17LHR@orclasm > set autotrace traceonly
18LHR@orclasm > select /*+result_cache*/ * from t_rc_20170611_lhr where object_id=10;
19
20
21Execution Plan
22----------------------------------------------------------
23Plan hash value: 3007689452
24
25-------------------------------------------------------------------------------------------------
26| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
27-------------------------------------------------------------------------------------------------
28| 0 | SELECT STATEMENT | | 13 | 2691 | 312 (1)| 00:00:04 |
29| 1 | RESULT CACHE | 4ns8fbzpm3a10agkanndg1mqbn | | | | |
30|* 2 | TABLE ACCESS FULL| T_RC_20170611_LHR | 13 | 2691 | 312 (1)| 00:00:04 |
31-------------------------------------------------------------------------------------------------
32
33Predicate Information (identified by operation id):
34---------------------------------------------------
35
36 2 - filter("OBJECT_ID"=10)
37
38Result Cache Information (identified by operation id):
39------------------------------------------------------
40
41 1 - column-count=15; dependencies=(LHR.T_RC_20170611_LHR); name="select /*+result_cache*/ * from t_rc_20170611_lhr where object_id=10"
42
43Note
44-----
45 - dynamic sampling used for this statement (level=2)
46
47
48Statistics
49----------------------------------------------------------
50 24 recursive calls
51 0 db block gets
52 1226 consistent gets
53 1440 physical reads
54 0 redo size
55 1609 bytes sent via SQL*Net to client
56 519 bytes received via SQL*Net from client
57 2 SQL*Net roundtrips to/from client
58 0 sorts (memory)
59 0 sorts (disk)
60 1 rows processed
61
62LHR@orclasm > select /*+result_cache*/ * from t_rc_20170611_lhr where object_id=10;
63
64
65Execution Plan
66----------------------------------------------------------
67Plan hash value: 3007689452
68
69-------------------------------------------------------------------------------------------------
70| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
71-------------------------------------------------------------------------------------------------
72| 0 | SELECT STATEMENT | | 13 | 2691 | 312 (1)| 00:00:04 |
73| 1 | RESULT CACHE | 4ns8fbzpm3a10agkanndg1mqbn | | | | |
74|* 2 | TABLE ACCESS FULL| T_RC_20170611_LHR | 13 | 2691 | 312 (1)| 00:00:04 |
75-------------------------------------------------------------------------------------------------
76
77Predicate Information (identified by operation id):
78---------------------------------------------------
79
80 2 - filter("OBJECT_ID"=10)
81
82Result Cache Information (identified by operation id):
83------------------------------------------------------
84
85 1 - column-count=15; dependencies=(LHR.T_RC_20170611_LHR); name="select /*+result_cache*/ * from t_rc_20170611_lhr where object_id=10"
86
87Note
88-----
89 - dynamic sampling used for this statement (level=2)
90
91
92Statistics
93----------------------------------------------------------
94 0 recursive calls
95 0 db block gets
96 0 consistent gets
97 0 physical reads
98 0 redo size
99 1609 bytes sent via SQL*Net to client
100 519 bytes received via SQL*Net from client
101 2 SQL*Net roundtrips to/from client
102 0 sorts (memory)
103 0 sorts (disk)
104 1 rows processed
105SYS@orclasm > SELECT d.NAMESPACE,d.DEPEND_COUNT,d.SCN,d.INVALIDATIONS FROM V$RESULT_CACHE_OBJECTS d WHERE d.CACHE_ID='4ns8fbzpm3a10agkanndg1mqbn';
106
107NAMES DEPEND_COUNT SCN INVALIDATIONS
108----- ------------ ---------- -------------
109SQL 1 66062134 0
110SYS@orclasm > select * from v$result_cache_statistics;
111
112 ID NAME VALUE
113---------- ---------------------------------------- -------------------
114 1 Block Size (Bytes) 1024
115 2 Block Count Maximum 2816
116 3 Block Count Current 32
117 4 Result Size Maximum (Blocks) 140
118 5 Create Count Success 5
119 6 Create Count Failure 0
120 7 Find Count 8
121 8 Invalidation Count 0
122 9 Delete Count Invalid 0
123 10 Delete Count Valid 0
124 11 Hash Chain Length 1
125 12 Find Copy Count 8
126
在第二次查询的执行计划中，发现逻辑读已经变成0了，说明Result Cache起作用了。（二）函数结果集缓存Oracle数据库用一个单独的缓存区为每一个函数同时保存输入和返回值。这个缓存区被连接到这个数据库实例的所有会话共享。每当函数被调用时，数据库就会检查是否已经缓存了相同的输入值。如果是，那么函数就不用重新执行了，而是把缓存中的值简单返回即可。每当发现要修改的是缓存所依赖的表，数据库就会自动把缓存失效。有两种函数缓存机制，分别是确定性函数缓存和函数结果集缓存。对于一个函数，如果有相同的IN和IN OUT参数，且函数的返回结果也相同，那么这个函数就是确定性的（DETERMINISTIC）。Oracle通过关键字DETERMINISTIC来表明一个函数是确定性的，确定性函数可以用于创建基于函数的索引。函数结果集缓存指的是Oracle通过关键字RESULT_CACHE对函数返回的结果进行缓存，缓存结果可以被所有会话共享。下面给出一个函数结果集缓存的示例：准备表：1CREATE TABLE T_FRC_20170611_LHR AS SELECT * FROM DBA_OBJECTS;2CREATE OR REPLACE FUNCTION FUN_OBJECTNAME_LHR(P_ID IN NUMBER) RETURN VARCHAR2 RESULT_CACHE AS3 V_OBJECT VARCHAR2(200);4BEGIN5 SELECT OBJECT_NAME INTO V_OBJECT FROM T_FRC_20170611_LHR WHERE OBJECT_ID = P_ID;6 RETURN V_OBJECT;7END FUN_OBJECTNAME_LHR;CREATE TABLE T_FRC_20170611_LHR AS SELECT * FROM DBA_OBJECTS;执行查询：
2CREATE OR REPLACE FUNCTION FUN_OBJECTNAME_LHR(P_ID IN NUMBER) RETURN VARCHAR2 RESULT_CACHE AS
3 V_OBJECT VARCHAR2(200);
4BEGIN
5 SELECT OBJECT_NAME INTO V_OBJECT FROM T_FRC_20170611_LHR WHERE OBJECT_ID = P_ID;
6 RETURN V_OBJECT;
7END FUN_OBJECTNAME_LHR;
1LHR@orclasm > SELECT FUN_OBJECTNAME_LHR(10) FROM DUAL; 2 3 4Execution Plan 5---------------------------------------------------------- 6Plan hash value: 1388734953 7 8----------------------------------------------------------------- 9| Id | Operation | Name | Rows | Cost (%CPU)| Time |10-----------------------------------------------------------------11| 0 | SELECT STATEMENT | | 1 | 2 (0)| 00:00:01 |12| 1 | FAST DUAL | | 1 | 2 (0)| 00:00:01 |13-----------------------------------------------------------------141516Statistics17----------------------------------------------------------18 30 recursive calls19 0 db block gets20 1216 consistent gets21 1117 physical reads22 0 redo size23 545 bytes sent via SQL*Net to client24 519 bytes received via SQL*Net from client25 2 SQL*Net roundtrips to/from client26 0 sorts (memory)27 0 sorts (disk)28 1 rows processed2930LHR@orclasm > SELECT FUN_OBJECTNAME_LHR(10) FROM DUAL;313233Execution Plan34----------------------------------------------------------35Plan hash value: 13887349533637-----------------------------------------------------------------38| Id | Operation | Name | Rows | Cost (%CPU)| Time |39-----------------------------------------------------------------40| 0 | SELECT STATEMENT | | 1 | 2 (0)| 00:00:01 |41| 1 | FAST DUAL | | 1 | 2 (0)| 00:00:01 |42-----------------------------------------------------------------434445Statistics46----------------------------------------------------------47 0 recursive calls48 0 db block gets49 0 consistent gets50 0 physical reads51 0 redo size52 545 bytes sent via SQL*Net to client53 519 bytes received via SQL*Net from client54 2 SQL*Net roundtrips to/from client55 0 sorts (memory)56 0 sorts (disk)57 1 rows processed58SYS@orclasm > SELECT d.NAMESPACE,d.DEPEND_COUNT,d.SCN,d.INVALIDATIONS FROM V$RESULT_CACHE_OBJECTS d WHERE D.NAMESPACE='PLSQL';5960NAMES DEPEND_COUNT SCN INVALIDATIONS61----- ------------ ---------- -------------62PLSQL 2 66067285 06364LHR@orclasm > DELETE FROM T_FRC_20170611_LHR WHERE ROWNUM<=1;65661 row deleted.67LHR@orclasm > COMMIT;6869Commit complete.7071LHR@orclasm > SELECT FUN_OBJECTNAME_LHR(10) FROM DUAL;（三）客户端结果集缓存初始化参数CLIENT_RESULT_CACHE_SIZE表示所有客户端的总缓存大小，有关客户端结果缓存本书不再详解，读者可以查阅相关的官方文档来学习。& 说明：有关结果集缓存的更多内容可以参考我的BLOG：http://blog.itpub.net/26736162/viewspace-2140589/
2
3
4Execution Plan
5----------------------------------------------------------
6Plan hash value: 1388734953
7
8-----------------------------------------------------------------
9| Id | Operation | Name | Rows | Cost (%CPU)| Time |
10-----------------------------------------------------------------
11| 0 | SELECT STATEMENT | | 1 | 2 (0)| 00:00:01 |
12| 1 | FAST DUAL | | 1 | 2 (0)| 00:00:01 |
13-----------------------------------------------------------------
14
15
16Statistics
17----------------------------------------------------------
18 30 recursive calls
19 0 db block gets
20 1216 consistent gets
21 1117 physical reads
22 0 redo size
23 545 bytes sent via SQL*Net to client
24 519 bytes received via SQL*Net from client
25 2 SQL*Net roundtrips to/from client
26 0 sorts (memory)
27 0 sorts (disk)
28 1 rows processed
29
30LHR@orclasm > SELECT FUN_OBJECTNAME_LHR(10) FROM DUAL;
31
32
33Execution Plan
34----------------------------------------------------------
35Plan hash value: 1388734953
36
37-----------------------------------------------------------------
38| Id | Operation | Name | Rows | Cost (%CPU)| Time |
39-----------------------------------------------------------------
40| 0 | SELECT STATEMENT | | 1 | 2 (0)| 00:00:01 |
41| 1 | FAST DUAL | | 1 | 2 (0)| 00:00:01 |
42-----------------------------------------------------------------
43
44
45Statistics
46----------------------------------------------------------
47 0 recursive calls
48 0 db block gets
49 0 consistent gets
50 0 physical reads
51 0 redo size
52 545 bytes sent via SQL*Net to client
53 519 bytes received via SQL*Net from client
54 2 SQL*Net roundtrips to/from client
55 0 sorts (memory)
56 0 sorts (disk)
57 1 rows processed
58SYS@orclasm > SELECT d.NAMESPACE,d.DEPEND_COUNT,d.SCN,d.INVALIDATIONS FROM V$RESULT_CACHE_OBJECTS d WHERE D.NAMESPACE='PLSQL';
59
60NAMES DEPEND_COUNT SCN INVALIDATIONS
61----- ------------ ---------- -------------
62PLSQL 2 66067285 0
63
64LHR@orclasm > DELETE FROM T_FRC_20170611_LHR WHERE ROWNUM<=1;
65
661 row deleted.
67LHR@orclasm > COMMIT;
68
69Commit complete.
70
71LHR@orclasm >
本文选自《Oracle程序员面试笔试宝典》，作者：李华荣。
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