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Oracle cursor_sharing 参数详解

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一. 官網(wǎng)的說明

http://download.oracle.com/docs/cd/E11882_01/server.112/e17110/initparams042.htm#REFRN10025

1.1 CURSOR_SHARING

Property	Description
Parameter type	String
Syntax	CURSOR_SHARING = { SIMILAR \| EXACT \| FORCE }
Default value	EXACT
Modifiable	ALTER SESSION, ALTER SYSTEM
Basic	No

?????? CURSOR_SHARING determines what kind of SQL statements can share the same cursors.

Values:

（1）FORCE

?????? Allows the creation of a new cursor if sharing an existing cursor, or if the cursor plan is not optimal.

（2）SIMILAR

?????? Causes statements that may differ in some literals, but are otherwise identical, to share a cursor, unless the literals affect either the meaning of the statement or the degree to which the plan is optimized.

（3）EXACT

?????? Only allows statements with identical text to share the same cursor.

?????? --只有SQL 語句完全相同的情況下，才會使用相同的cursor，即執(zhí)行計劃。

Notes:

?????? （1）If you set CURSOR_SHARING, then Oracle recommends the FORCE setting unless you are in a DSS environment. FORCE limits the growth of child cursors that can occur when the setting is SIMILAR.

?????? （2）The value of the CURSOR_SHARING parameter has performance implications. Refer to Oracle Database Performance Tuning Guide before setting this parameter.

1.2 When to Set CURSOR_SHARING to a Nondefault Value

?????? The best practice is to write sharable SQL and use the default of EXACT for CURSOR_SHARING. However, for applications with many similar statements, setting CURSOR_SHARING can significantly improve cursor sharing, resulting in reduced memory usage, faster parses, and reduced latch contention. Consider this approach when statements in the shared pool differ only in the values of literals, and when response time is poor because of a very high number of library cache misses.

Setting CURSOR_SHARING to FORCE or SIMILAR has the following drawbacks:

?????? （1）The database must perform extra work during the soft parse to find a similar statement in the shared pool.

?????? （2）There is an increase in the maximum lengths (as returned by DESCRIBE) of any selected expressions that contain literals in a SELECT statement. However, the actual length of the data returned does not change.

?????? （3）Star transformation is not supported.

?????? （4）If stored outlines were generated with CURSOR_SHARING set to EXACT, then the database does not use stored outlines generated with literals. To avoid this problem, generate outlines with CURSOR_SHARING set to FORCE or SIMILAR and use the CREATE_STORED_OUTLINES parameter.

?????? When deciding whether to set CURSOR_SHARING to FORCE or SIMILAR, consider the performance implications of each setting.

?????? When CURSOR_SHARING is set to FORCE, the database uses one parent cursor and one child cursor for each distinct SQL statement. The database uses the same plan for each execution of the same statement.

?????? When set to SIMILAR, database behavior depends on the presence of histograms:

?????? （1）Histogram absent for column with system-generated bind value

????????????? Only one parent cursor and one child cursor exists for each distinct SQL statement. In this case, all executions of a SQL statement use the same plan.

?????? （2）Histogram present for column with system-generated bind value

????????????? If the same SQL statement is executed multiple times, each execution has its own child cursor. In this case, the database peeks at bind variable values and create a new child cursor for each distinct value. Thus, each statement execution uses a plan based on the specific literals in the statement.

For example, consider the following statement:

?????? SELECT * FROM hr.employees WHERE employee_id = 101

?????? If FORCE is used, or if SIMILAR is used when no histogram exists, then the database optimizes this statement as if it contained a bind variable and uses bind peeking to estimate cardinality. Statements that differ only in the bind variable share the same execution plan.

?????? If SIMILAR is used, and if a histogram does exist, then the database does not treat the statement as if a bind variable were used. The same query for a different employee may not use the same plan.

?????? If you set CURSOR_SHARING, then Oracle recommends the FORCE setting unless you are in a DSS environment. FORCE limits the growth of child cursors that can occur when the setting is SIMILAR.

?????? Also, function-based indexes may not work when using SIMILAR because the database converts index parameters to bind variables.

?????? For example, if the index is SUBSTR(id,1,3), then the database converts it to SUBSTR("ID",:SYS_B_0,:SYS_B_1)=:id, rendering the index invalid.

Oracle 綁定變量詳解

http://blog.csdn.net/tianlesoftware/archive/2010/09/01/5856430.aspx

二. 測試

2.1 cursor_sharing=exact，這是cursor_sharing的默認值

2.1.1 查看cursor_sharing 值

SYS@anqing2(rac2)> show parameter cursor_sharing

NAME?????????? TYPE????????? VALUE

------------------------- -------------------- ---------------------

cursor_sharing???? ??string???????? EXACT

2.1.2 查看當(dāng)前硬解析值

SYS@anqing2(rac2)> select name,value from v$sysstat where name like '%parse%';

NAME??????????????????????????????? VALUE

------------------------------ ----------

parse time cpu??????????????????? 1882056

parse time elapsed??????????????? 2648194

parse count (total)????????????? 12780229

parse count (hard)??????????????? 9890010（硬解析次數(shù)）

parse count (failures)???????????????? 71

2.1.3 執(zhí)行一條select 語句，然后查看硬解析次數(shù)

SYS@anqing2(rac2)> select * from ta where id=168;

??????? ID NAME

---------- ------------------------------

?????? 168 dave

SYS@anqing2(rac2)> select name,value from v$sysstat where name like '%parse%';

NAME??????????????????????????????? VALUE

------------------------------ ----------

parse time cpu??????????????????? 1882061

parse time elapsed??????????????? 2648196

parse count (total)????????????? 12780360

parse count (hard)??????????????? 9890021

parse count (failures)???????????????? 71

-- 這里硬解析的次數(shù)加一，因為之前SQL 沒有解析過，所以需要進行硬解析之后才能執(zhí)行。

2.1.4 執(zhí)行與之前類似的SQL，謂詞值不一樣

SYS@anqing2(rac2)> select * from ta where id=198;

??????? ID NAME

---------- ------------------------------

?????? 198 dave

SYS@anqing2(rac2)> select name,value from v$sysstat where name like '%parse%';

NAME??????????????????????????????? VALUE

------------------------------ ----------

parse time cpu??????????????????? 1882061

parse time elapsed??????????????? 2648196

parse count (total)????????????? 12780482

parse count (hard)????? ??????????9890022

parse count (failures)???????????????? 71

-- 硬解析次數(shù)又加1了，沒有重用之前的執(zhí)行計劃

2.1.5 執(zhí)行相同的SQL 語句

SYS@anqing2(rac2)> select * from ta where id=198;

??????? ID NAME

---------- ------------------------------

?????? 198 dave

SYS@anqing2(rac2)> select name,value from v$sysstat where name like '%parse%';

NAME? ??????????????????????????????VALUE

------------------------------ ----------

parse time cpu??????????????????? 1882061

parse time elapsed??????????????? 2648196

parse count (total)????????????? 12780543

parse count (hard)??????????????? 9890022

parse count (failures)???????????????? 71

-- 測試硬解析沒有變化。重用之前的cursor。

總結(jié)：

?????? 在這種模式下,只有SQL 語句完全相同的情況下，才會使用相同的cursor，即執(zhí)行計劃。

?????? 這種模式下，表有統(tǒng)計信息和沒有統(tǒng)計信息的執(zhí)行計劃是有出入的。所以該模式下的表，需要定期的去收集統(tǒng)計信息。

2.2 cursor_sharing=force

--修改cursor_sharing 值

SYS@anqing2(rac2)> alter session set cursor_sharing='force';

Session altered.

SYS@anqing2(rac2)> show parameter cursor_sharing

NAME?????????? TYPE?????????? VALUE

------------------------- --------------------- ---------------

cursor_sharing???? string????????? force

--查看硬解析次數(shù)

SYS@anqing2(rac2)> select name,value from v$sysstat where name like '%parse%';

NAME??????????????????????????????? VALUE

------------------------------ ----------

parse time cpu??????????????????? 1882075

parse time elapsed??????????????? 2648219

parse count (total)????????????? 12782090

parse count (hard)??? ????????????9890067 （硬解析次數(shù)）

parse count (failures)???????????????? 71

-- select 查詢

SYS@anqing2(rac2)> select * from ta where id=88;

??????? ID NAME

---------- ------------------------------

??????? 88 dave

SYS@anqing2(rac2)> select name,value from v$sysstat where name like '%parse%';

NAME??????????????????????????????? VALUE

------------------------------ ----------

parse time cpu??????????????????? 1882075

parse time elapsed??????????????? 2648219

parse count (total)????????????? 12782215

parse count (hard)??????????????? 9890068 -- 硬解析次數(shù)加一

parse count (failures)???????????????? 71

-- 執(zhí)行相同的select，但謂詞值不一樣

SYS@anqing2(rac2)> select * from ta where id=99;

??????? ID NAME

---------- ------------------------------

??????? 99 dave

SYS@anqing2(rac2)> select name,value from v$sysstat where name like '%parse%';

NAME??????????????????????????????? VALUE

------------------------------ ----------

parse time cpu??????????????????? 1882075

parse time elapsed??????????????? 2648219

parse count (total)????????????? 12782285

parse count (hard) ???????????????9890068

parse count (failures)???????????????? 71

--注意，這里的硬解析次數(shù)沒有變化，這個就是force 的作用。只要sql語句相同，不管謂詞值是否相同，都會當(dāng)成相同的sql，重用之前的cursor，不會進行硬解析。

-- 查看child cursor 信息

SYS@anqing2(rac2)> select sql_text,child_number from v$sql where sql_text like 'select * from ta where%';

SQL_TEXT???????????????????????????????? CHILD_NUMBER

---------------------------------------- ------------

select * from ta where id=:"SYS_B_0" ???????????????0

select * from ta where id=:"SYS_B_0"??????????????? 1

select * from ta where id=:"SYS_B_0"??????????????? 2

注意：

?????? 對于相同的SQL，oracle 在這里將不同的謂詞值改成了變量，這樣SQL_TEXT 就相同，正常情況下，應(yīng)該使用同一個cursor，即執(zhí)行計劃，但是在我上面的查詢中，Oracle 并沒有重用，而是重新生成了一個child_cursor.? 這就說明Oracle 認為這個cursor 并不是最優(yōu)的，所有重新生成了一個。

可以通過如下SQL 查看為什么沒有重用之前的cursor：

?????? SQL>select * from v$sql_shared_cursor where sql_id='c9swtz4spq3xz';

如果這里有Y，就是導(dǎo)致不能重用的原因。

Oracle 性能相關(guān)的幾個視圖和參數(shù)

http://blog.csdn.net/tianlesoftware/archive/2010/09/06/5867276.aspx

總結(jié)：

?????? Allows the creation of a new cursor if sharing an existing cursor, or if the cursor plan is not optimal.

?????? FORCE limits the growth of child cursors that can occur when the setting is SIMILAR.

?????? 當(dāng)cursor_sharing 設(shè)置為force時， Oracle 會把相同SQL的不同謂詞值轉(zhuǎn)換成變量，這樣SQL_TEXT就看上去一樣。 Oracle 就會使用一個相同的cursor。這樣他們的執(zhí)行計劃也是一樣的。

?????? 當(dāng)Oracle 認為存在的cursor 不是最優(yōu)的時候，就會重新創(chuàng)建一個child cursor，而不重用之前的已經(jīng)存在cursor。可以通過v$sql_shared_cursor 查看為什么沒有重用。

?????? 這樣就會和我們上面查詢的一樣，會有多個child cursor，但是他們的parent cursor是一樣的。這個child cursor 不是無限增常的，force 和similar 都會限制child cursor 的增長。

2.3 cursor_sharing=similar

?????? 在這種模式下，對表做統(tǒng)計和沒做統(tǒng)計分析是不一樣的。有關(guān)統(tǒng)計信息，參考我的Blog：

?????? Oracle Statistic 統(tǒng)計信息小結(jié)

?????? http://blog.csdn.net/tianlesoftware/archive/2009/10/14/4668723.aspx

2.3.1 沒有統(tǒng)計信息，沒有直方圖的情況

-- 查詢ta的統(tǒng)計信息

SYS@anqing2(rac2)> exec dbms_stats.delete_table_stats('SYS','TA');

PL/SQL procedure successfully completed.

-- 修改cursor_sharing 模式

SYS@anqing2(rac2)> alter session set cursor_sharing='similar';

Session altered.

SYS@anqing2(rac2)> show parameter cursor_sharing

NAME????????? TYPE?????? VALUE

---------------------- ------------------ ----------------

cursor_sharing??? string??????? similar

在similer模式下，且沒有直方圖，等于force 模式。看測試：

-- 查看硬解析次數(shù)

SYS@anqing2(rac2)>? select name,value from v$sysstat where name like '%parse%';

NAME??????????????????????????????? VALUE

------------------------------ ----------

parse time cpu??????????????????? 1882222

parse time elapsed??????????????? 2648406

parse count (total)????????????? 12793472

parse count (hard)??????????????? 9890538

parse count (failures)???????????????? 74

--做一次select 查詢

SYS@anqing2(rac2)> select * from ta where id=238;

??????? ID NAME

---------- ------------------------------

?????? 238 dave

SYS@anqing2(rac2)> select name,value from v$sysstat where name like '%parse%';

NAME??????????????????????????????? VALUE

------------------------------ ----------

parse time cpu??????????????????? 1882222

parse time elapsed??????????????? 2648406

parse count (total)????????????? 12793476

parse count (hard)??????????????? 9890539 硬解析加一

parse count (failures)???????????????? 74

-- 在次select

SYS@anqing2(rac2)> select * from ta where id=2397;

??????? ID NAME

---------- ----------

????? 2397 dave

SYS@anqing2(rac2)> select name,value from v$sysstat where name like '%parse%';

NAME?? ?????????????????????????????VALUE

------------------------------ ----------

parse time cpu??????????????????? 1882337

parse time elapsed??????????????? 2648558

parse count (total)????????????? 12823605

parse count (hard)??????????????? 9890751

parse count (failures)???????????????? 75

-- 這里還是有增加硬解析，有點奇怪

-- 查看對應(yīng)的SQL

SYS@anqing2(rac2)> select sql_text,child_number from v$sql where sql_text like 'select * from ta where%';

SQL_TEXT???????????????????????????????? CHILD_NUMBER

---------------------------------------- ------------

select * from ta where id=238 ??????????????????????0

select * from ta where id=2396?? ???????????????????0

select * from ta where id=2391????????????????????? 0

select * from ta where id=2397????????????????????? 0

?????? 這里并沒有將謂詞值轉(zhuǎn)換成變量，也就是說similar 沒有生效，沒有達到force的效果。

在如下Blog 上有對Force 沒有生效的一個討論：

http://forums.oracle.com/forums/message.jspa?messageID=3569923#3569923

摘取一段如下：

?????? Interesting. You may need to flush the shared pool "harder"... Try your testcase again, but issue the ALTER SYSTEM FLUSH SHARED_POOL twice in a row before changing the cursor_sharing setting. It looks like that the statement doesn't show up in V$SQLAREA any longer but still seems not to be flushed entirely and I can find some left-overs of it in the underlying X$KGL library cache fixed tables with the same hash value. After the second "flush shared pool" these are gone and then the cursor sharing works as expected.

?????? 解決方法就是在指定cursor-_sharing 之前，執(zhí)行2次flush shared_pool. 測試一下看看。

SYS@anqing2(rac2)> alter system flush shared_pool;

System altered.

SYS@anqing2(rac2)> alter system flush shared_pool;

System altered.

SYS@anqing2(rac2)> alter session set cursor_sharing=similar;

Session altered.

SYS@anqing2(rac2)> select name,value from v$sysstat where name like '%parse%';

NAME??????????????????????????????? VALUE

------------------------------ ----------

parse count (hard)??????????????? 9890936

SYS@anqing2(rac2)> select * from ta where id=232;

??????? ID NAME

---------- ----------

?????? 232 dave

SYS@anqing2(rac2)> select name,value from v$sysstat where name like '%parse%';

NAME??????????????????????????????? VALUE

------------------------------ ----------

parse count (hard)??????????????? 9890946

--從硬解析來看，數(shù)量有增加

-- 在查詢一次

SYS@anqing2(rac2)> select * from ta where id=23218;

??????? ID NAME

---------- ----------

???? 23218 dave

SYS@anqing2(rac2)> select name,value from v$sysstat where name like '%parse%';

NAME??????????????????????????????? VALUE

------------------------------ ----------

parse count (hard)??????????????? 9890954

--硬解析又增加了，如果說只有這一條SQL執(zhí)行，不會增加硬解析。這個是其他的SQL 帶來的硬解析，我們可以查看v$sql驗證：

SYS@anqing2(rac2)> select sql_text,child_number,executions from v$sql where sql_text like 'select * from ta where%';

SQL_TEXT????????????????? CHILD_NUMBER EXECUTIONS

---------------------------------------- ------------ ----------

select * from ta where id=:"SYS_B_0" ????????0???????? ?2

-- 在這里，已經(jīng)使轉(zhuǎn)成成了變量，并且執(zhí)行了2次。這說明，我們的force是生效的。

2.3.2? 有統(tǒng)計信息和直方圖

?????? 當(dāng)cursor_sharing 設(shè)置為similar，并且表上存在histograms時，這時候的similar 等于默認值 exact.? 下面我們驗證一下。

-- 分析表

SYS@anqing2(rac2)> exec dbms_stats.gather_table_stats('SYS','TA', METHOD_OPT =>'FOR COLUMNS SIZE 10 ID');

PL/SQL procedure successfully completed.

--這里我手工的指定了收集的類型。

-- 查看直方圖: histograms

這部分內(nèi)容，具體可以參考我的blog：? Oracle Statistic 統(tǒng)計信息小結(jié)

?????? http://blog.csdn.net/tianlesoftware/archive/2009/10/14/4668723.aspx

SYS@anqing2(rac2)> SELECT column_name, num_distinct, num_buckets, histogram

? 2? FROM DBA_TAB_COL_STATISTICS WHERE table_name = 'TA' AND column_name = 'ID';

COLUMN_NAME?????? NUM_DISTINCT NUM_BUCKETS HISTOGRAM

------------------------------ ------------ ----------- ---------------

ID??????????????????? 1005510? ????????10 HEIGHT BALANCED

-- 先flush shared_pool 兩次

SYS@anqing2(rac2)>? alter system flush shared_pool;

System altered.

SYS@anqing2(rac2)>? alter system flush shared_pool;

System altered.

SYS@anqing2(rac2)> show parameter cursor_sharing

NAME????????? TYPE?????? VALUE

------------------------ -------------------------------- ----------

cursor_sharing???? string????? SIMILAR

-- select 查詢和對應(yīng)的解析

SYS@anqing2(rac2)> select sql_text,child_number,executions from v$sql where sql_text like 'select count(*) from ta %';

SQL_TEXT??????????????? CHILD_NUMBER EXECUTIONS

--------------------------------------------- ------------ ----------

select count(*) from ta where id>:"SYS_B_0"????????????? 0????????? 3

SYS@anqing2(rac2)> select count(*) from ta where id>10000;

? COUNT(*)

----------

??? 990000

SYS@anqing2(rac2)> select sql_text,child_number,executions from v$sql where sql_text like 'select count(*) from ta %';

SQL_TEXT????????????????????????????????????? CHILD_NUMBER EXECUTIONS

--------------------------------------------- ------------ ----------

select count(*) from ta where id>:"SYS_B_0"????????????? 0????????? 3

select count(*) from ta where id>:"SYS_B_0"????????????? 1????????? 1

--因為ID值和之前的不一樣，所以沒有重用之前的cursor，而是重新創(chuàng)建了一個

SYS@anqing2(rac2)> select count(*) from ta where id>100;

? COUNT(*)

----------

??? 999900

SYS@anqing2(rac2)> select sql_text,child_number,executions from v$sql where sql_text like 'select count(*) from ta %';

SQL_TEXT????????????????????????????????????? CHILD_NUMBER EXECUTIONS

--------------------------------------------- ------------ ----------

select count(*) from ta where id>:"SYS_B_0"????????????? 0????????? 3

select count(*) from ta where id>:"SYS_B_0"????????????? 1????????? 1

select count(*) from ta where id>:"SYS_B_0"??????????? ??2????????? 1

-- 這次ID 值又不一樣，又重新生成了一個cursor

..... 中間又生成了N 次...

SYS@anqing2(rac2)> select count(*) from ta where id>90808;

? COUNT(*)

----------

??? 909192

SYS@anqing2(rac2)> select sql_text,child_number,executions from v$sql where sql_text like 'select count(*) from ta %';

SQL_TEXT????????????????????????????????????? CHILD_NUMBER EXECUTIONS

--------------------------------------------- ------------ ----------

select count(*) from ta where id>:"SYS_B_0"????????????? 0????????? 3

select count(*) from ta where id>:"SYS_B_0"????????????? 1????????? 1

select count(*) from ta where id>:"SYS_B_0"????????????? 2????????? 1

select count(*) from ta where id>:"SYS_B_0"????????????? 3????????? 1

select count(*) from ta where id>:"SYS_B_0"????????????? 4????????? 1

select count(*) from ta where id>:"SYS_B_0"????????????? 5????????? 1

select count(*) from ta where id>:"SYS_B_0"????????????? 6????????? 1

select count(*) from ta where id>:"SYS_B_0"????????????? 7????????? 1

-- 這個就像我們開始說的，雖然會對變量進行轉(zhuǎn)換，但不會重用，只有謂詞值一樣的時候才重用之前的cursor。