对于自适应游标共享的一点补充
来源：互联网 发布：java中的类是什么意思 编辑：程序博客网 时间：2024/05/16 07:03
关于自适应游标共享请参加：http://blog.csdn.net/yidian815/article/details/17959907
对自适应游标共享的理解，本人认为难点在于对BIND_SENSITIVE 和BIND_AWARE的认识。再来复习一下：
bind_sensitive:oracle认为该语句可能会因为绑定变量的不同而需要采取不同的执行计划，因此需要监控该语句的执行并依据执行结果做出判断。
bind_aware:oracle通过监视已经认定该语句需要针对不同的绑定变量取值采取不同的执行计划。
现在就好出现如下的问题：
oracle认定什么样式的sql语句是bind_sensitve?
对于bind_aware的语句，oracle是如何根据不同的变量取值来选择执行计划的？
在回答这些问题之前，先来看一下测试环境
SQL> desc t2; 名称 是否为空? 类型 ----------------------------------------------------- -------- ------------------------------------ IDNUMBER RTYPEVARCHAR2(20) SELNUMBERSQL> select column_name,histogram from dba_tab_cols where table_name='T2' AND OWNER='EASY1';COLUMN_NAME HISTOGRAM------------------------------ ---------------ID NONERTYPE FREQUENCYSEL NONESQL> select rtype,count(1),min(sel),max(sel) from t2 group by rtype order by 3;RTYPE COUNT(1) MIN(SEL) MAX(SEL)-------------------- ---------- ---------- ----------1 1 7.6295E-06 7.6295E-062 2 .000015259 .0000152593 4 .000030518 .0000305184 8 .000061036 .0000610365 16 .000122072 .0001220726 32 .000244144 .0002441447 64 .000488289 .0004882898 128 .000976577 .0009765779 256 .001953155 .00195315510 512 .00390631 .0039063111 1024 .007812619 .007812619RTYPE COUNT(1) MIN(SEL) MAX(SEL)-------------------- ---------- ---------- ----------12 2048 .015625238 .01562523813 4096 .031250477 .03125047714 8192 .062500954 .06250095415 16384 .125001907 .12500190716 32768 .250003815 .25000381517 65536 .50000763 .50000763sel代表rype在整张表中的选择性。
首先我们猜测第一个问题的答案。
因为bind_sensitive是针对绑定变量的不同取值而论的，因此我们认为只有具有绑定变量的语句才可能是bind_sensitive的。
SQL> select sum(id) from t2 where rtype=16; SUM(ID)----------1610596352SQL> select sum(id) from t2 where rtype=1; SUM(ID)---------- 1
SQL> l 1* select sql_text,is_bind_sensitive,is_bind_aware from v$sql where sql_text like 'select sum(id) from t2%'SQL> /SQL_TEXT I I------------------------------------------------------------ - -select sum(id) from t2 where rtype=1 N Nselect sum(id) from t2 where rtype=16 N N是不是具有绑定变量就一定会是bind_sensitive?
SQL> var v varchar2(100)SQL> select sum(id) from t2 where rtype=:v; SUM(ID)----------SQL> select sum(id) from t2 where id=:v; SUM(ID)----------
SQL> l 1* select sql_text,is_bind_sensitive,is_bind_aware from v$sql where sql_text like 'select sum(id) from t2%'SQL> /SQL_TEXT I I------------------------------------------------------------ - -select sum(id) from t2 where rtype=1 N Nselect sum(id) from t2 where id=:v N Nselect sum(id) from t2 where rtype=:v Y Nselect sum(id) from t2 where rtype=16 N N看来不光需要具有绑定变量，还需要在绑定变量所在列上具有直方图统计信息才可以。当然这是在等值操作的情况下，在其他情况下那？在这里我们不做分析，至少在目前的情况下，oracle对bind_sensitive具有如下限制（官方文档）：
The optimizer has peeked at the bind values to generate selectivity estimates.
A histogram exists on the column containing the bind value.
A: Our goal is to consider many types of predicates where the selectivity can change when the bind value changes. In this first version of the feature, we only handle equality predicates where a histogram exists on the column and range predicates (with or without histogram). We do not currently consider LIKE predicates, but it is on the top of our list for future work.
下面我们来看看bind_aware的语句是如何工作的，为了简化操作，我们直接使用BIND_AWARE hint。关于该hint的使用，有如下解释：
From 11.1.0.7 onward it is possible to skip the monitoring that is required to detect bind-sensitive queries by using the BIND_AWARE hint. In the following example, the presence of the hint tells the optimizer that we believe the query is bind-sensitive, so it should use bind-aware cursor sharing from the first execution.
SELECT /*+ BIND_AWARE */ MAX(id) FROM acs_test_tab WHERE record_type = :l_record_type;
The hint will only work if the query uses bind variables in WHERE clause predicates referencing columns with histograms.
There is also a NO_BIND_AWARE hint that tells the optimizer to ignore bind-sensitive queries, effectively hiding the query from the adaptive cursor sharing functionality.
Bind-aware cursor sharing has a small overhead associated with it, which is why Oracle use the "adaptive" approach to identifying queries that would benefit from bind-aware cursor sharing. Adding the hint to queries that will not benefit from it is a waste.
在进一步实验之前，创建表t1
SQL> create table t1 as select * from t2 where 1 =2;表已创建。SQL> alter table t1 modify rtype number;表已更改。SQL> insert into t1 select * from t2;已创建 131071 行。SQL> commit;提交完成。SQL> create index i1 on t1(rtype);索引已创建。
SQL> exec dbms_stats.gather_table_stats(user,'t1',cascade=>true,estimate_percent=>null,method_opt=>'for all columns size auto,for columns rtype size 40');PL/SQL 过程已成功完成。SQL> select table_name,column_name,endpoint_number,to_char(endpoint_value),endpoint_actual_value from user_histograms where table_name='T1';TABLE_NAME COLUMN_NAMEENDPOINT_NUMBER TO_CHAR(ENDPOINT_VALUE) ENDPOINT_ACTUAL_VALUE---------- -------------------- --------------- ---------------------------------------- ------------------------------T1 RTYPE 1 1T1 RTYPE 3 2T1 RTYPE 7 3T1 RTYPE 15 4T1 RTYPE 31 5T1 RTYPE 63 6T1 RTYPE 127 7T1 RTYPE 255 8T1 RTYPE 511 9T1 RTYPE 1023 10T1 RTYPE 2047 11TABLE_NAME COLUMN_NAMEENDPOINT_NUMBER TO_CHAR(ENDPOINT_VALUE) ENDPOINT_ACTUAL_VALUE---------- -------------------- --------------- ---------------------------------------- ------------------------------T1 RTYPE 4095 12T1 RTYPE 8191 13T1 RTYPE 16383 14T1 RTYPE 131071 17T1 ID 0 1T1 SEL 0 .00000762951094834821T1 ID 1 131071T1 SEL 1 .87501335164416已选择19行。
SQL> select rtype,count(1),min(sel),max(sel) from t1 group by rtype order by 3; RTYPE COUNT(1)MIN(SEL) MAX(SEL)---------- ---------- ---------- ---------- 1 1 7.6295E-06 7.6295E-06 2 2 .000015259 .000015259 3 4 .000030518 .000030518 4 8 .000061036 .000061036 5 16 .000122072 .000122072 6 32 .000244144 .000244144 7 64 .000488289 .000488289 8 128 .000976577 .000976577 9 256 .001953155 .00195315510 512 .00390631 .0039063111 1024 .007812619 .007812619 RTYPE COUNT(1)MIN(SEL) MAX(SEL)---------- ---------- ---------- ----------12 2048 .015625238 .01562523813 4096 .031250477 .03125047714 8192 .062500954 .06250095417 114688 .875013352 .875013352对bind_aware的实验过程如下：
SQL> alter system flush shared_pool;系统已更改。SQL> var vr number;SQL> exec :vr := 1PL/SQL 过程已成功完成。SQL> select /*+ bind_aware */ sum(id) from t1 where rtype = :vr; SUM(ID)---------- 1SQL> @show_sqlSQL_TEXT EXECUTIONS I I I------------------------------------------------------------ ---------- - - -select /*+ bind_aware */ sum(id) from t1 where rtype = :vr 1 Y Y Y --由于使用了bind_aware HINT SQL> @show_selADDRESS HASH_VALUE SQL_ID CHILD_NUMBER PREDICATE RANGE_ID LOW HIGH---------------- ---------- ------------- ------------ ---------------------------------------- ---------- ---------- ----------00000000DD40C0E0 2679189014 082txyqgv2bhq 0 =VR 0 0.000007 0.000008 --根据直方图计算出rtype=1的选择性SQL> exec :vr := 2PL/SQL 过程已成功完成。SQL> select /*+ bind_aware */ sum(id) from t1 where rtype = :vr; SUM(ID)---------- 5SQL> @show_sqlSQL_TEXT EXECUTIONS I I I------------------------------------------------------------ ---------- - - -select /*+ bind_aware */ sum(id) from t1 where rtype = :vr 1 Y Y N --逐步淘汰出内存select /*+ bind_aware */ sum(id) from t1 where rtype = :vr 1 Y Y YSQL> @show_selADDRESS HASH_VALUE SQL_ID CHILD_NUMBER PREDICATE RANGE_ID LOW HIGH---------------- ---------- ------------- ------------ ---------------------------------------- ---------- ---------- ----------00000000DD40C0E0 2679189014 082txyqgv2bhq 1 =VR 0 0.000007 0.000017 --由于rtype=2的选择性不再0.00007~0.00008之间，所以生成新的子游标，由于新游标和旧游标的执行计划相同，所以进行合并，子游标0被设置为非共享，逐步淘汰出内存00000000DD40C0E0 2679189014 082txyqgv2bhq 0 =VR 0 0.000007 0.000008SQL> exec :vr := 1PL/SQL 过程已成功完成。SQL> select /*+ bind_aware */ sum(id) from t1 where rtype = :vr; SUM(ID)---------- 1SQL> @show_sqlSQL_TEXT EXECUTIONS I I I------------------------------------------------------------ ---------- - - -select /*+ bind_aware */ sum(id) from t1 where rtype = :vr 1 Y Y Nselect /*+ bind_aware */ sum(id) from t1 where rtype = :vr 2 Y Y Y --使用新的子游标，不再使用0号子游标SQL> @sho_selSP2-0310: 无法打开文件 "sho_sel.sql"SQL> @show_selADDRESS HASH_VALUE SQL_ID CHILD_NUMBER PREDICATE RANGE_ID LOW HIGH---------------- ---------- ------------- ------------ ---------------------------------------- ---------- ---------- ----------00000000DD40C0E0 2679189014 082txyqgv2bhq 1 =VR 0 0.000007 0.00001700000000DD40C0E0 2679189014 082txyqgv2bhq 0 =VR 0 0.000007 0.000008SQL> exec :vr := 3PL/SQL 过程已成功完成。SQL> select /*+ bind_aware */ sum(id) from t1 where rtype = :vr; SUM(ID)----------22SQL> @show_sqlSQL_TEXT EXECUTIONS I I I------------------------------------------------------------ ---------- - - -select /*+ bind_aware */ sum(id) from t1 where rtype = :vr 1 Y Y Nselect /*+ bind_aware */ sum(id) from t1 where rtype = :vr 2 Y Y Nselect /*+ bind_aware */ sum(id) from t1 where rtype = :vr 1 Y Y YSQL> @show_selADDRESS HASH_VALUE SQL_ID CHILD_NUMBER PREDICATE RANGE_ID LOW HIGH---------------- ---------- ------------- ------------ ---------------------------------------- ---------- ---------- ----------00000000DD40C0E0 2679189014 082txyqgv2bhq 2 =VR 0 0.000007 0.00003400000000DD40C0E0 2679189014 082txyqgv2bhq 1 =VR 0 0.000007 0.00001700000000DD40C0E0 2679189014 082txyqgv2bhq 0 =VR 0 0.000007 0.000008SQL> exec :vr := 1PL/SQL 过程已成功完成。SQL> select /*+ bind_aware */ sum(id) from t1 where rtype = :vr; SUM(ID)---------- 1SQL> @show_sqlSQL_TEXT EXECUTIONS I I I------------------------------------------------------------ ---------- - - -select /*+ bind_aware */ sum(id) from t1 where rtype = :vr 1 Y Y Nselect /*+ bind_aware */ sum(id) from t1 where rtype = :vr 2 Y Y Nselect /*+ bind_aware */ sum(id) from t1 where rtype = :vr 2 Y Y YSQL> exec :vr := 8PL/SQL 过程已成功完成。SQL> select /*+ bind_aware */ sum(id) from t1 where rtype = :vr; SUM(ID)---------- 24512SQL> @show_sqlSQL_TEXT EXECUTIONS I I I------------------------------------------------------------ ---------- - - -select /*+ bind_aware */ sum(id) from t1 where rtype = :vr 1 Y Y Nselect /*+ bind_aware */ sum(id) from t1 where rtype = :vr 2 Y Y Nselect /*+ bind_aware */ sum(id) from t1 where rtype = :vr 2 Y Y Nselect /*+ bind_aware */ sum(id) from t1 where rtype = :vr 1 Y Y YSQL> @show_selADDRESS HASH_VALUE SQL_ID CHILD_NUMBER PREDICATE RANGE_ID LOW HIGH---------------- ---------- ------------- ------------ ---------------------------------------- ---------- ---------- ----------00000000DD40C0E0 2679189014 082txyqgv2bhq 3 =VR 0 0.000007 0.00107400000000DD40C0E0 2679189014 082txyqgv2bhq 2 =VR 0 0.000007 0.00003400000000DD40C0E0 2679189014 082txyqgv2bhq 1 =VR 0 0.000007 0.00001700000000DD40C0E0 2679189014 082txyqgv2bhq 0 =VR 0 0.000007 0.000008SQL> exec :vr := 14PL/SQL 过程已成功完成。SQL> select /*+ bind_aware */ sum(id) from t1 where rtype = :vr; SUM(ID)---------- 100659200SQL> @show_sqlSQL_TEXT EXECUTIONS I I I------------------------------------------------------------ ---------- - - -select /*+ bind_aware */ sum(id) from t1 where rtype = :vr 1 Y Y Nselect /*+ bind_aware */ sum(id) from t1 where rtype = :vr 2 Y Y Nselect /*+ bind_aware */ sum(id) from t1 where rtype = :vr 2 Y Y Nselect /*+ bind_aware */ sum(id) from t1 where rtype = :vr 1 Y Y Nselect /*+ bind_aware */ sum(id) from t1 where rtype = :vr 1 Y Y YSQL> @show_selADDRESS HASH_VALUE SQL_ID CHILD_NUMBER PREDICATE RANGE_ID LOW HIGH---------------- ---------- ------------- ------------ ---------------------------------------- ---------- ---------- ----------00000000DD40C0E0 2679189014 082txyqgv2bhq 4 =VR 0 0.000007 0.06875100000000DD40C0E0 2679189014 082txyqgv2bhq 3 =VR 0 0.000007 0.00107400000000DD40C0E0 2679189014 082txyqgv2bhq 2 =VR 0 0.000007 0.00003400000000DD40C0E0 2679189014 082txyqgv2bhq 1 =VR 0 0.000007 0.00001700000000DD40C0E0 2679189014 082txyqgv2bhq 0 =VR 0 0.000007 0.000008SQL> exec :vr := 15PL/SQL 过程已成功完成。SQL> select /*+ bind_aware */ sum(id) from t1 where rtype = :vr; SUM(ID)----------SQL> @show_sqlSQL_TEXT EXECUTIONS I I I------------------------------------------------------------ ---------- - - -select /*+ bind_aware */ sum(id) from t1 where rtype = :vr 1 Y Y Nselect /*+ bind_aware */ sum(id) from t1 where rtype = :vr 2 Y Y Nselect /*+ bind_aware */ sum(id) from t1 where rtype = :vr 2 Y Y Nselect /*+ bind_aware */ sum(id) from t1 where rtype = :vr 1 Y Y Nselect /*+ bind_aware */ sum(id) from t1 where rtype = :vr 1 Y Y Nselect /*+ bind_aware */ sum(id) from t1 where rtype = :vr 1 Y Y Y已选择6行。SQL> @show_selADDRESS HASH_VALUE SQL_ID CHILD_NUMBER PREDICATE RANGE_ID LOW HIGH---------------- ---------- ------------- ------------ ---------------------------------------- ---------- ---------- ----------00000000DD40C0E0 2679189014 082txyqgv2bhq 5 =VR 0 0.000003 0.068751 --由于15不存在，所以选择性向下扩充00000000DD40C0E0 2679189014 082txyqgv2bhq 4 =VR 0 0.000007 0.06875100000000DD40C0E0 2679189014 082txyqgv2bhq 3 =VR 0 0.000007 0.00107400000000DD40C0E0 2679189014 082txyqgv2bhq 2 =VR 0 0.000007 0.00003400000000DD40C0E0 2679189014 082txyqgv2bhq 1 =VR 0 0.000007 0.00001700000000DD40C0E0 2679189014 082txyqgv2bhq 0 =VR 0 0.000007 0.000008已选择6行。SQL> exec :vr := 17PL/SQL 过程已成功完成。SQL> select /*+ bind_aware */ sum(id) from t1 where rtype = :vr; SUM(ID)----------8455659520SQL> @show_sqlSQL_TEXT EXECUTIONS I I I------------------------------------------------------------ ---------- - - -select /*+ bind_aware */ sum(id) from t1 where rtype = :vr 1 Y Y Nselect /*+ bind_aware */ sum(id) from t1 where rtype = :vr 2 Y Y Nselect /*+ bind_aware */ sum(id) from t1 where rtype = :vr 2 Y Y Nselect /*+ bind_aware */ sum(id) from t1 where rtype = :vr 1 Y Y Nselect /*+ bind_aware */ sum(id) from t1 where rtype = :vr 1 Y Y Nselect /*+ bind_aware */ sum(id) from t1 where rtype = :vr 1 Y Y Yselect /*+ bind_aware */ sum(id) from t1 where rtype = :vr 1 Y Y Y已选择7行。SQL> @show_selADDRESS HASH_VALUE SQL_ID CHILD_NUMBER PREDICATE RANGE_ID LOW HIGH---------------- ---------- ------------- ------------ ---------------------------------------- ---------- ---------- ----------00000000DD40C0E0 2679189014 082txyqgv2bhq 6 =VR 0 0.787503 0.962503 --新的子游标的执行计划于旧子游标不同，所以均保留00000000DD40C0E0 2679189014 082txyqgv2bhq 5 =VR 0 0.000003 0.06875100000000DD40C0E0 2679189014 082txyqgv2bhq 4 =VR 0 0.000007 0.06875100000000DD40C0E0 2679189014 082txyqgv2bhq 3 =VR 0 0.000007 0.00107400000000DD40C0E0 2679189014 082txyqgv2bhq 2 =VR 0 0.000007 0.00003400000000DD40C0E0 2679189014 082txyqgv2bhq 1 =VR 0 0.000007 0.00001700000000DD40C0E0 2679189014 082txyqgv2bhq 0 =VR 0 0.000007 0.000008已选择7行。由此可见，计算绑定变量的谓词选择性在bind_aware中扮演者重要角色
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