对于自适应游标共享的一点补充
- WBOY原創
- 2016-06-07 15:23:181269瀏覽
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
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 ----------------------------------------------------- -------- ------------------------------------ ID NUMBER RTYPE VARCHAR2(20) SEL NUMBER SQL> select column_name,histogram from dba_tab_cols where table_name='T2' AND OWNER='EASY1'; COLUMN_NAME HISTOGRAM ------------------------------ --------------- ID NONE RTYPE FREQUENCY SEL NONE SQL> 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-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 .001953155 10 512 .00390631 .00390631 11 1024 .007812619 .007812619 RTYPE COUNT(1) MIN(SEL) MAX(SEL) -------------------- ---------- ---------- ---------- 12 2048 .015625238 .015625238 13 4096 .031250477 .031250477 14 8192 .062500954 .062500954 15 16384 .125001907 .125001907 16 32768 .250003815 .250003815 17 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) ---------- 1610596352 SQL> 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$sql where sql_text like 'select sum(id) from t2%' SQL> / SQL_TEXT I I ------------------------------------------------------------ - - select sum(id) from t2 where rtype=1 N N select sum(id) from t2 where rtype=16 N N是不是具有绑定变量就一定会是bind_sensitive?
SQL> var v varchar2(100) SQL> select sum(id) from t2 where rtype=:v; SUM(ID) ---------- SQL> select sum(id) from t2 where id=:v; SUM(ID) ----------
SQL> l 1* select sql_text,is_bind_sensitive,is_bind_aware from v$sql where sql_text like 'select sum(id) from t2%' SQL> / SQL_TEXT I I ------------------------------------------------------------ - - select sum(id) from t2 where rtype=1 N N select sum(id) from t2 where id=:v N N select sum(id) from t2 where rtype=:v Y N select 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.
- 下面一段文字来自网络，尽快参考
Q: What triggers a cursor to be marked "bind sensitive"?
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_NAME ENDPOINT_NUMBER TO_CHAR(ENDPOINT_VALUE) ENDPOINT_ACTUAL_VALUE ---------- -------------------- --------------- ---------------------------------------- ------------------------------ T1 RTYPE 1 1 T1 RTYPE 3 2 T1 RTYPE 7 3 T1 RTYPE 15 4 T1 RTYPE 31 5 T1 RTYPE 63 6 T1 RTYPE 127 7 T1 RTYPE 255 8 T1 RTYPE 511 9 T1 RTYPE 1023 10 T1 RTYPE 2047 11 TABLE_NAME COLUMN_NAME ENDPOINT_NUMBER TO_CHAR(ENDPOINT_VALUE) ENDPOINT_ACTUAL_VALUE ---------- -------------------- --------------- ---------------------------------------- ------------------------------ T1 RTYPE 4095 12 T1 RTYPE 8191 13 T1 RTYPE 16383 14 T1 RTYPE 131071 17 T1 ID 0 1 T1 SEL 0 .00000762951094834821 T1 ID 1 131071 T1 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 .001953155 10 512 .00390631 .00390631 11 1024 .007812619 .007812619 RTYPE COUNT(1) MIN(SEL) MAX(SEL) ---------- ---------- ---------- ---------- 12 2048 .015625238 .015625238 13 4096 .031250477 .031250477 14 8192 .062500954 .062500954 17 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 := 1 PL/SQL 过程已成功完成。 SQL> select /*+ bind_aware */ sum(id) from t1 where rtype = :vr; SUM(ID) ---------- 1 SQL> @show_sql SQL_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_sel ADDRESS 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 := 2 PL/SQL 过程已成功完成。 SQL> select /*+ bind_aware */ sum(id) from t1 where rtype = :vr; SUM(ID) ---------- 5 SQL> @show_sql SQL_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 Y SQL> @show_sel ADDRESS 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 0.000008 SQL> exec :vr := 1 PL/SQL 过程已成功完成。 SQL> select /*+ bind_aware */ sum(id) from t1 where rtype = :vr; SUM(ID) ---------- 1 SQL> @show_sql SQL_TEXT EXECUTIONS I I I ------------------------------------------------------------ ---------- - - - select /*+ bind_aware */ sum(id) from t1 where rtype = :vr 1 Y Y N select /*+ 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_sel SP2-0310: 无法打开文件 "sho_sel.sql" SQL> @show_sel ADDRESS HASH_VALUE SQL_ID CHILD_NUMBER PREDICATE RANGE_ID LOW HIGH ---------------- ---------- ------------- ------------ ---------------------------------------- ---------- ---------- ---------- 00000000DD40C0E0 2679189014 082txyqgv2bhq 1 =VR 0 0.000007 0.000017 00000000DD40C0E0 2679189014 082txyqgv2bhq 0 =VR 0 0.000007 0.000008 SQL> exec :vr := 3 PL/SQL 过程已成功完成。 SQL> select /*+ bind_aware */ sum(id) from t1 where rtype = :vr; SUM(ID) ---------- 22 SQL> @show_sql SQL_TEXT EXECUTIONS I I I ------------------------------------------------------------ ---------- - - - select /*+ bind_aware */ sum(id) from t1 where rtype = :vr 1 Y Y N select /*+ bind_aware */ sum(id) from t1 where rtype = :vr 2 Y Y N select /*+ bind_aware */ sum(id) from t1 where rtype = :vr 1 Y Y Y SQL> @show_sel ADDRESS HASH_VALUE SQL_ID CHILD_NUMBER PREDICATE RANGE_ID LOW HIGH ---------------- ---------- ------------- ------------ ---------------------------------------- ---------- ---------- ---------- 00000000DD40C0E0 2679189014 082txyqgv2bhq 2 =VR 0 0.000007 0.000034 00000000DD40C0E0 2679189014 082txyqgv2bhq 1 =VR 0 0.000007 0.000017 00000000DD40C0E0 2679189014 082txyqgv2bhq 0 =VR 0 0.000007 0.000008 SQL> exec :vr := 1 PL/SQL 过程已成功完成。 SQL> select /*+ bind_aware */ sum(id) from t1 where rtype = :vr; SUM(ID) ---------- 1 SQL> @show_sql SQL_TEXT EXECUTIONS I I I ------------------------------------------------------------ ---------- - - - select /*+ bind_aware */ sum(id) from t1 where rtype = :vr 1 Y Y N select /*+ bind_aware */ sum(id) from t1 where rtype = :vr 2 Y Y N select /*+ bind_aware */ sum(id) from t1 where rtype = :vr 2 Y Y Y SQL> exec :vr := 8 PL/SQL 过程已成功完成。 SQL> select /*+ bind_aware */ sum(id) from t1 where rtype = :vr; SUM(ID) ---------- 24512 SQL> @show_sql SQL_TEXT EXECUTIONS I I I ------------------------------------------------------------ ---------- - - - select /*+ bind_aware */ sum(id) from t1 where rtype = :vr 1 Y Y N select /*+ bind_aware */ sum(id) from t1 where rtype = :vr 2 Y Y N select /*+ bind_aware */ sum(id) from t1 where rtype = :vr 2 Y Y N select /*+ bind_aware */ sum(id) from t1 where rtype = :vr 1 Y Y Y SQL> @show_sel ADDRESS HASH_VALUE SQL_ID CHILD_NUMBER PREDICATE RANGE_ID LOW HIGH ---------------- ---------- ------------- ------------ ---------------------------------------- ---------- ---------- ---------- 00000000DD40C0E0 2679189014 082txyqgv2bhq 3 =VR 0 0.000007 0.001074 00000000DD40C0E0 2679189014 082txyqgv2bhq 2 =VR 0 0.000007 0.000034 00000000DD40C0E0 2679189014 082txyqgv2bhq 1 =VR 0 0.000007 0.000017 00000000DD40C0E0 2679189014 082txyqgv2bhq 0 =VR 0 0.000007 0.000008 SQL> exec :vr := 14 PL/SQL 过程已成功完成。 SQL> select /*+ bind_aware */ sum(id) from t1 where rtype = :vr; SUM(ID) ---------- 100659200 SQL> @show_sql SQL_TEXT EXECUTIONS I I I ------------------------------------------------------------ ---------- - - - select /*+ bind_aware */ sum(id) from t1 where rtype = :vr 1 Y Y N select /*+ bind_aware */ sum(id) from t1 where rtype = :vr 2 Y Y N select /*+ bind_aware */ sum(id) from t1 where rtype = :vr 2 Y Y N select /*+ bind_aware */ sum(id) from t1 where rtype = :vr 1 Y Y N select /*+ bind_aware */ sum(id) from t1 where rtype = :vr 1 Y Y Y SQL> @show_sel ADDRESS HASH_VALUE SQL_ID CHILD_NUMBER PREDICATE RANGE_ID LOW HIGH ---------------- ---------- ------------- ------------ ---------------------------------------- ---------- ---------- ---------- 00000000DD40C0E0 2679189014 082txyqgv2bhq 4 =VR 0 0.000007 0.068751 00000000DD40C0E0 2679189014 082txyqgv2bhq 3 =VR 0 0.000007 0.001074 00000000DD40C0E0 2679189014 082txyqgv2bhq 2 =VR 0 0.000007 0.000034 00000000DD40C0E0 2679189014 082txyqgv2bhq 1 =VR 0 0.000007 0.000017 00000000DD40C0E0 2679189014 082txyqgv2bhq 0 =VR 0 0.000007 0.000008 SQL> exec :vr := 15 PL/SQL 过程已成功完成。 SQL> select /*+ bind_aware */ sum(id) from t1 where rtype = :vr; SUM(ID) ---------- SQL> @show_sql SQL_TEXT EXECUTIONS I I I ------------------------------------------------------------ ---------- - - - select /*+ bind_aware */ sum(id) from t1 where rtype = :vr 1 Y Y N select /*+ bind_aware */ sum(id) from t1 where rtype = :vr 2 Y Y N select /*+ bind_aware */ sum(id) from t1 where rtype = :vr 2 Y Y N select /*+ bind_aware */ sum(id) from t1 where rtype = :vr 1 Y Y N select /*+ bind_aware */ sum(id) from t1 where rtype = :vr 1 Y Y N select /*+ bind_aware */ sum(id) from t1 where rtype = :vr 1 Y Y Y 已选择6行。 SQL> @show_sel ADDRESS 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 0.068751 00000000DD40C0E0 2679189014 082txyqgv2bhq 3 =VR 0 0.000007 0.001074 00000000DD40C0E0 2679189014 082txyqgv2bhq 2 =VR 0 0.000007 0.000034 00000000DD40C0E0 2679189014 082txyqgv2bhq 1 =VR 0 0.000007 0.000017 00000000DD40C0E0 2679189014 082txyqgv2bhq 0 =VR 0 0.000007 0.000008 已选择6行。 SQL> exec :vr := 17 PL/SQL 过程已成功完成。 SQL> select /*+ bind_aware */ sum(id) from t1 where rtype = :vr; SUM(ID) ---------- 8455659520 SQL> @show_sql SQL_TEXT EXECUTIONS I I I ------------------------------------------------------------ ---------- - - - select /*+ bind_aware */ sum(id) from t1 where rtype = :vr 1 Y Y N select /*+ bind_aware */ sum(id) from t1 where rtype = :vr 2 Y Y N select /*+ bind_aware */ sum(id) from t1 where rtype = :vr 2 Y Y N select /*+ bind_aware */ sum(id) from t1 where rtype = :vr 1 Y Y N select /*+ bind_aware */ sum(id) from t1 where rtype = :vr 1 Y Y N select /*+ bind_aware */ sum(id) from t1 where rtype = :vr 1 Y Y Y select /*+ bind_aware */ sum(id) from t1 where rtype = :vr 1 Y Y Y 已选择7行。 SQL> @show_sel ADDRESS 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 0.068751 00000000DD40C0E0 2679189014 082txyqgv2bhq 4 =VR 0 0.000007 0.068751 00000000DD40C0E0 2679189014 082txyqgv2bhq 3 =VR 0 0.000007 0.001074 00000000DD40C0E0 2679189014 082txyqgv2bhq 2 =VR 0 0.000007 0.000034 00000000DD40C0E0 2679189014 082txyqgv2bhq 1 =VR 0 0.000007 0.000017 00000000DD40C0E0 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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