对于自适应游标共享的一点补充 - 数据库编程

redicates 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

对于自适应游标共享的一点补充(二)