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【DM版本】:8.1，安全版，DB Version: 0x7000c
【操作系统】：麒麟 V10
【CPU】:16核 ARMv8 CPU
【问题描述】*：
刚从oracle搬迁到云达梦，发现类似如下半连接sql只能走NEST LOOP SEMI JOIN2,在oracle里边0.1s，在达梦要15s多。
类似SQL:
select * from dgd
where exists (select 1
from p, s
where p.id = s.id
and dgd.hsstr || dgd.exstr like s.hs_str || '%');
无论是达梦还是Oracle，HASH算法都不支持连接条件为like。

所以在oracle里边走了filter，但也比达梦的nl要快的多。
怎么让达梦不使用NEST LOOP SEMI JOIN2。

另外补充说明：关联条件均无索引存在，也就是涉及的3个表都是全表扫描。
Oracle里边的表统计信息都是没问题的并没有使用了动态采样技术。

简单创建表进行测试，脚本如下：
create table dgd (hsstr varchar2(20),exstr varchar2(20));
create table p (id number);
create table s (id number,hs_str varchar2(40));
在Oracle里边执行计划：

-----------------------------------------------------------------------------------------------------------------
| Id  | Operation           | Name | Starts | E-Rows | A-Rows |   A-Time   | Buffers |  OMem |  1Mem | Used-Mem |
-----------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT    |      |      1 |        |      0 |00:00:00.01 |       3 |       |       |          |
|*  1 |  FILTER             |      |      1 |        |      0 |00:00:00.01 |       3 |       |       |          |
|   2 |   TABLE ACCESS FULL | DGD  |      1 |      1 |      0 |00:00:00.01 |       3 |       |       |          |
|*  3 |   HASH JOIN         |      |      0 |      1 |      0 |00:00:00.01 |       0 |  1393K|  1393K|          |
|   4 |    TABLE ACCESS FULL| P    |      0 |      1 |      0 |00:00:00.01 |       0 |       |       |          |
|*  5 |    TABLE ACCESS FULL| S    |      0 |      1 |      0 |00:00:00.01 |       0 |       |       |          |
-----------------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - filter( IS NOT NULL)
   3 - access("P"."ID"="S"."ID")
   5 - filter(:B1||:B2 LIKE "S"."HS_STR"||'%')

在达梦里边执行计划：

SQL> alter session set 'MONITOR_SQL_EXEC'=1;
DMSQL executed successfully
used time: 1.810(ms). Execute id is 11707.
SQL> select * from dgd
2   where exists (select 1
3   from p, s
4   where p.id = s.id
5   and dgd.hsstr || dgd.exstr like s.hs_str || '%');
no rows

used time: 0.641(ms). Execute id is 11708.
SQL> set linesize 5000
SQL> et(11708);

LINEID  OP    TIME(US)   PERCENT RANK    SEQ   N_ENTER     MEM_USED(KB)  DISK_USED(KB)  HASH_USED_CELLS   HASH_CONFLICT   DHASH3_USED_CELLS DHASH3_CONFLICT HASH_SAME_VALUE     
------- ----- ---------- ------- ------- ----- ----------- ------------- -------------- ----------------- --------------- ----------------- --------------- --------------------
1       PRJT2 2          1.18%   5       2     2           0             0              0                 0               NULL              NULL            0
2       DLCK  7          4.12%   4       0     2           0             0              0                 0               NULL              NULL            0
3       NLS2  10         5.88%   3       3     2           0             0              0                 0               NULL              NULL            0
4       NSET2 59         34.71%  2       1     2           0             0              0                 0               NULL              NULL            0
5       CSCN2 92         54.12%  1       4     1           0             0              0                 0               NULL              NULL            0

used time: 35.821(ms). Execute id is 11721.

SQL> explain select * from dgd
2   where exists (select 1
3   from p, s
4   where p.id = s.id
5   and dgd.hsstr || dgd.exstr like s.hs_str || '%');

1   #NSET2: [1, 1, 108] 
2     #PRJT2: [1, 1, 108]; exp_num(3), is_atom(FALSE) 
3       #NEST LOOP SEMI JOIN2: [1, 1, 108];  join condition(DGD.HSSTR || DGD.EXSTR LIKE S.HS_STR || '%')[with var]
4         #CSCN2: [1, 1, 108]; INDEX33557305(DGD)
5         #SLCT2: [1, 1, 108]; var4 LIKE S.HS_STR || '%'
6           #HASH2 INNER JOIN: [1, 1, 108];  KEY_NUM(1); KEY(P.ID=S.ID) KEY_NULL_EQU(0)
7             #CSCN2: [1, 1, 30]; INDEX33557306(P)
8             #CSCN2: [1, 1, 78]; INDEX33557308(S)