同层参数化路径的Hint

功能描述

通过predpush_same_level Hint来指定同层表或物化视图之间参数化路径生成。

语法格式

    
         predpush_same_level(src, dest)
predpush_same_level(src1 src2 ..., dest)

本参数仅在rewrite_rule中的predpushforce选项打开时生效。

示例

准备参数和表及索引：

    
         openGauss=# set rewrite_rule = 'predpushforce';
SET
openGauss=# create table t1(a int, b int);
CREATE TABLE
openGauss=# create table t2(a int, b int);
CREATE TABLE
openGauss=# create index idx1 on t1(a);
CREATE INDEX
openGauss=# create index idx2 on t2(a);
CREATE INDEX

执行语句查看计划：

    
         openGauss=# explain select * from t1, t2 where t1.a = t2.a;
                            QUERY PLAN
------------------------------------------------------------------
 Hash Join  (cost=27.50..56.25 rows=1000 width=16)
   Hash Cond: (t1.a = t2.a)
   ->  Seq Scan on t1  (cost=0.00..15.00 rows=1000 width=8)
   ->  Hash  (cost=15.00..15.00 rows=1000 width=8)
         ->  Seq Scan on t2  (cost=0.00..15.00 rows=1000 width=8)
(5 rows)

可以看到t1.a = t2.a条件过滤在Join上面，此时可以通过predpush_same_level(t1, t2)将条件下推至t2的扫描算子上：

    
         openGauss=# explain select /*+predpush_same_level(t1, t2)*/ * from t1, t2 where t1.a = t2.a;
                             QUERY PLAN
---------------------------------------------------------------------
 Nested Loop  (cost=0.00..335.00 rows=1000 width=16)
   ->  Seq Scan on t1  (cost=0.00..15.00 rows=1000 width=8)
   ->  Index Scan using idx2 on t2  (cost=0.00..0.31 rows=1 width=8)
         Index Cond: (a = t1.a)
(4 rows)