LightDB-no_push_pred hint

对于不能合并的视图，有两种连接方式，一种是不下推连接谓词，与视图的结果集进行连接；一种是下推连接谓词，对每个驱动行都获取对应的视图结果集（nestloop）。
no_push_pred hint 在oracle 中用于不下推连接谓词到不能合并的视图中。

下面介绍使用示例：

示例

初始化

create table test_no_push_pred1 (key1 int primary key, key2 int);
create table test_no_push_pred2 (key1 int primary key, key2 int);
create table test_no_push_pred3 (key1 int primary key, key2 int);

set enable_hashjoin to off;
set enable_mergejoin to off;

案例

lightdb@test_o=# EXPLAIN (COSTS false)
lightdb@test_o-# select /*+leading(a)*/* from test_no_push_pred1 a join (select * from test_no_push_pred2 order by key1 limit 1) b on a.key1 =b.key2;
                                    QUERY PLAN                              
      
----------------------------------------------------------------------------
------
 Nested Loop
   Join Filter: (a.key1 = test_no_push_pred2.key2)
   ->  Seq Scan on test_no_push_pred1 a @"lt#1"
   ->  Materialize
         ->  Limit
               ->  Index Scan using test_no_push_pred2_pkey on test_no_push_pred2
(6 rows)

lightdb@test_o=# EXPLAIN (COSTS false)
lightdb@test_o-# select /*+leading(a) no_push_pred(b)*/* from test_no_push_pred1 a join (select * from test_no_push_pred2 order by key1 limit 1) b on a.key1 =b.key2;
                                    QUERY PLAN                              
      
----------------------------------------------------------------------------
------
 Nested Loop
   Join Filter: (a.key1 = test_no_push_pred2.key2)
   ->  Seq Scan on test_no_push_pred1 a @"lt#1"
   ->  Materialize
         ->  Limit
               ->  Index Scan using test_no_push_pred2_pkey on test_no_push_pred2
(6 rows)

lightdb@test_o=# EXPLAIN (COSTS false)
lightdb@test_o-# select /*+leading(a) no_push_pred(@qb)*/* from test_no_push_pred1 a join (select/*+ qb_name(qb)*/ * from test_no_push_pred2 order by key1 limit 1) b on a.key1 =b.key2;
                                      QUERY PLAN                            
          
----------------------------------------------------------------------------
----------
 Nested Loop
   Join Filter: (a.key1 = test_no_push_pred2.key2)
   ->  Seq Scan on test_no_push_pred1 a @"lt#0"
   ->  Materialize
         ->  Limit
               ->  Index Scan using test_no_push_pred2_pkey on test_no_push_pred2 @qb
(6 rows)

lightdb@test_o=# EXPLAIN (COSTS false)
lightdb@test_o-# select /*+leading(a) */* from test_no_push_pred1 a join (select/*+ no_push_pred*/ * from test_no_push_pred2 order by key1 limit 1) b on a.key1 =b.key2;
                                        QUERY PLAN                          
              
----------------------------------------------------------------------------
--------------
 Nested Loop
   Join Filter: (a.key1 = test_no_push_pred2.key2)
   ->  Seq Scan on test_no_push_pred1 a @"lt#1"
   ->  Materialize
         ->  Limit
               ->  Index Scan using test_no_push_pred2_pkey on test_no_push_pred2 @"lt#0"
(6 rows)