Help Center/ GaussDB/ Developer Guide(Centralized_2.x)/ Performance Tuning/ SQL Optimization/ Hint-based Tuning/ Hint of Parameterized Paths at the Same Level
Updated on 2023-10-23 GMT+08:00
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Hint of Parameterized Paths at the Same Level

Function

The predpush_same_level hint is used to specify the generation of parameterized paths between tables or MVs at the same level.

Syntax

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

This parameter takes effect only when the predpushforce option in rewrite_rule is enabled.

Examples

Prepare parameters, tables, and indexes.

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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

Run the following statement to view the plan:

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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)

The filter condition t1.a = t2.a is displayed on Join. In this case, predpush_same_level(t1, t2) can be used to push the condition down to the scan operator of t2.

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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)
  • predpush_same_level can specify multiple src parameters in the same condition.
  • If the specified src and dest conditions do not exist or do not meet the parameterized path requirements, this hint does not take effect.
Parent topic: Hint-based Tuning