Help Center/ GaussDB/ Developer Guide(Centralized_8.x)/ SQL Optimization/ Optimization Cases/ Case: Rewriting SQL Statements to Eliminate Subqueries
Updated on 2024-08-20 GMT+08:00
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Case: Rewriting SQL Statements to Eliminate Subqueries

Symptom

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select 
    1,
    (select count(*) from normal_date n where n.id = a.id) as GZCS 
from normal_date a;

This SQL performance is poor. SubPlan exists in the execution plan as follows:

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                                                              QUERY PLAN
---------------------------------------------------------------------------------------------------------------------------------------
 Seq Scan on normal_date a  (cost=0.00..888118.42 rows=5129 width=4) (actual time=2.394..22194.907 rows=10000 loops=1)
   SubPlan 1
     ->  Aggregate  (cost=173.12..173.12 rows=1 width=8) (actual time=22179.496..22179.942 rows=10000 loops=10000)
           ->  Seq Scan on normal_date n  (cost=0.00..173.11 rows=1 width=0) (actual time=11279.349..22159.608 rows=10000 loops=10000)
                 Filter: (id = a.id)
                 Rows Removed by Filter: 99990000
 Total runtime: 22196.415 ms
(7 rows)

Optimization

The core of this optimization is to eliminate subqueries. Based on the service scenario analysis, a.id is not NULL. In terms of SQL syntax, you can rewrite the SQL statement as follows:

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select 
count(*) 
from normal_date n, normal_date a
where n.id = a.id
group by  a.id;
The plan is as follows:
                                                            QUERY PLAN
----------------------------------------------------------------------------------------------------------------------------------
 HashAggregate  (cost=480.86..532.15 rows=5129 width=12) (actual time=21.539..24.356 rows=10000 loops=1)
   Group By Key: a.id
   ->  Hash Join  (cost=224.40..455.22 rows=5129 width=4) (actual time=6.402..13.484 rows=10000 loops=1)
         Hash Cond: (n.id = a.id)
         ->  Seq Scan on normal_date n  (cost=0.00..160.29 rows=5129 width=4) (actual time=0.087..1.459 rows=10000 loops=1)
         ->  Hash  (cost=160.29..160.29 rows=5129 width=4) (actual time=6.065..6.065 rows=10000 loops=1)
                Buckets: 32768  Batches: 1  Memory Usage: 352kB
               ->  Seq Scan on normal_date a  (cost=0.00..160.29 rows=5129 width=4) (actual time=0.046..2.738 rows=10000 loops=1)
 Total runtime: 26.844 ms
(9 rows)

To ensure that the modified statements have the same functions, NOT NULL is added to normal_date.id.

Parent topic: Optimization Cases