Re: How to improve the performance of my SQL query？

[Erik Wienhold <ewie@xxxxxxxxx>]

> On 20/07/2023 09:09 CEST gzh <gzhcoder@xxxxxxx> wrote:
>
> I'm running into some performance issues with my SQL query.
> The following SQL query is taking a long time to execute.
>
> explain analyze
> select COUNT(ET_CD)
> from TBL_SHA
> WHERE TBL_SHA.MS_CD = '009'
> and TBL_SHA.ETRYS in
> (select TBL_INF.RY_CD
> from TBL_INF
> WHERE TBL_INF.MS_CD = '009'
> AND TBL_INF.RY_CD = '000001'
> )
> ----- Execution Plan -----
> Limit (cost=2738709.57..2738709.58 rows=1 width=8) (actual time=124168.769..124168.771 rows=1 loops=1)
> -> Aggregate (cost=2738709.57..2738709.58 rows=1 width=8) (actual time=124168.767..124168.769 rows=1 loops=1)
> -> Nested Loop (cost=0.29..2730702.63 rows=3202774 width=9) (actual time=97264.166..123920.769 rows=3200000 loops=1)
> -> Index Only Scan using TBL_INF_pkc on TBL_INF (cost=0.29..8.31 rows=1 width=9) (actual time=0.025..0.030 rows=1 loops=1)
> Index Cond: ((MS_CD = '009'::bpchar) AND (RY_CD = '000001'::bpchar))
> Heap Fetches: 1
> -> Seq Scan on TBL_SHA (cost=0.00..2698666.58 rows=3202774 width=18) (actual time=97264.138..123554.792 rows=3200000 loops=1)
> Filter: ((MS_CD = '009'::bpchar) AND (ETRYS = '000001'::bpchar))
> Rows Removed by Filter: 32000325
> Planning Time: 0.162 ms
> Execution Time: 124168.838 ms
> --------------------------------------------------------------------------------
>
> The index is defined as follows.
>
> CREATE INDEX index_search_01 ON mdb.TBL_SHA USING btree (MS_CD, ETRYS);
>
>
> When I take the following sql statement, the index works fine and the query
> is fast.
>
> select COUNT(ET_CD)
> from TBL_SHA
> WHERE MS_CD = '009'
> AND ETRYS = '000001'
>
> The amount of data in the table is as follows.
> TBL_SHA 38700325
> TBL_INF 35546
>
> Any suggestions for improving the performance of the query would be greatly
> appreciated.

You can try EXISTS instead of IN to correlate the subquery and the outer query:

	SELECT count(et_cd)
	FROM tbl_sha
	WHERE tbl_sha.ms_cd = '009'
	AND tbl_sha.etrys = '000001'
	AND EXISTS (
	  SELECT
	  FROM tbl_inf
	  WHERE tbl_inf.ms_cd = tbl_sha.ms_cd
	  AND tbl_inf.ry_cd = tbl_sha.etrys
	)

--
Erik