MySQL 8.0

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CREATE TABLE tb_kse(id_no INT, ss_no INT )
create index IDX_KSE_NO_01 on tb_kse(ss_no);

Records: 0  Duplicates: 0  Warnings: 0


create index IDX_KSE_NO_02 on tb_kse(id_no);

Records: 0  Duplicates: 0  Warnings: 0


explain
SELECT *
FROM tb_kse WHERE ss_no = 10
UNION ALL
SELECT *
FROM tb_kse
WHERE ss_no IS NULL AND id_no = 10;


id
select_type
table
partitions
type
possible_keys
key
key_len
ref
rows
filtered
Extra


1
PRIMARY
tb_kse
null
ref
IDX_KSE_NO_01
IDX_KSE_NO_01
5
const
1
100.00
null

2
UNION
tb_kse
null
ref
IDX_KSE_NO_01,IDX_KSE_NO_02
IDX_KSE_NO_01
5
const
1
100.00
Using index condition; Using where

id	select_type	table	partitions	type	possible_keys	key	key_len	ref	rows	filtered	Extra
1	PRIMARY	tb_kse	null	ref	IDX_KSE_NO_01	IDX_KSE_NO_01	5	const	1	100.00	null
2	UNION	tb_kse	null	ref	IDX_KSE_NO_01,IDX_KSE_NO_02	IDX_KSE_NO_01	5	const	1	100.00	Using index condition; Using where