MySQL 8.4

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select version();


version()


8.4.3


create table t1 (
  a int, b int, 
  index i1(a), index i2(b)
);

set @c=1;
insert into t1 select @c:=@c+1, @c from performance_schema.session_variables;

Records: 646  Duplicates: 0  Warnings: 1


explain select * from t1 where a<3 or b<3;


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


1
SIMPLE
t1
null
index_merge
i1,i2
i1,i2
5,5
null
2
100.00
Using sort_union(i1,i2); Using where


explain select /*+ NO_INDEX_MERGE(t1 i1,i2) */ * from t1 where a<3 or b<3;


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


1
SIMPLE
t1
null
ALL
i1,i2
null
null
null
646
55.55
Using where


explain select /*+ NO_INDEX_MERGE(t1@`select#1` i1,i2) */ * from t1 where a<3 or b<3;


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


1
SIMPLE
t1
null
ALL
i1,i2
null
null
null
646
55.55
Using where


explain select /*+ NO_INDEX_MERGE(t1@`select#2` i1,i2) */ * 
from 
(select * from t1 where a<3 or b<3 ) T;


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


1
SIMPLE
t1
null
ALL
i1,i2
null
null
null
646
55.55
Using where


explain select /*+ NO_INDEX_MERGE(t1@`select#1` i1,i2) */ * 
from 
(select * from t1 where a<3 or b<3 ) T;


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


1
SIMPLE
t1
null
index_merge
i1,i2
i1,i2
5,5
null
2
100.00
Using sort_union(i1,i2); Using where