MySQL 8.0

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 create table DEMO as select 1 A, 1 B, 1 C union all select 1 A, 1 B , 1 C

Records: 2  Duplicates: 0  Warnings: 0


insert into DEMO select 1,1,1 from DEMO a, DEMO b, DEMO c, DEMO d, DEMO e, DEMO f, DEMO g, DEMO h

Records: 256  Duplicates: 0  Warnings: 0


insert into DEMO values (2,0,0);
insert into DEMO select a.A, a.B, a.C from DEMO a,DEMO b

Records: 67081  Duplicates: 0  Warnings: 0


create index DEMO_AB on DEMO(A,B);

Records: 0  Duplicates: 0  Warnings: 0


select A,count(*) from DEMO group by A;


A
count(*)


1
67080

2
260


select sum(A) from DEMO where A=2;


sum(A)


520


explain select sum(A) from DEMO where A=2;


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


1
SIMPLE
DEMO
null
ref
DEMO_AB
DEMO_AB
8
const
260
100.00
Using index


select sum(B) from DEMO where A=2;


sum(B)


0


explain select sum(B) from DEMO where A=2;


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


1
SIMPLE
DEMO
null
ref
DEMO_AB
DEMO_AB
8
const
260
100.00
Using index


explain select sum(C) from DEMO where A=2;


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


1
SIMPLE
DEMO
null
ref
DEMO_AB
DEMO_AB
8
const
260
100.00
null


select sum(C) from DEMO where A=2;


sum(C)


0