MySQL 8.0

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create table t1 (id int primary key);
create table t2 (id int primary key);
explain select * from t1 inner join t2 on t1.id=t2.id;


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


1
SIMPLE
t1
null
index
PRIMARY
PRIMARY
4
null
1
100.00
Using index

1
SIMPLE
t2
null
eq_ref
PRIMARY
PRIMARY
4
fiddle_NTBFHZQPZPTPOTLPGUEV.t1.id
1
100.00
Using index


explain select * from t1, t2 where t1.id=t2.id;


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


1
SIMPLE
t1
null
index
PRIMARY
PRIMARY
4
null
1
100.00
Using index

1
SIMPLE
t2
null
eq_ref
PRIMARY
PRIMARY
4
fiddle_NTBFHZQPZPTPOTLPGUEV.t1.id
1
100.00
Using index

id	select_type	table	partitions	type	possible_keys	key	key_len	ref	rows	filtered	Extra
1	SIMPLE	t1	null	index	PRIMARY	PRIMARY	4	null	1	100.00	Using index
1	SIMPLE	t2	null	eq_ref	PRIMARY	PRIMARY	4	fiddle_NTBFHZQPZPTPOTLPGUEV.t1.id	1	100.00	Using index