MySQL query plan not using Index - mysql

Question

I have a nested query and I trying to see if there is any full table scan in my query.
explain delete from ACCESS where ACCESS.MESSAGEID in (select ID from MESSAGE where MESSAGE.CID = 'xzy67sd’)\G
The sub query is hitting index but the first is not using index. Here is the query plan.
*************************** 1. row ***************************
id: 1
select_type: PRIMARY
table: ACCESS
type: ALL
possible_keys: NULL
key: NULL
key_len: NULL
ref: NULL
rows: 18295
Extra: Using where
*************************** 2. row ***************************
id: 2
select_type: DEPENDENT SUBQUERY
table: MESSAGE
type: unique_subquery
possible_keys: PRIMARY
key: PRIMARY
key_len: 8
ref: func
rows: 1
Extra: Using where
But if I separate the query and check the query plan then it is using index. I am not able to understand why and looking for some hints
explain delete from ACCESS where ACCESS.MESSAGEID in (2,3)\G
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: ACCESS
type: range
possible_keys: ACCESS_ID1
key: ACCESS_ID1
key_len: 8
ref: const
rows: 2
Extra: Using where
Subquery select statement returns constant, so rather than using select statement I type integer and the query plan start picking index
select ID from MESSAGE where MESSAGE.CID = 'xzy67sd’)\G
Thanks in advance

Answer 1

You don't need a subquery, here, and as a general rule, you shouldn't use one in MySQL unless you actually do need one.
DELETE a
FROM ACCESS a
JOIN MESSAGE m ON m.ID = a.MESSAGEID
WHERE m.CID = 'xzy67sd’;
This will delete the rows from ACCESS while leaving MESSAGE alone because only ACCESS is listed (by its alias "a") between DELETE and FROM, which is where you specify which tables you want to delete matching rows from.
The optimizer should use the indexes appropriately.
https://dev.mysql.com/doc/refman/5.6/en/delete.html (multi-table syntax)