Query 1:
SELECT cid,
dl
FROM chal
WHERE cid IN (
SELECT cid
FROM c_users
WHERE uid = 636587
);
Query 2:
SELECT chal.cid AS cid,
chal.dl AS dl
FROM chal,
c_users
WHERE uid = 808
AND chal.cid = c_users.cid;
cid is primary key in chal cid and uid are indexed in c_users, cid is not unique;
Which of the above query is better?
Explain says the following
Query 1 uses two types of index namely ALL and index_subquery
Query 2 users two types of index namely ALL and ref
I wonder why both queries say ALL as type of index though cid is primary key in table chal.