Re: SQL Query Performance tips

If I understand well a person has all the free weapons which have a level
<= to his own level, and of course all the weapons he bought.

1) get da weapons
One query can only use one index. Bad for you !
Let's split the free and non-free weapons.

1a) free weapons
SELECT weapon_alignment, count(1) as cnt
FROM weapons
WHERE weapon_level < (user_level)
AND weapon_cost = 0
GROUP BY weapon_alignment;

No need for distinct anymore ! Note also that distinct'ing on weapon_name
is a slower than on weapon_id.
You can create an index on (weapon_cost,weapon_level) but I don't think
it'll be useful.
For ultimate speed, as this does not depend on the user_id, only the
level, you can store the results of this in a table, precalculating the
results for all levels (if there are like 10 levels, it'll be a big win).

1b) weapons bought by the user

SELECT w.weapon_alignment, count(1) as cnt
FROM weapons w, user_weapons uw
WHERE w.weapon_id = uw.weapon_id
AND uw.user_id = (the user_id)
AND w.weapon_cost > 0
GROUP BY weapon_alignment;

You'll note that the weapons in 1a) had cose=0 so they cannot appear
here, no need to distinct the two.

2) combine the two

SELECT weapon_alignment, sum(cnt) FROM
(SELECT weapon_alignment, count(1) as cnt
FROM weapons
WHERE weapon_level < (user_level)
AND weapon_cost = 0
GROUP BY weapon_alignment)
UNION ALL
SELECT w.weapon_alignment, count(1) as cnt
FROM weapons w, user_weapons uw
WHERE w.weapon_id = uw.weapon_id
AND uw.user_id = (the user_id)
AND w.weapon_cost > 0
GROUP BY weapon_alignment)
GROUP BY weapon_alignment;

You can also do this :

SELECT weapon_alignment, count(1) as cnt FROM
(SELECT weapon_alignment
FROM weapons
WHERE weapon_level < (user_level)
AND weapon_cost = 0)
UNION ALL
SELECT w.weapon_alignment
FROM weapons w, user_weapons uw
WHERE w.weapon_id = uw.weapon_id
AND uw.user_id = (the user_id)
AND w.weapon_cost > 0)
GROUP BY weapon_alignment;

How does it turn out ?