MySQL query running *extremely* slowly - mysql

Question

I have a sql query that is running really slow and I am perplexed as to why. The query is:
SELECT DISTINCT(c.ID),c.* FROM `content` c
LEFT JOIN `content_meta` cm1 ON c.id = cm1.content_id
WHERE 1=1
AND c.site_id IN (14)
AND c.type IN ('a','t')
AND c.status = 'visible'
AND (c.lock = 0 OR c.site_id = 14)
AND c.level = 0
OR
(
( c.site_id = 14
AND cm1.meta_key = 'g_id'
AND cm1.meta_value IN ('12','13','7')
)
OR
( c.status = 'visible'
AND (
(c.type = 'topic' AND c.parent_id IN (628,633,624))
)
)
)
ORDER BY c.date_updated DESC LIMIT 20
The content table has about 1250 rows and the content meta table has about 3000 rows. This isn't a lot of data and I'm not quite sure what may be causing it to run so slow. Any thoughts/opinions would be greatly appreciated.
Thanks!

Answer 1

Is you where clause correct? You are making a series of AND statements and later you are executing a OR.
Wouldn't the correct be something like:
AND (c.lock = 0 OR c.site_id = 14)
AND (
( ... )
OR
( ... )
)
If it is indeed correct, you could think on changing the structure or treating the result in an script or procedure.

Answer 2

It might have to do with your "OR" clause at the end... your up-front are being utilized by the indexes where possible, but then you throw this huge OR condition at the end that can be either one or the other. Not knowing more of the underlying content, I would adjust to have a UNION on the inside so each entity can utilize its own indexes, get you qualified CIDs, THEN join to final results.
select
c2.*
from
( select distinct
c.ID
from
`content` c
where
c.site_id in (14)
and c.type in ('a', 't' )
and c.status = 'visible'
and c.lock in ( 0, 14 )
and c.level = 0
UNION
select
c.ID
from
`content` c
where
c.status = 'visible'
and c.type = 'topic'
and c.parent_id in ( 628, 633, 624 )
UNION
select
c.ID
from
`content` c
join `content_meta` cm1
on c.id = cm1.content_id
AND cm1.meta_key = 'g_id'
AND cm1.meta_value in ( '12', '13', '7' )
where
c.site_id = 14 ) PreQuery
JOIN `content` c2
on PreQuery.cID = c2.cID
order by
c2.date_updated desc
limit
20
I would ensure content table has an index on ( site_id, type, status ) another on (parent_id, type, status)
and the meta table, an index on ( content_id, meta_key, meta_value )