Mysql Sub Select Query Optimization - php

Question

I'm running a query daily to compile stats - but it seems really inefficient. This is the Query:
SELECT a.id, tstamp, label_id, (SELECT author_id FROM b WHERE b.tid = a.id ORDER BY b.tstamp DESC LIMIT 1) AS author_id
FROM a, b
WHERE (status = '2' OR status = '3')
AND category != 6
AND a.id = b.tid
AND (b.type = 'C' OR b.type = 'R')
AND a.tstamp1 BETWEEN {$timestamp_start} AND {$timestamp_end}
ORDER BY b.tstamp DESC
LIMIT 500
This query seems to run really slow. Apologies for the crap naming - I've been asked to not reveal the actual table names.
The reason there is a sub select is because the outer select gets one row from the table a and it gets a row from table b. But also need to know the latest author_id from table b as well, so I run a subselect to return that one. I don't want to run another select inside a php loop - as that is also inefficient.
It works correctly - I just need to find a much faster way of getting this data set.

Answer 1

If b.tstamp is unique within b.tid, take OMG Ponies' solution.
Otherwise you could try this solution. It sorts the whole result by b.tstamp DESC and adds a ranking per author_id. The outer selects takes only the row with rank = 1, which is the one with the greatest tstamp per author_id.
SELECT id, tstamp, label_id, author_id
FROM (SELECT id,
tstamp,
label_id,
author_id,
CASE
WHEN #author_id != author_id THEN #row_num := 1
ELSE #row_num := #row_num + 1
END AS rank,
#author_id := b.author_id
FROM a,
b,
(SELECT #row_num := 0, #author_id := NULL) y
WHERE a.id = b.tid
AND (status = '2' OR status = '3')
AND category != 6
AND (b.type = 'C' OR b.type = 'R')
AND a.tstamp1 BETWEEN {$timestamp_start} AND {$timestamp_end}
ORDER BY b.author_id, b.tstamp DESC
) x
WHERE x.rank = 1
LIMIT 500
I have not tried it, so please comment if it does not work.

Answer 2

Try:
SELECT a.id,
b.tstamp,
label_id,
y.author_id
FROM TABLE_A a
JOIN TABLE_B b ON b.tid = a.id
JOIN (SELECT b.tid,
MAX(b.tstamp) 'm_tstamp'
FROM TABLE_B b
GROUP BY b.tid) x ON x.tid = a.id
JOIN (SELECT b.tid,
b.author_id,
b.tstamp
FROM TABLE_B b
GROUP BY b.tid) y ON y.tid = a.id
AND y.tstamp = x.m_tstamp
WHERE status IN ('2', '3')
AND b.type IN ('C', 'R')
AND category != 6
AND a.tstamp1 BETWEEN {$timestamp_start} AND {$timestamp_end}
ORDER BY b.tstamp DESC
LIMIT 500