Mysql Sub Select Query Optimization

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

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

Answer 2

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.