Question

I’m trying to write a stored procedure in MySQL. For complicated reasons I have an aggregate query with a bunch of joins to look object ids, and then do another query with more joins (including same tables) to get some data

So, something like this:

CREATE TEMPORARY TABLE ids ENGINE=MEMORY
SELECT MAX(child.id)
    FROM parent 
    JOIN child ON child.parent_id = parent.id
    WHERE
    GROUP BY child.parent_id;

SELECT *
    # MUST SELECT STUFF FROM PARENT, CHILD, AND STUFF JOINED ON CHILD
    FROM child 
    JOIN parent ON parent.id = child.parent_id
    # A BUNCH MORE JOINS HERE
    WHERE child.id IN (SELECT * FROM ids);

DROP TEMPORARY TABLE  IF EXISTS ids;

Now I noticed that first select takes 0.000 seconds. Second select if I replace (SELECT * FROM ids) with a constant list like (1435,2352,43653,34534,…) also takes 0.000 seconds.

However running them both with the temporary table creation takes .6 seconds. 0.o WTH?

So my question is how to skip this temporary table creation, and just pass the list of ids?

Also, if I move the entire first select as a sub-select to the second select then the whole procedure takes way-way longer. More than a minute at which point I give up waiting.

Answer 1

Try this one:

SELECT *
FROM (
  SELECT parent.*, MAX(child.id) as max_id
  FROM parent 
  JOIN  child ON child.parent_id = parent.id
  WHERE -- some where cond
  GROUP BY parent.id
) as parents
JOIN  child ON child.id = parents.max_id
-- other joins

and show performance results.