Question

I used to do this:

SELECT layerID FROM layers WHERE ownerID = ? AND collectionID = ? 

Which would give me an array of layerID’s, and then I’d loop and do this for each one:

SELECT DATA FROM drawings WHERE layerID = ? 

And it all worked fine. So now I’m trying to do it in one step, so I try this:

SELECT DATA , layerID FROM drawings WHERE layerID = ANY (   SELECT layerID   FROM layers   WHERE ownerID = ?   AND collectionID = ? )  

But for some reason, it doesn’t use the index, for the main query, SELECT DATA etc! So this one combined query takes much much longer to complete, versus the separate queries I was doing before. (By theway, the subquery, SELECT layerID etc still uses the index).

I’ve determined if it’s using a query or not by using the ‘EXPLAIN’ statement.

I have individual indexes on the ownerID and collectionID columns in the layers table, and on the layerID column in the drawings table.

What am I doing wrong with my query?

Answer 1

Try a join. ANY ends up looking a lot like an unoptimizable UNION to the query optimizer.

SELECT d.DATA, d.layerID   FROM drawings AS d   INNER JOIN layers AS l ON d.layerID = l.layerID   WHERE l.ownerID = ? AND l.collectionID = ?