Outer query is very slow when inner query returns no results-CodePudding

I'm trying to fetch a row from a table called export with random weights. It should then fetch one row from another table export_chunk which references the first row. This is the query:

SELECT * FROM export_chunk
WHERE export_id=(
    SELECT id FROM export 
    WHERE schedulable=1 
    ORDER BY -LOG(1 - RAND())/export.weight LIMIT 1)
AND status='PENDING' 
LIMIT 2;

The export table can have 1000 rows while the export_chunk table can have millions of rows.

The query is very fast when the inner query returns a row. However, if there are no rows with schedulable=1, the outer query performs a full table scan on export_chunk. Why does this happen and is there any way to prevent it?

EDIT: Trying COALESCE()

Akina in the comments suggested using COALESCE, ie.:

SELECT * FROM export_chunk
WHERE export_id=COALESCE(
    SELECT id FROM export 
    WHERE schedulable=1 
    ORDER BY -LOG(1 - RAND())/export.weight LIMIT 1)
    ,-1)
AND status='PENDING' 
LIMIT 2;

This should work. When I run:

SELECT COALESCE((SELECT id FROM export WHERE schedulable=1 ORDER BY -LOG(1-RAND())/export.weight LIMIT 1), -1) FROM export;

It does return -1 for each row which Akina predicted. And if I manually search for -1 instead of the inner query it returns no rows very quickly. However, when I try to use COALESCE on the inner query it is still really slow. I do not understand why.

CodePudding user response：

Test this:

SELECT export_chunk.* 
FROM export_chunk
JOIN ( SELECT id 
       FROM export 
       WHERE schedulable=1 
       ORDER BY -LOG(1 - RAND())/export.weight 
       LIMIT 1 ) AS random_row ON export_chunk.export_id=random_row.id
WHERE export_chunk.status='PENDING' 
LIMIT 2;

Does this matches needed logic? especially when no matching rows in the subquery - do you need none output rows (like now) or any 2 rows in this case?

PS. LIMIT without ORDER BY in outer query is strange.