Hallo Andreas,
I reduced the problem to the innermost query:
1) SELECT DISTINCT trainer_id, trainer_name FROM student
This results in a sequential table scan. Execution time 7500ms.
2) I created an INDEX ON (trainer_id, trainer_name). Then I had an index
scan instead, which still cost 7000ms.
3) Next, I changed from DISTINCT to GROUP BY:
SELECT trainer_id, trainer_name FROM student
GROUP BY trainer_id, trainer_name
This resulted in an index scan @ 6750ms
4) I filtered out NULL trainer_ids
WHERE trainer_id IS NOT NULL
Amazingly, this resulted in a sequential table scan, which only took
1300ms!!
Please, explain (pun not intended)! How can this be. Only 11000/250000
rows have a null trainer_id.
That's an impressive improvement...
Personally I have no idea what caused it, specially when you say it was
sequential :-|
Warmed caches ?
Best,
Oliver
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