select s.*, ss.* from shipment s, shipment_status ss, release_code r where s.current_status_id = ss.id and ss.release_code_id = r.id and r.filtered_column = '5' order by ss.date desc limit 100;
Release code is just a very small table of 8 rows by looking at the production data, hence the 0.125 filter ratio. However, the data distribution is not normal since the filtered column actually pulls out about 54% of the rows in shipment_status when it joins. Postgres seems to be doing a sequencial scan to pull out all of these rows. Next, it joins approx 17550 rows to shipment. Since this query has a limit, it only returns the first 100, which seems like a waste.
Well, postgres does what you asked. It will be slow, because you have a full table join. LIMIT does not change this because the rows have to be sorted first.
The date is in shipment_status so you should first get the shipment_status.id that you need and later join to shipment. This will avoid the big join :
SELECT s.*, ss.* FROM (SELECT * FROM shipment_status WHERE release_code_id IN (SELECT r.id FROM release_code WHERE r.filtered_column = '5') ORDER BY date DESC LIMIT 100 ) as ss, shipment s WHERE s.current_status_id = ss.id ORDER BY date DESC LIMIT 100
Is this better ?
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