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From: Israel Brewster <[email protected]>
To: PostgreSQL General <[email protected]>
Subject: PostgreSQL 15-->18 slowdown?
Date: Fri, 26 Jun 2026 09:58:30 -0800
Message-ID: <[email protected]> (raw)

In postgreSQL 15, I had the below query that worked quickly. Now, I make no claims that the query is the best possible, or even a good query, but it DID work, and it did so quickly enough to be un-noticable when running.

Then I upgrade to PostgreSQL 18 - and now the query never completes (as in, I get a command timeout after at least half an hour before I get a result). Looking at the EXPLAIN (https://explain.depesz.com/s/llAQ) makes it pretty obvious why: we have a sequence scan on a large table inside a nested loop - and that sequence scan is apparently not short circuiting.

I tried the obvious: REINDEX database and VACUUM ANALYZE, but neither helped. I have my default_statistics_target set to 500 at the moment.

Then I tried SET enable_seqscan = off; Lo and behold, the query ran in only 123.888 ms (fun number :-D ) - https://explain.depesz.com/s/K2K9

What did I do wrong here? Thanks!

The query in question:

    SELECT
        stations.id,
        stations.name as name,
        latitude::float as lat,
        longitude::float as lon,
        elevation::float alt,
        type,
        coalesce(sites.name, 'Unknown') as site,
        sites.id as siteid,
        coalesce((SELECT true
                  FROM tilt_data
                  INNER JOIN tilt_orientation
                  ON tilt_data.station=tilt_orientation.station
                  WHERE tilt_data.station=stations.id
                  LIMIT 1),
                false) as has_tilt,
        array_to_json(volcano) as stationvolcs,
        coalesce(
            (SELECT array_agg(s.volcano_name ORDER BY t.ord)
                FROM volcano s
                JOIN unnest( volcano ) WITH ORDINALITY t(id,ord)
                ON (t.id=s.volcano_id)
            ), '{}' ) volcnames
    FROM stations
    LEFT JOIN sites ON stations.siteref=sites.id
    ORDER BY site, name


---
Israel Brewster
Software Engineer
Alaska Volcano Observatory 
Geophysical Institute - UAF 
2156 Koyukuk Drive 
Fairbanks AK 99775-7320
Work: 907-474-5172
cell:  907-328-9145

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