Re: PostgreSQL 15-->18 slowdown?

On Jun 26, 2026, at 10:14 AM, Adrian Klaver <adrian(dot)klaver(at)aklaver(dot)com> wrote:
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> On 6/26/26 10:58 AM, Israel Brewster wrote:
>> 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 <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.
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> The link provided shows no times or rows, did you pick the correct one?
>> 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 <https://explain.depesz.com/s/K2K9>
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> This one does not show the actual query.

Right, sorry. It’s the same query as in the first one though. Only difference is sequence scan off.

---
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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>> 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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