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From: Clive Boughton <[email protected]>
To: David Rowley <[email protected]>
Cc: Joe Conway <[email protected]>
Cc: [email protected]
Subject: Re: A serious change in performance between PG 15 and PG 16, 17, 18.
Date: Wed, 18 Feb 2026 12:05:26 +1100
Message-ID: <[email protected]> (raw)
In-Reply-To: <CAApHDvouSSC+koUjuMHx7gCTNFWrJOGtrVR+kpSn5=KLDtRX9A@mail.gmail.com>
References: <[email protected]>
	<[email protected]>
	<CAApHDvouSSC+koUjuMHx7gCTNFWrJOGtrVR+kpSn5=KLDtRX9A@mail.gmail.com>

David,

I've handed over further communication with you (and others) to Ji Zhang, my (better) technical colleague.
He is able to determine any issues surrounding open/closed publication.

Clive.

Dr Clive Boughton

Software Improvements

Street address:
97 Bankers Road, NSW 2621
Australia

Mobile Phone: +61 (0)410 632 055
Telephone: +61 (0)2 6230 3195




> On 18 Feb 2026, at 11:01 am, David Rowley <[email protected]> wrote:
> 
> On Tue, 17 Feb 2026 at 04:42, Joe Conway <[email protected]> wrote:
>> Are some of your indexes on collatable columns? If so, what version of
>> glibc is on each of your systems (i.e. did you change OS major versions)?
> 
> For the record, Clive did send me the query and EXPLAIN output
> offline. I asked him if he'd anonymise it and post it here, but no
> response yet.
> 
> I'll just describe what I saw so as not to share details that Clive
> isn't happy making public. The query is to a single table, no joins.
> The filter is equality on an INT column. The v15 plan uses a Seq Scan
> and runs in 175ms. v18 is using a Bitmap Heap Scan using the table's
> primary key index. The PK must be a composite key and the WHERE clause
> must be on the first column of that key. That query runs in 305ms, so
> more like x2 rather than the reported 300-600x. v18 estimates slightly
> fewer rows than v15, so that might be why it switched plans to the
> bitmap heap scan. That could be down to random sampling from ANALYZE
> finding fewer rows that match the query's WHERE clause in v18.
> 
> I'd recommend Clive to try running ANALYZE on the table and seeing if
> the plan changes. Repeat that a few times to see if the Seq Scan plan
> is ever picked. I'd also check if the random_page_cost and
> seq_page_cost settings are the same on both instances and ensure
> enable_seqscan is on.
> 
> David

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