From: Tom Lane <tgl@sss.pgh.pa.us>
To: Mark Kirkwood <mark.kirkwood@gmail.com>
cc: David Rowley <dgrowleyml@gmail.com>,
        pgsql-performance@lists.postgresql.org
Subject: Re: Unexpected planner choice in simple JOIN
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Comments: In-reply-to Mark Kirkwood <mark.kirkwood@gmail.com>
	message dated "Thu, 08 Jan 2026 17:44:25 +1300"
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Date: Wed, 07 Jan 2026 23:51:53 -0500
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Mark Kirkwood <mark.kirkwood@gmail.com> writes:
> A point comes to mind - this is not a particularly unusual setup (i.e 
> relatively small parent table with big child one), so maybe the defaults 
> are not ideal here?

Very probably.  To my mind, the default costs for parallel query and
JIT are both unduly optimistic and tend to drive the planner to use
those features when you'd be better off without.  The reason there's
not been more argument about them is that the downside of using those
features on a too-small query is bounded, while the upside of using
them on very-big queries isn't.  So nobody's invested the effort to
gather enough evidence to back choosing a different set of defaults.

			regards, tom lane