From: Tom Lane <tgl@sss.pgh.pa.us>
To: Bruce Momjian <bruce@momjian.us>
cc: PostgreSQL-documentation <pgsql-docs@postgresql.org>
Subject: Re: PREPARE and GUC plan_cache_mode
In-reply-to: <20190930155505.GA21095@momjian.us>
References: <20190930155505.GA21095@momjian.us>
Comments: In-reply-to Bruce Momjian <bruce@momjian.us>
	message dated "Mon, 30 Sep 2019 11:55:05 -0400"
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Date: Mon, 30 Sep 2019 12:05:07 -0400
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Bruce Momjian <bruce@momjian.us> writes:
> Our current docs have this text for PREPARE:
>        Prepared statements can use generic plans rather than re-planning
>        with each set of supplied EXECUTE values. This occurs immediately
>        for prepared statements with no parameters; otherwise it occurs
>        only after five or more executions produce plans whose estimated
>        cost average (including planning overhead) is more expensive than
>        the generic plan cost estimate. Once a generic plan is chosen, it
>        is used for the remaining lifetime of the prepared statement. Using
>        EXECUTE values which are rare in columns with many duplicates can
>        generate custom plans that are so much cheaper than the generic
>        plan, even after adding planning overhead, that the generic plan
>        might never be used.

> There is no mention that PG 12's plan_cache_mode can modify this
> behavior.  I think this needs a doc patch.

Yeah, agreed.  I can do it, or do you want to?

			regards, tom lane