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
Date: Mon, 30 Sep 2019 12:05:07 -0400
Message-ID: <13155.1569859507@sss.pgh.pa.us> (raw)
In-Reply-To: <20190930155505.GA21095@momjian.us>
References: <20190930155505.GA21095@momjian.us>
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
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Reply to all the recipients using the --to and --cc options:
reply via email
To: pgsql-docs@postgresql.org
Cc: tgl@sss.pgh.pa.us, bruce@momjian.us
Subject: Re: PREPARE and GUC plan_cache_mode
In-Reply-To: <13155.1569859507@sss.pgh.pa.us>
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
This inbox is served by DDX for PostgreSQL; see mirroring instructions
for how to clone and mirror all data and code used for this inbox