From: Bruce Momjian <bruce@momjian.us>
To: PostgreSQL-documentation <pgsql-docs@postgresql.org>
Subject: PREPARE and GUC plan_cache_mode
Date: Mon, 30 Sep 2019 11:55:05 -0400
Message-ID: <20190930155505.GA21095@momjian.us> (raw)
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.
--
Bruce Momjian <bruce@momjian.us> http://momjian.us
EnterpriseDB http://enterprisedb.com
+ As you are, so once was I. As I am, so you will be. +
+ Ancient Roman grave inscription +
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: bruce@momjian.us
Subject: Re: PREPARE and GUC plan_cache_mode
In-Reply-To: <20190930155505.GA21095@momjian.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