Date: Mon, 30 Sep 2019 11:55:05 -0400
From: Bruce Momjian <bruce@momjian.us>
To: PostgreSQL-documentation <pgsql-docs@postgresql.org>
Subject: PREPARE and GUC plan_cache_mode
Message-ID: <20190930155505.GA21095@momjian.us>
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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 +