[doc] adding way to examine the plan type of prepared statements

[torikoshia]

Hi,

Currently, EXPLAIN is the only way to know whether the plan is generic 
or custom according to the manual of PREPARE.

  https://www.postgresql.org/docs/devel/sql-prepare.html

After commit d05b172, we can also use pg_prepared_statements view to 
examine the plan types.

How about adding this explanation like the attached patch?


Regards,

--
Atsushi TorikoshiFrom 2c8f66637075fcb2f802a2b9cfd354f2efbbbb18 Mon Sep 17 00:00:00 2001
From: Atsushi Torikoshi <torikos...@oss.nttdata.com>
Date: Thu, 12 Nov 2020 17:00:19 +0900
Subject: [PATCH v1] After commit d05b172, we can use pg_prepared_statements
 view to examine whether the plan is generic or custom. This patch adds this
 explanation in the manual of PREPARE.

---
 doc/src/sgml/ref/prepare.sgml | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/doc/src/sgml/ref/prepare.sgml b/doc/src/sgml/ref/prepare.sgml
index 57a34ff83c..2268c222a9 100644
--- a/doc/src/sgml/ref/prepare.sgml
+++ b/doc/src/sgml/ref/prepare.sgml
@@ -179,6 +179,13 @@ EXPLAIN EXECUTE <replaceable>name</replaceable>(<replaceable>parameter_values</r
    documentation.
   </para>
 
+  <para>
+   To examine how many times each prepared statement chose generic and
+   custom plan cumulatively in the current session, refer
+   <link linkend="view-pg-prepared-statements"><structname>pg_prepared_statements</structname></link>
+   system view.
+  </para>
+
   <para>
    Although the main point of a prepared statement is to avoid repeated parse
    analysis and planning of the statement, <productname>PostgreSQL</productname> will
-- 
2.18.1