From 319f4d54e82d15d4a0c3f4cc1328c40dba024b5c Mon Sep 17 00:00:00 2001
From: Tom Lane <tgl@sss.pgh.pa.us>
Date: Wed, 30 Dec 2020 17:48:43 -0500
Subject: [PATCH] Doc: spell out comparison behaviors for the date/time types.

The behavior of cross-type comparisons among date/time data types was
not really explained anywhere.  You could probably infer it if you
recognized the applicability of comments elsewhere about datatype
conversions, but it seems worthy of explicit documentation.

Per bug #16797 from Dana Burd.

Discussion: https://postgr.es/m/16797-f264b0b980b53b8b@postgresql.org
---
 doc/src/sgml/func.sgml | 20 ++++++++++++++++++--
 1 file changed, 18 insertions(+), 2 deletions(-)

diff --git a/doc/src/sgml/func.sgml b/doc/src/sgml/func.sgml
index 5021ac1ca9..1e044b51a1 100644
--- a/doc/src/sgml/func.sgml
+++ b/doc/src/sgml/func.sgml
@@ -8190,14 +8190,30 @@ SELECT regexp_match('abc01234xyz', '(?:(.*?)(\d+)(.*)){1,1}');
    linkend="datatype-datetime"/>.
   </para>
 
+  <para>
+   In addition, the usual comparison operators shown in
+   <xref linkend="functions-comparison-op-table"/> are available for the
+   date/time types.  Dates and timestamps (with or without time zone) are
+   all comparable, while times (with or without time zone) and intervals
+   can only be compared to other values of the same data type.  When
+   comparing a timestamp without time zone to a timestamp with time zone,
+   the former value is assumed to be given in the time zone specified by
+   the <xref linkend="guc-timezone"/> configuration parameter, and is
+   rotated to UTC for comparison to the latter value (which is already
+   in UTC internally).  Similarly, a date value is assumed to represent
+   midnight in the <varname>TimeZone</varname> zone when comparing it
+   to a timestamp.
+  </para>
+
   <para>
    All the functions and operators described below that take <type>time</type> or <type>timestamp</type>
    inputs actually come in two variants: one that takes <type>time with time zone</type> or <type>timestamp
    with time zone</type>, and one that takes <type>time without time zone</type> or <type>timestamp without time zone</type>.
    For brevity, these variants are not shown separately.  Also, the
    <literal>+</literal> and <literal>*</literal> operators come in commutative pairs (for
-   example both date + integer and integer + date); we show only one of each
-   such pair.
+   example both <type>date</type> <literal>+</literal> <type>integer</type>
+   and <type>integer</type> <literal>+</literal> <type>date</type>); we show
+   only one of each such pair.
   </para>
 
     <table id="operators-datetime-table">