From 30f017626308a06cf0c0c82a706a1ba1b07df34a Mon Sep 17 00:00:00 2001
From: Andrew Dunstan <andrew@dunslane.net>
Date: Fri, 3 Jan 2025 09:23:46 -0500
Subject: [PATCH] Document strange jsonb sort order for empty top level arrays

Slightly faulty logic in the original jsonb code (commit d9134d0a355)
results in an empty top level array sorting less than a json null. We
can't change the sort order now since it would affect btree indexes over
jsonb, so document the anomaly.

Backpatch to all live branches (13 .. 17)

In master, also add a code comment noting the anomaly.

Reported-by: Yan Chengpen
Reviewed-by: Jian He

Discussion: https://postgr.es/m/OSBPR01MB45199DD8DA2D1CECD50518188E272@OSBPR01MB4519.jpnprd01.prod.outlook.com
---
 doc/src/sgml/json.sgml             | 3 ++-
 src/backend/utils/adt/jsonb_util.c | 7 +++++++
 2 files changed, 9 insertions(+), 1 deletion(-)

diff --git a/doc/src/sgml/json.sgml b/doc/src/sgml/json.sgml
index 54648c459c1..206eadb8f7b 100644
--- a/doc/src/sgml/json.sgml
+++ b/doc/src/sgml/json.sgml
@@ -584,12 +584,13 @@ SELECT jdoc->'guid', jdoc->'name' FROM api WHERE jdoc @@ '$.tags[*] == "qui"';
     The <literal>btree</literal> ordering for <type>jsonb</type> datums is seldom
     of great interest, but for completeness it is:
 <synopsis>
-<replaceable>Object</replaceable> > <replaceable>Array</replaceable> > <replaceable>Boolean</replaceable> > <replaceable>Number</replaceable> > <replaceable>String</replaceable> > <replaceable>Null</replaceable>
+<replaceable>Object</replaceable> > <replaceable>Array</replaceable> > <replaceable>Boolean</replaceable> > <replaceable>Number</replaceable> > <replaceable>String</replaceable> > <replaceable>null</replaceable>
 
 <replaceable>Object with n pairs</replaceable> > <replaceable>object with n - 1 pairs</replaceable>
 
 <replaceable>Array with n elements</replaceable> > <replaceable>array with n - 1 elements</replaceable>
 </synopsis>
+      with the exception that (for historical reasons) an empty top level array sorts less than <replaceable>null</replaceable>.
       Objects with equal numbers of pairs are compared in the order:
 <synopsis>
 <replaceable>key-1</replaceable>, <replaceable>value-1</replaceable>, <replaceable>key-2</replaceable> ...
diff --git a/src/backend/utils/adt/jsonb_util.c b/src/backend/utils/adt/jsonb_util.c
index 806d1aa3a98..773f3690c7b 100644
--- a/src/backend/utils/adt/jsonb_util.c
+++ b/src/backend/utils/adt/jsonb_util.c
@@ -246,6 +246,13 @@ compareJsonbContainers(JsonbContainer *a, JsonbContainer *b)
 						 */
 						if (va.val.array.rawScalar != vb.val.array.rawScalar)
 							res = (va.val.array.rawScalar) ? -1 : 1;
+
+						/*
+						 * There should be an "else" here, to prevent us from
+						 * overriding the above, but we can't change the sort
+						 * order now, so there is a mild anomaly that an empty
+						 * top level array sorts less than null.
+						 */
 						if (va.val.array.nElems != vb.val.array.nElems)
 							res = (va.val.array.nElems > vb.val.array.nElems) ? 1 : -1;
 						break;