Document strange jsonb sort order for empty top level arrays

adunstan · adunstan · commit 30f017626308 · 2025-01-03T10:36:30.000-05:00
Slightly faulty logic in the original jsonb code (commit d9134d0) 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
diff --git 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
@@ -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;
