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Commit30f0176

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Document strange jsonb sort order for empty top level arrays
Slightly faulty logic in the original jsonb code (commitd9134d0)results in an empty top level array sorting less than a json null. Wecan't change the sort order now since it would affect btree indexes overjsonb, so document the anomaly.Backpatch to all live branches (13 .. 17)In master, also add a code comment noting the anomaly.Reported-by: Yan ChengpenReviewed-by: Jian HeDiscussion:https://postgr.es/m/OSBPR01MB45199DD8DA2D1CECD50518188E272@OSBPR01MB4519.jpnprd01.prod.outlook.com
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‎doc/src/sgml/json.sgml

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@@ -584,12 +584,13 @@ SELECT jdoc->'guid', jdoc->'name' FROM api WHERE jdoc @@ '$.tags[*] == "qui"';
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The <literal>btree</literal> ordering for <type>jsonb</type> datums is seldom
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of great interest, but for completeness it is:
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<synopsis>
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<replaceable>Object</replaceable> > <replaceable>Array</replaceable> > <replaceable>Boolean</replaceable> > <replaceable>Number</replaceable> > <replaceable>String</replaceable> > <replaceable>Null</replaceable>
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<replaceable>Object</replaceable> > <replaceable>Array</replaceable> > <replaceable>Boolean</replaceable> > <replaceable>Number</replaceable> > <replaceable>String</replaceable> > <replaceable>null</replaceable>
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<replaceable>Object with n pairs</replaceable> > <replaceable>object with n - 1 pairs</replaceable>
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<replaceable>Array with n elements</replaceable> > <replaceable>array with n - 1 elements</replaceable>
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</synopsis>
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with the exception that (for historical reasons) an empty top level array sorts less than <replaceable>null</replaceable>.
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Objects with equal numbers of pairs are compared in the order:
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<synopsis>
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<replaceable>key-1</replaceable>, <replaceable>value-1</replaceable>, <replaceable>key-2</replaceable> ...

‎src/backend/utils/adt/jsonb_util.c

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@@ -246,6 +246,13 @@ compareJsonbContainers(JsonbContainer *a, JsonbContainer *b)
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*/
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if (va.val.array.rawScalar!=vb.val.array.rawScalar)
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res= (va.val.array.rawScalar) ?-1 :1;
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/*
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* There should be an "else" here, to prevent us from
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* overriding the above, but we can't change the sort
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* order now, so there is a mild anomaly that an empty
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* top level array sorts less than null.
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*/
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if (va.val.array.nElems!=vb.val.array.nElems)
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res= (va.val.array.nElems>vb.val.array.nElems) ?1 :-1;
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break;

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