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Document strange jsonb sort order for empty top level arrays

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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
This commit is contained in:
Andrew Dunstan 2025-01-03 09:23:46 -05:00
parent 297b280abd
commit 591128f9c9
1 changed files with 2 additions and 1 deletions
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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"';
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> ...