Thread: Re: [PATCH] Fix jsonb comparison for raw scalar pseudo arrays

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Dear PostgreSQL Hackers,

 

Problem Description

 

I encountered an issue with the B-Tree ordering of `jsonb` values. According to the PostgreSQL documentation[1], the ordering should follow this precedence:

 

`Object > Array > Boolean > Number > String > Null`

 

However, empty arrays (`[]`) are currently considered smaller than `null`, which violates the documented rules. This occurs due to improper handling of the `rawScalar` flag when comparing arrays in the `compareJsonbContainers()` function in `src/backend/utils/adt/jsonb_util.c`.

I agree that this is a (10 year old) bug:

-                        if (va.val.array.nElems != vb.val.array.nElems)
+                        else if (va.val.array.nElems != vb.val.array.nElems)

But I don't think we can fix it, because there could well be indexes that would no longer be valid if we change the sort order. Given that, I think the best we can do is adjust the documentation to mention the anomaly.

So the actual sort order as implemented is, AIUI,

Object > Non-Empty-Array > Boolean > Number > String > Null > Empty-Array

which is ugly, but fortunately not many apps rely on jsonb sort order.

cheers

andrew

--
Andrew Dunstan
EDB: https://www.enterprisedb.com

On Sun, Dec 8, 2024 at 10:58 PM Andrew Dunstan <andrew@dunslane.net> wrote:
>
> So the actual sort order as implemented is, AIUI,
>
> Object > Non-Empty-Array > Boolean > Number > String > Null > Empty-Array
>
> which is ugly, but fortunately not many apps rely on jsonb sort order.
>
> Nobody else has commented, so I propose to apply this patch documenting the anomaly.
>

while at it. we can fix the appearance of jsonb null.

since
select jsonb 'Null';
select jsonb 'NULL';
will fail.

so maybe change
<replaceable>Null</replaceable> in <synopsis> section and
<replaceable>NULL</replaceable>
to
<replaceable>null</replaceable>