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From: Peter Eisentraut <[email protected]>
To: Alexandra Wang <[email protected]>
To: PostgreSQL Hackers <[email protected]>
Cc: Andrew Dunstan <[email protected]>
Subject: Re: SQL:2023 JSON simplified accessor support
Date: Mon, 16 Sep 2024 21:44:54 +0200
Message-ID: <[email protected]> (raw)
In-Reply-To: <CAK98qZ1JNNAx4QneJG+eX7iLesOhd6A68FNQVvvHP6Up_THf3A@mail.gmail.com>
References: <CAK98qZ1JNNAx4QneJG+eX7iLesOhd6A68FNQVvvHP6Up_THf3A@mail.gmail.com>

On 29.08.24 18:33, Alexandra Wang wrote:
> I’ve implemented the member and array accessors and attached two
> alternative patches:
> 
> 1. v1-0001-Add-JSON-JSONB-simplified-accessor.patch: This patch
> enables dot access to JSON object fields and subscript access to
> indexed JSON array elements by converting "." and "[]" indirection
> into a JSON_QUERY JsonFuncExpr node.
> 
> 2. v2-0001-Transform-JSON-dot-access-to-arrow-operator.txt: This
> alternative patch implements dot access to JSON object fields by
> transforming the "." indirection into a "->" operator.
> 
> The upside of the v1 patch is that it strictly aligns with the SQL
> standard, which specifies that the simplified access is equivalent to:
> 
> JSON_QUERY (VEP, 'lax $.JC' WITH CONDITIONAL ARRAY WRAPPER NULL ON
> EMPTY NULL ON ERROR)
> 
> However, the performance of JSON_QUERY might be suboptimal due to
> function call overhead. Therefore, I implemented the v2 alternative
> using the "->" operator.

Using the operator approach would also allow taking advantage of 
optimizations such as 
<https://www.postgresql.org/message-id/flat/CAKU4AWoqAVya6PBhn%2BBCbFaBMt3z-2%3Di5fKO3bW%3D6HPhbid2Dw...;.

> There is some uncertainty about the semantics of conditional array
> wrappers. Currently, there is at least one subtle difference between
> the "->" operator and JSON_QUERY, as shown:

That JSON_QUERY bug has been fixed.

I suggest you rebase both of your patches over this, just to double 
check everything.  But then I think you can drop the v1 patch and just 
submit a new version of v2.

The patch should eventually contain some documentation.  It might be 
good starting to look for a good spot where to put that documentation. 
It might be either near the json types documentation or near the general 
qualified identifier syntax, not sure.

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