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From: David G. Johnston <[email protected]>
To: Laurenz Albe <[email protected]>
Cc: Phillip Diffley <[email protected]>
Cc: [email protected] <[email protected]>
Subject: Re: Convert JSON value back to postgres representation
Date: Thu, 19 Jun 2025 22:17:55 -0700
Message-ID: <CAKFQuwZJCdJZAAJJnmU8cwiD=-=dLcnaRfJ-LxAL8zdH5Ft3QA@mail.gmail.com> (raw)
In-Reply-To: <[email protected]>
References: <CAGAwPgSCTk=HWKtrzf4L+DOdCFDNOZY0Np9go0nsT6wqe6jd=w@mail.gmail.com>
	<[email protected]>

On Thursday, June 19, 2025, Laurenz Albe <[email protected]> wrote:

> On Thu, 2025-06-19 at 23:05 +0200, Phillip Diffley wrote:
> > Postgres has a to_jsonb function that will convert a value into its
> jsonb representation.
> > I am now trying to turn a json value back into its postgres type. I was
> hoping there would
> > be something like a from_jsonb function that, along with a type hint,
> could be used as an
> > inverse of to_jsonb, like
> >
> > from_jsonb(to_jsonb('{1,2,3}'::int[]) as int[]
> >
> > but I do not see a function like this. I was able to convert a json
> value back to its
> > postgres representation using the jsonb_to_record function, as used in
> the WHERE expression
> > below, but I feel like there might be a better way to do this.
> >
> > CREATE TABLE mytable (id int, col1 int[]);
> > INSERT INTO mytable VALUES (1, '{1, 2, 3}'), (2, '{3, 4, 5}');
> > SELECT * from mytable WHERE col1 = (select col1 from
> json_to_record('{"col1": [1, 2, 3]}'::JSON) as x(col1 int[]));
> >
> > Is there a preferred method for turning a JSON value back to its
> postgres representation?
>
> I think jsonb_populate_record() is the closest thing to what you envision.
>

jsonb_to_record avoids the temporary type.

select * from jsonb_to_record('{"ia":[1,2,3]}'::jsonb) as r (ia integer[]);

There is a gap for arrays.  Scalars you can just cast and composites have
these functions.  But no simple/direct way to go from json array to sql
array is presently implemented.

Though since 17 json_query can apparently do it.

select pg_typeof( json_query('[1,2,3]'::jsonb, '$' returning integer[]) )
-> integer[]


David J.

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