Re: Cast jsonb to numeric, int, float, bool

From: Teodor Sigaev <teodor(at)sigaev(dot)ru>
To: Robert Haas <robertmhaas(at)gmail(dot)com>
Cc: Aleksander Alekseev <a(dot)alekseev(at)postgrespro(dot)ru>, David Steele <david(at)pgmasters(dot)net>, Nikita Glukhov <n(dot)gluhov(at)postgrespro(dot)ru>, Darafei Praliaskouski <me(at)komzpa(dot)net>, Anastasia Lubennikova <a(dot)lubennikova(at)postgrespro(dot)ru>, PostgreSQL Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org>
Subject: Re: Cast jsonb to numeric, int, float, bool
Date: 2018-03-30 12:28:40
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> I would like to complain about this patch. First, I think that it
> would've been a better idea to use functions for this rather than
> operators, because now ::text does something totally unlike what ::int
> does, and that's confusing. If we had json_to_WHATEVER for various
> values of WHATEVER then all of the conversions could be spelled
> similarly; as the commit message right points out, the cast can only
> do one thing.

From another point of view, casting jsonb to text produces completely grounded
result: we get a text correctly formatted as json. Other casts produce correct
json but with non-text type.

Casting jsonb with text is two-way casting:
# select '123'::jsonb::text::jsonb, '"xxx"'::jsonb::text::jsonb;
jsonb | jsonb
123 | "xxx"

But casting with numeric types and bool is not, but it could be done with
intermediate cast to text (uppercase cast):
# select '123'::jsonb::int::TEXT::jsonb;

For completeness it's possible to add direct cast from numeric/boolean types to
jsonb. Then such casts will be mutual.

> Also, I think the error messages aren't great:
> +select '[]'::jsonb::bool;
> +ERROR: jsonb value must be boolean
> In this simple scenario, it's clear enough what has gone wrong, but in
> a more complicated case I suspect people will have a hard time
> figuring out what the source of that error message is. It seems like
> it would be better to say something about casting or converting in the
> error message, to give users a clue.

Agree, something like "could not convert jsonb value to boolean type. jsonb
value must be scalar boolean type"?

Teodor Sigaev E-mail: teodor(at)sigaev(dot)ru

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