Horton

Re: pgsql: Add more SQL/JSON constructor functions

Tom Lane <tgl@sss.pgh.pa.us>

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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Alvaro Herrera <alvherre@alvh.no-ip.org>
Cc: Amit Langote <amitlangote09@gmail.com>, jian he <jian.universality@gmail.com>, Peter Eisentraut <peter@eisentraut.org>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2024-06-29T18:56:15Z
Lists: pgsql-hackers 
Alvaro Herrera <alvherre@alvh.no-ip.org> writes:
>> +	/*
>> +	 * For domains, consider the base type's typmod to decide whether to setup
>> +	 * an implicit or explicit cast.
>> +	 */
>> +	if (get_typtype(returning->typid) == TYPTYPE_DOMAIN)
>> +		(void) getBaseTypeAndTypmod(returning->typid, &baseTypmod);

> TBH I'm not super clear on why we decide on explicit or implicit cast
> based on presence of a typmod.  Why isn't it better to always use an
> implicit one?

Hmm ... there are a bunch of existing places that seem to have similar
logic, but they are all in new-ish SQL/JSON functionality, and I would
not be surprised if they are all wrong.  parse_coerce.c is quite
opinionated about what a domain's typtypmod means (see comments in
coerce_type() for instance); see also the logic in coerce_to_domain:

     * If the domain applies a typmod to its base type, build the appropriate
     * coercion step.  Mark it implicit for display purposes, because we don't
     * want it shown separately by ruleutils.c; but the isExplicit flag passed
     * to the conversion function depends on the manner in which the domain
     * coercion is invoked, so that the semantics of implicit and explicit
     * coercion differ.  (Is that really the behavior we want?)

I don't think that this SQL/JSON behavior quite matches that.

While I'm bitching ... this coding style is bogus anyway:

>> +	if (get_typtype(returning->typid) == TYPTYPE_DOMAIN)
>> +		(void) getBaseTypeAndTypmod(returning->typid, &baseTypmod);

because it results in two syscache lookups not one.  You are supposed
to apply getBaseTypeAndTypmod unconditionally, as is done everywhere
except in the SQL/JSON logic.  I am also wondering how it can possibly
be sensible to throw away the function result of getBaseTypeAndTypmod
in this context.

			regards, tom lane

Commits

  1. SQL/JSON: Avoid initializing unnecessary ON ERROR / ON EMPTY steps

  2. SQL/JSON: Fix default ON ERROR behavior for JSON_TABLE

  3. SQL/JSON: Fix JSON_TABLE() column deparsing

  4. Update comment about ExprState.escontext

  5. SQL/JSON: Fix casting for integer EXISTS columns in JSON_TABLE

  6. SQL/JSON: Some fixes to JsonBehavior expression casting

  7. SQL/JSON: Remove useless code in ExecInitJsonExpr()

  8. SQL/JSON: Respect OMIT QUOTES when RETURNING domains over jsonb

  9. SQL/JSON: Improve error-handling of JsonBehavior expressions

  10. SQL/JSON: Fix error-handling of some JsonBehavior expressions

  11. SQL/JSON: Rethink c2d93c3802b

  12. SQL/JSON: Always coerce JsonExpr result at runtime

  13. SQL/JSON: Fix coercion of constructor outputs to types with typmod