Horton

Re: [HACKERS] [PATCH] Generic type subscripting

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

View thread View on postgr.es
From: Tom Lane <tgl@sss.pgh.pa.us>
To: Pavel Stehule <pavel.stehule@gmail.com>
Cc: Dmitry Dolgov <9erthalion6@gmail.com>, Andres Freund <andres@anarazel.de>, Alexander Korotkov <aekorotkov@gmail.com>, Justin Pryzby <pryzby@telsasoft.com>, David Steele <david@pgmasters.net>, Nikita Glukhov <n.gluhov@postgrespro.ru>, Alvaro Herrera <alvherre@2ndquadrant.com>, David Fetter <david@fetter.org>, Thomas Munro <thomas.munro@gmail.com>, Oleksandr Shulgin <oleksandr.shulgin@zalando.de>, Robert Haas <robertmhaas@gmail.com>, Michael Paquier <michael.paquier@gmail.com>, Oleg Bartunov <obartunov@gmail.com>, Peter Eisentraut <peter.eisentraut@2ndquadrant.com>, PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Date: 2020-12-22T19:21:22Z
Lists: pgsql-hackers 
Pavel Stehule <pavel.stehule@gmail.com> writes:
> But maybe we try to design some that are designed already. Is there some
> info about index specification in SQL/JSON?

We do have precedent for this, it's the rules about resolving argument
types for overloaded functions.  But the conclusion that that precedent
leads to is that we should check whether the subscript expression can
be *implicitly* coerced to either integer or text, and fail if neither
coercion or both coercions succeed.  I'd be okay with that from a system
design standpoint, but it's hard to say without trying it whether it
will work out nicely from a usability standpoint.  In a quick trial
it seems it might be okay:

regression=# create function mysub(int) returns text language sql
regression-# as $$select 'int'$$;
CREATE FUNCTION
regression=# create function mysub(text) returns text language sql
as $$select 'text'$$;
CREATE FUNCTION
regression=# select mysub(42);
 mysub 
-------
 int
(1 row)

regression=# select mysub('foo');
 mysub 
-------
 text
(1 row)

But there are definitely cases that will fail when an assignment
coercion would have succeeded, eg

regression=# select mysub(42::bigint);
ERROR:  function mysub(bigint) does not exist

Maybe that's okay.  (As I said earlier, we can't use assignment
coercion when there's two possible coercion targets, because it'd
be too likely that they both succeed.)

			regards, tom lane

Commits

  1. Throw error when assigning jsonb scalar instead of a composite object

  2. Filling array gaps during jsonb subscripting

  3. Implementation of subscripting for jsonb

  4. Allow ALTER TYPE to update an existing type's typsubscript value.

  5. Allow subscripting of hstore values.

  6. Support subscripting of arbitrary types, not only arrays.

  7. jit: Reference function pointer types via llvmjit_types.c.

  8. Teach contain_leaked_vars that assignment SubscriptingRefs are leaky.

  9. jit: Correct parameter type for generated expression evaluation functions.

  10. Renaming for new subscripting mechanism

  11. Fix assertion failure for SSL connections.

  12. Teach eval_const_expressions() to handle some more cases.