Horton

Re: [HACKERS] [PATCH] Generic type subscripting

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

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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Chapman Flack <chap@anastigmatix.net>
Cc: pgsql-hackers@lists.postgresql.org
Date: 2020-12-17T21:47:04Z
Lists: pgsql-hackers 
Chapman Flack <chap@anastigmatix.net> writes:
> That's likely to be what a programmer intends when writing
> (variable explicitly typed integer) := js['n'] and
> (variable explicitly types varchar) := js['v']

I think that what we want, if we're to support that sort of thing,
is that the js[] constructs produce jsonb by definition, and then an
assignment-level cast is applied to get from jsonb to integer or text.
I see we already have most of the necessary casts, but they're currently
marked explicit-only.  Downgrading them to assignment level might be
okay though.  If we don't want to do that, it means we have to write
	integervar := js['n']::integer
which is a bit more wordy but also unmistakable as to intent.  (I think
the "intent" angle might be the reason we insisted on these things
being explicit to start with.)

It's somewhat interesting to speculate about whether we could optimize
the combination of the subscripting function and the cast function.
But (a) that's an optimization, not something that should be part of
the user-visible semantics, and (b) it should not be part of the initial
feature.  I think a large part of the reason this patch is still not
done after four years is that it's been biting off more than it could
chew all along.  Let's try to get it to completion and then optimize
later.

As far as "treat as" is concerned, we already have a spelling for
that, it's called a cast.

			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.