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Re: [HACKERS] [PATCH] Generic type subscripting

Pavel Stehule <pavel.stehule@gmail.com>

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From: Pavel Stehule <pavel.stehule@gmail.com>
To: Dmitry Dolgov <9erthalion6@gmail.com>
Cc: Justin Pryzby <pryzby@telsasoft.com>, Tom Lane <tgl@sss.pgh.pa.us>, 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>, Thomas Munro <thomas.munro@enterprisedb.com>, Oleksandr Shulgin <oleksandr.shulgin@zalando.de>, Robert Haas <robertmhaas@gmail.com>, Andres Freund <andres@anarazel.de>, Michael Paquier <michael.paquier@gmail.com>, Oleg Bartunov <obartunov@gmail.com>, Peter Eisentraut <peter.eisentraut@2ndquadrant.com>, PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Date: 2020-09-17T15:19:19Z
Lists: pgsql-hackers 
čt 17. 9. 2020 v 15:56 odesílatel Dmitry Dolgov <9erthalion6@gmail.com>
napsal:

> > On Thu, Sep 17, 2020 at 02:47:54PM +0200, Pavel Stehule wrote:
> > > I have my concerns about the performance side of this implementation as
> > > well as how surprising this would be for users, but at the same time
> the
> > > patch already does something similar and the code change should not be
> > > that big, so why not - I can include this change into the next rebased
> > > version. But it still can cause some confusion as it's not going to
> work
> > > for negative indices, so
> > >
> > >     update foo set a[-10] = 1;
> > >
> > > and
> > >
> > >     select a[-10] from foo;
> > >
> > > can return different value from what was assigned. Otherwise, if we
> will
> > > try to fix a[-10] assignment in the same way, it will prepend the array
> > > and a[10] will not return the same value.
> >
> > What is semantic of negative index? It has clean semantic in C, but in
> > PLpgSQL?
>
> It's just a common pattern for jsonb when a negative index count from
> the end of an array. I believe it was like that from the very earlier
> implementations, although can't comment on that from the semantic point
> of view.
>

ok, then I think we can design some workable behaviour

My first rule - there should not be any implicit action that shifts
positions in the array. It can be explicit, but not implicit. It is true
for positive indexes, and it should be true for negative indexes too.

then I think so some like this can work

if (idx < 0)
{
  if (abs(idx) > length of array)
    exception("index is of of range");
  array[length of array - idx] := value;
}
else
{
   /* known behave for positive index */
}

Regards

Pavel

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.