Horton

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: 2020-09-17T12:47:54Z
Lists: pgsql-hackers 
> There should be consistency
> >
> > postgres=# create table foo2(a text[]);
> > CREATE TABLE
> > postgres=# insert into foo2 values('{}');
> > INSERT 0 1
> > postgres=# update foo set a[10] = 'AHOJ';
> > UPDATE 1
> > postgres=# select (a)[10] from foo;
> > ┌────────┐
> > │   a    │
> > ╞════════╡
> > │ "AHOJ" │
> > └────────┘
> > (1 row)
> >
> > and some natural behaviour - any special case with different behaviour
> is a
> > bad thing generally.
>
> Yeah, I see your point. IIRC there is no notion of an arbitrary index in
> jsonb array, so it needs to be done within an assignment operation
> similar to how the last patch fills the gaps between elements. Taking
> into account, that if there are more than one elements in the array, all
> the gaps should be filled and the behaviour is already the same as you
> described, what needs to be changed is more nulls need to be added
> around before the first element depending on the assignment index.
>

> 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?

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.