Horton

Re: [HACKERS] [PATCH] Generic type subscripting

Dmitry Dolgov <9erthalion6@gmail.com>

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From: Dmitry Dolgov <9erthalion6@gmail.com>
To: Pavel Stehule <pavel.stehule@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: 2020-09-18T11:02:04Z
Lists: pgsql-hackers 
> On Thu, Sep 17, 2020 at 05:19:19PM +0200, Pavel Stehule wrote:
>
> 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 */
> }

In this way (returning an error on a negative indices bigger than the
number of elements) functionality for assigning via subscripting will be
already significantly differ from the original one via jsonb_set. Which
in turn could cause a new wave of something similar to "why assigning an
SQL NULL as a value returns NULL instead of jsonb?". Taking into account
that this is not absolutely new interface, but rather a convenient
shortcut for the existing one it probably makes sense to try to find a
balance between both consistency with regular array and similarity with
already existing jsonb modification functions.

Having said that, my impression is that this balance should be not fully
shifted towards consistensy with the regular array type, as jsonb array
and regular array are fundamentally different in terms of
implementation. If any differences are of concern, they should be
addressed at different level. At the same time I've already sort of gave
up on this patch in the form I wanted to see it anyway, so anything goes
if it helps bring it to the finish point. In case if there would be no
more arguments from other involved sides, I can post the next version
with your suggestion included.

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.