From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
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To: | Robert Haas <robertmhaas(at)gmail(dot)com> |
Cc: | Andres Freund <andres(at)anarazel(dot)de>, Dmitry Dolgov <9erthalion6(at)gmail(dot)com>, Michael Paquier <michael(dot)paquier(at)gmail(dot)com>, Arthur Zakirov <a(dot)zakirov(at)postgrespro(dot)ru>, Oleg Bartunov <obartunov(at)gmail(dot)com>, Peter Eisentraut <peter(dot)eisentraut(at)2ndquadrant(dot)com>, David Steele <david(at)pgmasters(dot)net>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: [HACKERS] [PATCH] Generic type subscripting |
Date: | 2018-01-11 19:20:52 |
Message-ID: | 12567.1515698452@sss.pgh.pa.us |
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Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> On Thu, Jan 11, 2018 at 1:37 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> Right, but in the case of stored arrays, we've decided that it *is*
>> our problem (as indeed it must be, because the user has no tools with
>> which they could fix a representation change for stored data). The
>> question is to what extent that need would propagate to pseudo array
>> types.
> I think I view the rationale a bit differently. Let's say that a user
> defines a composite type as (a int, b text) and uses that composite
> type as a column type. Then, somebody tries to change column a to
> have type text, and suppose we don't throw an error but simply permit
> the operation. If the user now tries to select from the offending
> column, the server will very likely crash. In contrast, in the case
> where the user has defined an SQL function that selects $1.a and
> returns it as an int, they will get a runtime error when they try to
> use the function. In my mind, that is the critical difference.
There are two critical differences --- that's one, and the other is
that there are SQL-level ways to fix the problem, ie change the function
text with CREATE OR REPLACE FUNCTION. We don't have a SQL command that
says "now go update the representation of table T column C".
But I think we've probably beaten this topic to death ...
regards, tom lane
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