BUG #14953: pg_blocking_pids / array_length inconsistency
The following bug has been logged on the website:
Bug reference: 14953
Logged by: Steven Winfield
Email address: steven.winfield@cantabcapital.com
PostgreSQL version: 10.0
Operating system: Linux
Description:
It seems as though pg_blocking_pids() can return a zero-length,
one-dimensional array whereas users can only create NULL-length,
zero-dimensional arrays (even though the type suggests it is one
dimensional: 'integer[]')
test=# SELECT pg_blocking_pids(pg_backend_pid()),
array_length(pg_blocking_pids(pg_backend_pid()), 1), '{}'::integer[],
array_length('{}'::integer[], 1);
pg_blocking_pids | array_length | int4 | array_length
------------------+--------------+------+--------------
{} | 0 | {} |
(1 row)
This means that, for example, the code to check for an empty array of PIDs
must be special-cased.
For what it's worth, I think having array_length('{}'::integer[], 1) = 0 is
far more sensible, and
making differently-dimensioned empty arrays compare equal (as mentioned
previously[1]/messages/by-id/tencent_5D502954744B7F744ED6F34E@qq.com) is quite confusing.
[1]: /messages/by-id/tencent_5D502954744B7F744ED6F34E@qq.com
/messages/by-id/tencent_5D502954744B7F744ED6F34E@qq.com
On Thu, Dec 7, 2017 at 3:40 PM, <steven.winfield@cantabcapital.com> wrote:
For what it's worth, I think having array_length('{}'::integer[], 1) = 0 is
far more sensible, and
making differently-dimensioned empty arrays compare equal (as mentioned
previously[1]) is quite confusing.
I'm not saying this isn't a bug, but you might want to start using the
cardinality() function for comparisons like this.
.m
Thanks - I hadn’t spotted that, even though it was added in 9.4.
Steven.
From: Marko Tiikkaja [mailto:marko@joh.to]
Sent: 07 December 2017 15:07
To: Steven Winfield
Cc: Pg Bugs
Subject: Re: BUG #14953: pg_blocking_pids / array_length inconsistency
On Thu, Dec 7, 2017 at 3:40 PM, <steven.winfield@cantabcapital.com<mailto:steven.winfield@cantabcapital.com>> wrote:
For what it's worth, I think having array_length('{}'::integer[], 1) = 0 is
far more sensible, and
making differently-dimensioned empty arrays compare equal (as mentioned
previously[1]) is quite confusing.
I'm not saying this isn't a bug, but you might want to start using the cardinality() function for comparisons like this.
.m
steven.winfield@cantabcapital.com writes:
It seems as though pg_blocking_pids() can return a zero-length,
one-dimensional array whereas users can only create NULL-length,
zero-dimensional arrays (even though the type suggests it is one
dimensional: 'integer[]')
This has been fixed recently in HEAD:
https://git.postgresql.org/gitweb/?p=postgresql.git&a=commitdiff&h=716ea626a
but we decided not to back-patch the change.
regards, tom lane