BUG #14953: pg_blocking_pids / array_length inconsistency

From: steven(dot)winfield(at)cantabcapital(dot)com
To: pgsql-bugs(at)postgresql(dot)org
Cc: steven(dot)winfield(at)cantabcapital(dot)com
Subject: BUG #14953: pg_blocking_pids / array_length inconsistency
Date: 2017-12-07 14:40:28
Message-ID: 20171207144028.21691.76008@wrigleys.postgresql.org
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The following bug has been logged on the website:

Bug reference: 14953
Logged by: Steven Winfield
Email address: steven(dot)winfield(at)cantabcapital(dot)com
PostgreSQL version: 10.0
Operating system: Linux

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]) is quite confusing.



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