On 2024-03-18 09:21 +0100, PG Doc comments form wrote:
> The following documentation comment has been logged on the website:
>
> Page: https://www.postgresql.org/docs/16/intarray.html
> Description:
>
> Hi,
> I recently ran into an unusual issue with the intarray extension where if
> you subtract one array from another the result is *also* sorted and
> de-duplicated. The documentation does not seem to imply that this should be
> the case, stating only that the operator "removes elements of the right
> array from the left array" and not that it also de-duplicates and sorts the
> result... It seems to only occur when subtracting an array. Is this the
> intended behavior?
>
> SELECT '{3,1,1,2,2,2}'::int[] - 1; --> {3,2,2,2} as you would expect
> SELECT '{3,1,1,2,2,2}'::int[] - '{1}'::int[]; --> {2,3} instead of
> {3,2,2,2}
>
> I have confirmed that I get the same result when using PostgreSQL 9 through
> 16 on DBFiddle. See here: https://dbfiddle.uk/i-eXKhFR
> I am using the official PostgreSQL 16.2 Docker image for Debian (PostgreSQL
> 16.2 (Debian 16.2-1.pgdg120+2) on x86_64-pc-linux-gnu, compiled by gcc
> (Debian 12.2.0-14) 12.2.0, 64-bit).
I don't know if it's intended behavior but it's implemented that way
since its inception (see intset_subtract in [1]). Also the intersection
and union operators behave similarly (sorted result without duplicates).
The attached patch changes the docs to state that the operator computes
the difference.
Operator integer[] - integer OTOH appears to be more in line with
integer[] + integer[] and integer[] + integer in that it doesn't treat
the arguments as sets.
It's unfortunate that both operations use the same operator and not
something like integer[] / integer[] which would be closer to the usual
notation for set difference.
[1]
https://git.postgresql.org/gitweb/?p=postgresql.git;a=commitdiff;h=181ca96e7a730ba35e973d3361422e6d8a460f88
--
Erik
>From 171ba158a1857e8d3323c758c73eb804609a939a Mon Sep 17 00:00:00 2001
From: Erik Wienhold <e...@ewie.name>
Date: Mon, 18 Mar 2024 17:04:09 +0100
Subject: [PATCH v1] Document intarray subtraction as set difference
The subtract operator provided by intarray actually has set semantics
and computes the difference of the provided arguments instead of just
removing elements. Document that analogous to the intersection and
union operators next to it.
---
doc/src/sgml/intarray.sgml | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/doc/src/sgml/intarray.sgml b/doc/src/sgml/intarray.sgml
index c72d49b01d..25838ff023 100644
--- a/doc/src/sgml/intarray.sgml
+++ b/doc/src/sgml/intarray.sgml
@@ -312,7 +312,7 @@
<returnvalue>integer[]</returnvalue>
</para>
<para>
- Removes elements of the right array from the left array.
+ Computes the difference of the arguments.
</para></entry>
</row>
--
2.44.0
