Subject: make \d table Collation field showing domains collation if that attribute is type of domain.

From: jian he <[email-masked]>
Date: Thu, 3 Oct 2024 22:16:00 +0800

$subject related demos:
CREATE COLLATION case_insensitive (provider = icu, locale =
'@colStrength=secondary', deterministic = false);
create domain d_colli as text collate case_insensitive;
create domain d_collc as d_colli collate "C";
create table  s1(a d_colli);
create table  s2(a d_colli[] collate "C");
create table  s3(a d_collc);

currently in HEAD:
\d s1
                 Table "public.s1"
 Column |  Type   | Collation | Nullable | Default
--------+---------+-----------+----------+---------
 a      | d_colli |           |          |

\d s2
                  Table "public.s2"
 Column |   Type    | Collation | Nullable | Default
--------+-----------+-----------+----------+---------
 a      | d_colli[] | C         |          |

\d s3
                 Table "public.s3"
 Column |  Type   | Collation | Nullable | Default
--------+---------+-----------+----------+---------
 a      | d_collc |           |          |

we can change it to
\d s1
                    Table "public.s1"
 Column |  Type   |    Collation     | Nullable | Default
--------+---------+------------------+----------+---------
 a      | d_colli | case_insensitive |          |

\d s2
                  Table "public.s2"
 Column |   Type    | Collation | Nullable | Default
--------+-----------+-----------+----------+---------
 a      | d_colli[] | C         |          |

\d s3
                 Table "public.s3"
 Column |  Type   | Collation | Nullable | Default
--------+---------+-----------+----------+---------
 a      | d_collc | C         |          |

as you can see:
For non-domain types, everything is the same as before.
but domains with COLLATION clauses behave differently.

main changes:
@@ -1866,7 +1866,7 @@ describeOneTableDetails(const char *schemaname,
                attrdef_col = cols++;
                attnotnull_col = cols++;
                appendPQExpBufferStr(&buf, ",\n  (SELECT c.collname
FROM pg_catalog.pg_collation c, pg_catalog.pg_type t\n"
-                                                        "   WHERE
c.oid = a.attcollation AND t.oid = a.atttypid AND a.attcollation <>
t.typcollation) AS attcollation");
+                                                        "   WHERE
c.oid = a.attcollation AND t.oid = a.atttypid and c.collname <>
'default' ) AS attcollation");

it will make a table column with type "name", display Collation "C",
since "name" type typcollation is 'C',
other than this all behavior is the same as HEAD.

what do you think?

---

From: Tom Lane <[email-masked]>
Date: Thu, 03 Oct 2024 10:46:46 -0400

> 'default' ) AS attcollation");

I doubt that this is an improvement.  It will create as many
weird cases as it removes.  (To cite just one, there is nothing
preventing somebody from creating a collation named "default".)

The existing definition seems fine to me, anyway.  What's so
wrong with treating the domain's collation as being an implicit
property of the domain type?  What you want to do hopelessly confuses
this display between attributes of the table and attributes of the
column type.

A nearby comparable case is that the "Default" column only tells
about default values applied in the table definition, not any
that might have come from the domain.  Nor does the "Check
constraints" footer tell about constraints coming from a
domain column.

			regards, tom lane