Обсуждение: BUG #18962: Type Conversion incorrect when performing UNION of queries.

The following bug has been logged on the website:

Bug reference:      18962
Logged by:          Sundar Mudupalli
Email address:      sundar@mudupalli.com
PostgreSQL version: 17.5
Operating system:   Ubuntu I believe (using a Cloud SQL instance in GC
Description:

Take the following table definitions:
```
create table public.test_table_1 as
SELECT
   'AA'::character(2) AS text_type
UNION ALL
SELECT
   'BB'::character(2) AS text_type
UNION ALL
SELECT
   'CC'::TEXT AS text_type
;
create table public.test_table_2 (
        char_fixed character,
        char_fixed_len character(5),
        char_var character varying(5),
        text_column text );
```
Based on the [type conversion rules for
Union](https://www.postgresql.org/docs/current/typeconv-union-case.html),
the column `text_type` in `test_table_1` should resolve to the postgres data
type `TEXT`. However running the following query produces the following:
```
select table_name, column_name, data_type, character_maximum_length from
information_schema.columns where table_name like 'test_table_%' order by
table_name;
  table_name  |  column_name   |     data_type     |
character_maximum_length
--------------+----------------+-------------------+--------------------------
 test_table_1 | text_type      | character         |
 test_table_2 | char_fixed     | character         |
1
 test_table_2 | char_fixed_len | character         |
5
 test_table_2 | char_var       | character varying |
5
 test_table_2 | text_column    | text              |
(5 rows)
```
The data type for text says `character` (fixed length character string) of
undetermined length, when it should really be of type `text` as the column
`text_column` in `test_table_2`.

PG Bug reporting form <noreply@postgresql.org> writes:
> create table public.test_table_1 as
> SELECT
>    'AA'::character(2) AS text_type
> UNION ALL
> SELECT
>    'BB'::character(2) AS text_type
> UNION ALL
> SELECT
>    'CC'::TEXT AS text_type
> ;

> Based on the [type conversion rules for
> Union](https://www.postgresql.org/docs/current/typeconv-union-case.html),
> the column `text_type` in `test_table_1` should resolve to the postgres data
> type `TEXT`.

I wonder how you read those rules to arrive at that result.  The
relevant step is

    5. Select the first non-unknown input type as the candidate type,
    then consider each other non-unknown input type, left to right.
    If the candidate type can be implicitly converted to the other
    type, but not vice-versa, select the other type as the new
    candidate type. Then continue considering the remaining
    inputs. If, at any stage of this process, a preferred type is
    selected, stop considering additional inputs.

So we start with char(2) as the candidate type, and nothing changes
when we consider the second UNION arm.  When we consider the third
UNION arm, there are implicit casts in both directions between
char(2) and text, so we will not change the candidate type there
either.

I'd be the first to agree that this behavior sometimes yields
non-intuitive results, but we've been using it for ~25 years.
Changing it now seems out of the question.

            regards, tom lane