BUG #5926: information schema dtd_identifier for element_types, columns, parameters views inconsistent

The following bug has been logged online:

Bug reference: 5926
Logged by: Ingmar Brouns
Email address: swingi(at)gmail(dot)com
PostgreSQL version: 9.0.3
Operating system: Ubuntu 9.0.4
Description: information schema dtd_identifier for element_types,
columns, parameters views inconsistent
Details:

Hi,

I am writing a function that needs to retrieve information with respect to
the types of parameters of functions. I use the information schema for that.
The parameters view documentation states:

data_type: Data type of the parameter, if it is a built-in type, or ARRAY if
it is some array (in that case, see the view element_types)

So for arrays I will have to join with information_schema.element_types

http://www.postgresql.org/docs/9.0/static/infoschema-element-types.html

At the top op that documentation is some example code, it joins on
dtd_identifier, the code does not work:

create table public.test_table(a varchar array, b integer, c integer
array);

SELECT c.column_name, c.data_type, e.data_type AS element_type
FROM information_schema.columns c
LEFT JOIN information_schema.element_types e
ON ((c.table_catalog, c.table_schema, c.table_name, 'TABLE',
c.dtd_identifier)
= (e.object_catalog, e.object_schema, e.object_name, e.object_type,
e.dtd_identifier))
WHERE c.table_schema = 'public' AND c.table_name = 'test_table'
ORDER BY c.ordinal_position;

column_name | data_type | element_type
-------------+-----------+--------------
a | ARRAY |
b | integer |
c | ARRAY |
(3 rows)

The same holds when joining with the parameters view. The reason seems to be
that the dtd_identifier of the element_types view has prepended 'a's whereas
the dtd_identifiers of the columns and parameter views do not:

select column_name,dtd_identifier
from information_schema.columns c
where c.table_schema = 'public'
and c.table_name = 'test_table';

column_name | dtd_identifier
-------------+----------------
a | 1
b | 2
c | 3
(3 rows)

select dtd_identifier
from information_schema.element_types e
where e.object_schema = 'public'
and e.object_name = 'test_table';

dtd_identifier
----------------
a1
a3
(2 rows)

The element_types view has a column 'collection_type_identifier', this
column is not present in the documentation. It is defined exactly as the
dtd_identifier, only then without the prepended 'a':

('a'::text || x.objdtdid::text)::information_schema.sql_identifier AS
dtd_identifier
x.objdtdid ::information_schema.sql_identifier AS
collection_type_identifier

When I modify the example code to join on this column instead, I get the
expected results:

SELECT c.column_name, c.data_type, e.data_type AS element_type
FROM information_schema.columns c LEFT JOIN information_schema.element_types
e
ON ((c.table_catalog, c.table_schema, c.table_name, 'TABLE',
c.dtd_identifier)
= (e.object_catalog, e.object_schema, e.object_name, e.object_type,
e.collection_type_identifier))
WHERE c.table_schema = 'public' AND c.table_name = 'test_table'
ORDER BY c.ordinal_position;

column_name | data_type | element_type
-------------+-----------+-------------------
a | ARRAY | character varying
b | integer |
c | ARRAY | integer
(3 rows)

Kind regards,

Ingmar Brouns