Thread: Getting the column to a which a sequence belongs.

Hi,

I'm trying to extend the Postgres support in my SQL tool. I'm trying to recreate the SQL for a sequence, and I wonder
ifthere is a way to find out the column to which a sequence "belongs".  

I'm talking either about sequences that are created automatically by PG when using the serial datatype or sequences
thathave been changed using ALTER SEQUENCE ... OWNED BY ... 

I know I can get the sequence that belongs to a column using pg_get_serial_sequence() (although only in 8.4), but I'm
lookingfor the other way: given a sequence find out if it's "owned" by a column.  

So far "debugging" psql using the -E option didn't show up anything and I couldn't find any hints in the system
catalogsdocumentation. 

Is this possible at all? As neither psql nor pgAdmin display this information, I suspect it's not.

Regards
Thomas

On Thu, Aug 27, 2009 at 09:18:57PM +0200, Thomas Kellerer wrote:
> I'm trying to extend the Postgres support in my SQL tool. I'm trying to
> recreate the SQL for a sequence, and I wonder if there is a way to find
> out the column to which a sequence "belongs".

The information is all in the system catalogs; I've not had much
opportunity to fiddle with them so far but the following may be a start
to help get things out for you.

  SELECT c.relname, a.attname, t.relname
  FROM pg_class c, pg_depend d, pg_class t, pg_attribute a
  WHERE c.relkind     = 'S'
    AND d.objid       = c.oid
    AND d.refobjid    = t.oid
    AND (d.refobjid,d.refobjsubid) = (a.attrelid,a.attnum);

The first reference to "pg_class" can probably be dropped as you can
convert the names of tables/sequences into their oid by using literals
of type "regclass".  For example, to pull out all the column names from
table "foo", you can do:

  SELECT attname
  FROM pg_attribute
  WHERE attrelid = 'foo'::regclass;

Have a look here for docs:

  http://www.postgresql.org/docs/current/static/catalogs.html

--
  Sam  http://samason.me.uk/

Sam Mason wrote on 27.08.2009 21:51:
> The information is all in the system catalogs; I've not had much
> opportunity to fiddle with them so far but the following may be a start
> to help get things out for you.
>
>   SELECT c.relname, a.attname, t.relname
>   FROM pg_class c, pg_depend d, pg_class t, pg_attribute a
>   WHERE c.relkind     = 'S'
>     AND d.objid       = c.oid
>     AND d.refobjid    = t.oid
>     AND (d.refobjid,d.refobjsubid) = (a.attrelid,a.attnum);
>
Ah great, I didn't realize I could use pg_depend for this.

Thanks, works like a charm! This is exactly what I was looking for.

Thomas