To: Peter Eisentraut <peter_e@gmx.net>
cc: saera87@hotmail.com, pgsql-bugs@postgresql.org
Subject: Re: BUG #5662: Incomplete view 
In-reply-to: <1284918458.20510.0.camel@vanquo.pezone.net> 
References: <201009190941.o8J9fT6H048689@wwwmaster.postgresql.org>
	<1284918458.20510.0.camel@vanquo.pezone.net>
Comments: In-reply-to Peter Eisentraut <peter_e@gmx.net>
	message dated "Sun, 19 Sep 2010 20:47:38 +0300"
Date: Sun, 19 Sep 2010 14:28:18 -0400
Message-ID: <6630.1284920898@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>

Peter Eisentraut <peter_e@gmx.net> writes:
> On sön, 2010-09-19 at 09:41 +0000, saera87@hotmail.com wrote:
>> The Sequence view in the information schema is incomplete. It does not
>> return a Sequence's maximum_value, minimum_value or increment. Please
>> complete the view.

> This is known and documented:
> http://www.postgresql.org/docs/8.4/static/infoschema-sequences.html

> Should still be fixed eventually, of course.

I think the difficulty is in the fact that you can't join to a sequence
whose name isn't predetermined.  In the past we've speculated about
creating a single catalog or view containing all sequences' parameters,
so that information_schema.sequences could be implemented with a join
to that.  However, there's never been any movement on that, and it seems
less than trivial to do.

What about inventing a function to extract a sequence's parameters?
Perhaps something like

	pg_sequence_parameter(seq regclass, colname text) returns bigint

which would do an appropriate permissions check and then fetch the named
column.  (This could actually be implemented in a line or two in
plpgsql, but I think we want it in C because information_schema
shouldn't depend on plpgsql.)  This would work OK for all the bigint
columns, and we could cheat a bit for the boolean columns by returning
0 or 1.  You couldn't fetch the sequence_name column this way, but
that's okay with me --- we don't maintain that anyway.

Given that, the sequence view would include outputs like

           CAST(pg_sequence_parameter(c.oid, 'max_value') AS cardinal_number) AS maximum_value,

The main objection I can see to this is that fetching multiple column
values would involve multiple accesses to the sequence.  But it's not
clear that a solution based on a single view would be any better
performance-wise.

Another possibility, if we had LATERAL, would be a function that
takes just the sequence OID and returns all its parameters as a row.
But again, if we want to do it that way then fixing the view will
involve waiting for a complex feature that might or might not
show up anytime soon.

Or maybe we could implement that function, call it like this

           CAST((pg_sequence_parameters(c.oid)).max_value AS cardinal_number) AS maximum_value,

and plan on optimizing the view when we get LATERAL.

			regards, tom lane