Surprising (?) Sequence Behavior

["Richard M. Kues" <software(at)riva(dot)at>]

Hallo all

During a performance tuning session I had a complex
query that gives some form of ranking. The "correct" way to
solve this, is the use of a scalar subquery that provides
the rank (or use "dense_rank over" in oracle).

But in my case the query is much too slow in this special case.
Even with small number of records that fit into memory (no IO).

So I'am searching for a faster solution and tried
also to use temporary sequences to achieve the same effect.

Example 1:
DROP SEQUENCE IF EXISTS s;
CREATE TEMPORARY SEQUENCE s;

SELECT
   nextval('s'), t.name
FROM
(
   SELECT
      tablename AS name
   FROM
      pg_tables
   ORDER BY
      tablename
) AS t;

gives:
   1 pg_aggregate
   2 pg_am
   3 pg_amop
   4 pg_amproc
   5 pg_attrdef
   6 pg_attribute
   7 pg_auth_members


But if this query is combined with a simple extension it does
not work as expected.

DROP SEQUENCE IF EXISTS s;
CREATE TEMPORARY SEQUENCE s;

SELECT
   nextval('s'), t.name
FROM
(
   SELECT
      tablename AS name
   FROM
      pg_tables
   ORDER BY
      tablename
) AS t
WHERE
   t.name = 'pg_am'
;

The result is:
   1 pg_am

instead of:
   2 pg_am


At least for me this is surprising!
Any hints? Or do I miss something obvious?

thanks a lot, richard