Re: Best database structure for timely ordered values

Reiner Dassing <dassing@wettzell.ifag.de> writes:
> Here is another example, which is even worse:
> select value from table where id=1 order by epoch desc limit 1;
> to ask for the last stored value.
> This request needs about 5 minutes to complete.

Hm.  That should produce a decent plan given the right indexes.
On 7.0.2 I see:

play=> create table foo (id int, epoch timestamp primary key);
NOTICE:  CREATE TABLE/PRIMARY KEY will create implicit index 'foo_pkey' for table 'foo'
CREATE
play=> explain select * from foo where id=1 order by epoch desc limit 1;
NOTICE:  QUERY PLAN:

Index Scan Backward using foo_pkey on foo  (cost=0.00..62.50 rows=10 width=12)

EXPLAIN

which ought to work pretty well unless id=1 is very rare.

> The explain statements results in:

> explain select * from table where id=1 order by epoche desc limit 1;
> NOTICE:  QUERY PLAN:

> Sort  (cost=12692.74 rows=202175 width=16)
>   -> Index Scan using wetter_pkey on table  (cost=12692.74 rows=202175 width=16)

That's not very informative, since you haven't told us what that index is...
        regards, tom lane