Your hypothesis about index usage of count() and max() is correct.
As for why you see index usage in your first example query and not your
second: compare the number of rows in question. An index is extremely
useful if 19 rows will be returned. But when 62350411 rows will be
returned, you're talking about a substantial fraction of the table. A
sequential scan will probably correctly be judged to be faster by the
planner.
-tfo
--
Thomas F. O'Connell
Co-Founder, Information Architect
Sitening, LLC
http://www.sitening.com/
110 30th Avenue North, Suite 6
Nashville, TN 37203-6320
615-260-0005
On Mar 8, 2005, at 12:35 PM, Rick Schumeyer wrote:
> I have two index questions. The first is about an issue that has been
> recently discussed,
> and I just wanted to be sure of my understanding. Functions like
> count(), max(), etc. will
> use sequential scans instead of index scans because the index doesn’t
> know which rows
> are actually visible…is this correct?
>
>
>
> Second:
>
>
>
> I created an index in a table with over 10 million rows.
> The index is on field x, which is a double.
>
> The following command, as I expected, results in an index scan:
>
> =# explain select * from data where x = 0;
>
> QUERY PLAN
>
> -----------------------------------------------------------------------
> --
> Index Scan using data_x_ix on data (cost=0.00..78.25 rows=19
> width=34)
> Index Cond: (x = 0::double precision)
> (2 rows)
>
>
> But this command, in which the only difference if > instead of =, is a
> sequential scan.
>
>
> =# explain select * from data where x > 0;
>
> QUERY PLAN
>
> ------------------------------------------------------------------
>
> Seq Scan on data (cost=0.00..1722605.20 rows=62350411 width=34)
> Filter: (x > 0::double precision)
> (2 rows)
>
> Why is this?
>
> (This is with pg 8.0.1 on a PC running FC3 with 1GB ram…if it matters)