index use again and again

I read the Postgres boog, browsed the mailing list archives and have
still no clue.

I have a table with about 150.000 rows and put some indexes (not unique)
on it. If I use "=" in the where clause tha index is used, if I use ">"
or "between" then it is not used. It happens both with 7.1.3 and the new
7.2:

|db1=# select count(*) from verfuegbarkeiten;
| count
|--------
| 152428
|(1 row)
|
|db1=# explain select red from verfuegbarkeiten where datum =
|'2002-01-01';
|NOTICE:  QUERY PLAN:
|
|Index Scan using verfuegbarkeiten_datum_idx on verfuegbarkeiten
|(cost=0.00..1489.67 rows=566 width=4)
|
|EXPLAIN
|db1=# explain select red from verfuegbarkeiten where datum >
|'2002-01-01';
|NOTICE:  QUERY PLAN:
|
|Seq Scan on verfuegbarkeiten  (cost=0.00..3820.35 rows=22322 width=4)
|
|EXPLAIN

"vacuum analyze" has be done before (and is done daily). CASTs like
"where datum > '2002-01-01'::date" don't help.

Are 152428 rows not enough to use the index?


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