Optimizing >= and <= for numbers and dates

Hi all,

I haven't found any official documentation about the postgres sql optimizer
on the web, so please forgive me if there is such a document and point me to
the right direction.

I've got the following problem: I cannot make the postgres SQL Optimizer use
an index on a date field to filter out a date range, e.g.

select * from mytable where mydate >= '2003-10-01';

 Seq Scan on mytable  (cost=0.00..2138.11 rows=12203 width=543)
   Filter: (mydate >= '2003-09-01'::date)


the index is created as follows:

create index query on mytable(mydate);

Testing for equality gives me the index optimization:

select * from mytable where mydate = '2003-10-01';

Index Scan using query on mytable  (cost=0.00..54.93 rows=44 width=543)
   Index Cond: (mydate = '2003-09-01'::date)


I have run vacuum analyze on the table. Also the table contains 25.000
records, so the index should be used in my opinion. Am I missing something ?
The
same seems to apply to integers.

Thank you very much in advance
Dimi

PS The postgres version is as follows:

 PostgreSQL 7.3.2 on i386-redhat-linux-gnu, compiled by GCC
i386-redhat-linux-gcc (GCC) 3.2.2 20030213 (Red Hat Linux 8.0 3.2.2-1)



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