Performance problem

Hi there,

maybe anyone can help me with the following problem.
Using PostgreSQL 7.03 on a Suse 6.3 Linux (kernel 2.2.13) with a P3/550
MHz,
I get a performance problem with a select command which takes up to 10
seconds.
The following tables are involved in this command:

CREATE TABLE "property_list" (
"number" float8,
"level_1_de" text,
"level_1_en" text,
"level_2_de" text,
"level_2_en" text,

...

);

CREATE TABLE "properties" (
"material_oid" oid NOT NULL,
"property_list_oid" oid NOT NULL,

...

);

CREATE TABLE "materials" (
"user_oid" oid NOT NULL,
"material_class" oid,
"is_new" bool DEFAULT 't'::bool,

...

);

and the command is

SELECT DISTINCT level_1_de from property_list pl, properties p,
materials m where m.oid=p.material_oid and p.property_list_oid=pl.oid
and m.is_new=FALSE;

The command returns about 10 items.
The tables mentioned contain the following number of entries:

materials: 2476
properties: 30 323
property_list: 349

I hope that using INDEXes should speed up the search, but although I
made
several attempts, the search is too slow. Since I am not an expert on
databases and PostgreSQL, maybe somebody who had similar problems is
able to help me.
Also, I do not know whether the DISTINCT in the SELECT statement forces
the database to
use algorithms which have not been optimized yet.

Thanks in advance for your help,

kind regards,

Andreas