Execution plan problem

hi all,

I have a strange problem and really wish to get some help in here.

I have the following tables

create table shr_objects(
objectid int8 not null
companyid int4 not null
name varchar(255)
description varchar(255)
)
primary key : object id
foreign key on companyid references shr_companies(companyid)
Index on companyid
Number of rows ~ 1,410,000

create table smb_contacts{
contactid int4 not null
objectid int8 not null
firstname varchar(255)
lastname varchar(255)
)
primary key : contactid
foreign key on objectid references shr_objects9objectid)
index on : objectid
Number of rows ~ 10,000

I am trying to execute a query that joins the 2 tables on object id , it
works fine but when i add an order clause the performance is degarded
dramatically. I have included both quiries with their excution plan.

=============================================================================

intranet=# explain analyze SELECT obj.companyid, obj.name,
obj.description,
intranet-# cnt.firstname, cnt.lastname
intranet-# FROM smb_contacts cnt JOIN shr_objects obj ON cnt.objectid =
obj.objectid
intranet-# order by obj.companyid
intranet-# limit 90;
NOTICE: QUERY PLAN:

Limit (cost=44459.46..44459.46 rows=90 width=566) (actual
time=14426.92..14427.26 rows=90 loops=1)
-> Sort (cost=44459.46..44459.46 rows=10101 width=566) (actual
time=14426.91..14427.05 rows=91 loops=1)
-> Merge Join (cost=853.84..41938.61 rows=10101 width=566)
(actual time=123.25..14396.31 rows=10101 loops=1)
-> Index Scan using shr_objects_pk on shr_objects obj
(cost=0.00..37386.55 rows=1418686 width=544) (actual time=6.19..11769.85
rows=1418686 loops=1)
-> Sort (cost=853.84..853.84 rows=10101 width=22)
(actual time=117.02..134.60 rows=10101 loops=1)
-> Seq Scan on smb_contacts cnt (cost=0.00..182.01
rows=10101 width=22) (actual time=0.03..27.14 rows=10101 loops=1)
Total runtime: 14435.77 msec

EXPLAIN
==============================================================================
intranet=#
intranet=# explain analyze SELECT obj.companyid, obj.name,
obj.description,
intranet-# cnt.firstname, cnt.lastname
intranet-# FROM smb_contacts cnt JOIN shr_objects obj ON cnt.objectid =
obj.objectid
intranet-# limit 90;
NOTICE: QUERY PLAN:

Limit (cost=0.00..382.72 rows=90 width=566) (actual time=15.87..25.39
rows=90 loops=1)
-> Merge Join (cost=0.00..42954.26 rows=10101 width=566) (actual
time=15.86..25.08 rows=91 loops=1)
-> Index Scan using objectid_fk on smb_contacts cnt
(cost=0.00..1869.48 rows=10101 width=22) (actual time=15.76..16.32
rows=91 loops=1)
-> Index Scan using shr_objects_pk on shr_objects obj
(cost=0.00..37386.55 rows=1418686 width=544) (actual time=0.09..7.81
rows=193 loops=1)
Total runtime: 25.60 msec

EXPLAIN

It is obvious that in the order by query the company index is not used
and also it had to go thru all records in shr_objects.
Can someone please tell me how is this happening and how to fix it.

--
Hesham Gowaily <heshamgowaily(at)yahoo(dot)com>