Non-unique index performance

Hello!

I have a table eith name person as described below. It has an unique
index for id column (it is also primary key) and has an index for parent
column.

If I run a query with where clause on id column it uses the index (look
at the first explain analyze result; it says "Index Scan using...") and
the query for 582856 rows table results in 225,893 ms.

But, if I run another query with where clause on parent column it does
not use the index (look at the second explain analyze result; it says
"Seq Scan using...") and the query for 582856 rows table results in
11192.913 ms.

Why the difference of both queries is so dramatical for unique and
non-unique indexed columns? Why PostgreSQL does not use the non-unique
indexes (it says that it does sequential scan)?

I have to use an index on non-unique column. What is the solution for
that? Is there a way to speed up non-unique indexes?

*****************************************************************
test=> \d person
Table "public.person"
Column | Type | Modifiers
---------+-----------------------+-----------
name | character varying(30) |
surname | character varying(30) |
id | integer | not null
parent | integer |
Indexes:
"person_pkey" primary key, btree (id)
"parent_ndx" btree (parent)

test=> explain analyze select id,name from person where id in ('17201',
'338191', '244319', '515209', '20415');
QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Index Scan using person_pkey, person_pkey, person_pkey, person_pkey,
person_pkey on person (cost=0.00..30.12 rows=5 width=18) (actual
time=56.817..225.760 rows=5 loops=1)
Index Cond: ((id = 17201) OR (id = 338191) OR (id = 244319) OR (id =
515209)OR (id = 20415))
Total runtime: 225.893 ms
(3 rows)

test=> explain analyze select * from person where parent in ('17201',
'338191', '244319', '515209', '20415');
QUERY PLAN
-----------------------------------------------------------------------------------------------------------------
Seq Scan on person (cost=0.00..35871.26 rows=14427 width=32) (actual
time=0.063..11192.809 rows=5 loops=1)
Filter: ((parent = 17201) OR (parent = 338191) OR (parent = 244319)
OR (parent = 515209) OR (parent = 20415))
Total runtime: 11192.913 ms
(3 rows)

*****************************************************************

Thanks!

--
sy