Index use and slow queries

<p><font size="2">Hello,</font><p><font size="2">My version of Postgresql is 7.4.3. <br />I have a simple table with 2
indexes:<br />                             Table "public.tst" <br /> Column |            Type            
|             Modifiers <br />--------+-----------------------------+------------------------------------- <br
/> tst_id| bigint                      | default nextval('tst_id_seq'::text) <br /> mmd5   | character
varying(32)      | not null <br /> active | character(1)                | not null <br /> lud    | timestamp without
timezone | default now() <br />Indexes: <br />    "tst_idx" unique, btree (mmd5, active) <br />    "tst_tst_id_key"
unique,btree (tst_id) <br /></font><p><font size="2">There are exactly 1,000,000 (one million) rows in the table
(tst). There are no NULLS, empty columns in any row.</font><p><font size="2">I get really fast response
times when usingthe following select statement (Less than 1 second). <br /></font><font size="2">maach=# explain select
*from tst where mmd5 = '71e1c18cbc708a0bf28fe106e03256c7' and active = 'A'; <br
/>                                             QUERY PLAN <br
/>------------------------------------------------------------------------------------------------------<br /> Index
Scanusing tst_idx on tst  (cost=0.00..6.02 rows=1 width=57) <br />   Index Cond: (((mmd5)::text =
'71e1c18cbc708a0bf28fe106e03256c7'::text)AND (active = 'A'::bpchar)) <br />(2 rows) <br /></font><p><font size="2">I
get reallyslow repoonse times when using the following select statement (About 20 seconds). <br />maach=# explain
select* from tst where tst_id = 639246; <br />                       QUERY PLAN <br
/>--------------------------------------------------------<br /> Seq Scan on tst  (cost=0.00..23370.00 rows=1 width=57)
<br/>   Filter: (tst_id = 639246) <br />(2 rows) <br /></font><p><font size="2">Why is the second select statement so
slow,it should be using the "tst_tst_id_key" unique, btree (tst_id) index, but instead EXPLAIN says it's using a Seq
Scan. If it was using the index, this select statement should be as fast if not faster than the above select
statement.</font><p><fontsize="2">When I turned off,  maach=# SET ENABLE_SEQSCAN TO OFF; <br />The slow select
statementgets even slower. <br />maach=# explain select * from tst where tst_id = 639246; <br
/>                            QUERY PLAN <br />-------------------------------------------------------------------- <br
/> SeqScan on tst  (cost=100000000.00..100023370.00 rows=1 width=57) <br />   Filter: (tst_id = 639246) <br />(2 rows)
<br/></font><p><font size="2">Why do I have to use 2 columns to create a fast/efficient index?  I want to get the
singlecolumn index to be the fastest index for my select statements.  How do I accomplish this.</font><p><font
size="2">Thanks,<br />Tom</font>