Query with Max, Order by is very slow.......

Hi,

we have table having 23 million rows.
This is the table structure.
Table Request:

Column | Type | Modifiers
-----------+-----------------------------+-----------
origindb | character(1) | not null
uid | integer | not null
rtype | integer |
senderid | integer |
destaddr | character varying(15) |
opid | integer |
devmodel | integer |
ikind | integer |
itemid | character varying(10) |
tranfk | integer |
enteredon | timestamp without time zone |
status | integer |
accountid | integer |
Indexes:
"request_pkey" primary key, btree (origindb, uid)

I do max Query like this

select max(uid) from request where originDB=1;

it took around 20 min to return the result.. Since
max, count functions do the full table scan, i tried
the workaround given..

select uid from request where originDB=1 order by uid
desc limit 1;

this query runs forever.. i tried even without where
condition..no result..

I'm not able to figure out what could be the reason..
can anybody help?

Thanks in Advance

Regards
Priya

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