cannot use multicolumn index

dear all,

i have a table with (approx) 500.000.000 rows. it has several indexes, one
of which is a multicolumn index
for a column that has an id (integer) and a column that has a timestamp. i
have read in the manual that the multicolumn index can be used only if the
clauses of the query are in the same order as the columns of the index. so i
am trying the following simple query ->

server=# explain select count(*) from temp_by_hour where xid > 100 and xdate
> now() - interval '1 week';
*                                           QUERY PLAN
------------------------------------------------------------------------------------------------
 Aggregate  (cost=29356607.86..29356607.87 rows=1 width=0)
   ->  Seq Scan on temp_by_hour i  (cost=0.00..29342531.72 rows=5630456
width=0)
         Filter: ((xid > 100) AND (xdate > (now() - '7 days'::interval)))*

and the index is this ->
*"temp_by_hour_idx" btree (xid, xdate)*

the size of the index is this ->
* public | temp_by_hour_idx                    | index | mydb| temp_by_hour
| 35 GB *

and the size of the table is this ->
* public |temp_by_hour                            | table    | mydb| 115 GB
*


any ideas on how i should write to query to use this index? thx in advance



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