SQL to Check whether "AN HOUR PERIOD" is between start and end timestamps

Dear all,

I have a table that records User Login Sessions with two timestamp fields. Basically Start of Session and End of a Session (start_ts and end_ts). Each row in the table identifies a session which a customer has used. 

Data from the table (session):
-----------------------------
 customer_id | log_session_id  |          start_ts          |           end_ts
-------------+-----------------+----------------------------+----------------------------
 1006100716  | 209571229340224 | 15/12/2008 16:53:52.665327 | 15/12/2008 16:58:52.665327
 1006100789  | 112061228488202 | 05/12/2008 20:13:32.773065 | 09/12/2008 22:59:02.770218
 1006100888  | 214221233045949 | 27/01/2009 14:15:16.289626 | 27/01/2009 14:58:59.989182
 1006000008  | 205221236839534 | 12/03/2009 12:02:15.947509 | 12/03/2009 12:07:15.947509
 1006100825  | 112331229068049 | 12/12/2008 13:17:37.229249 | 12/12/2008 13:56:58.394577

The requirement is as follows,

I have to find out how many User Sessions that were present in any given "1 HOUR TIME PERIOD".  A single User Session can span across many days.
Example:
             start_ts          |           end_ts
    05/12/2008 20:13:32.773065 | 09/12/2008 22:59:02.770218

-----------------------------------------------------------------------------------------------------

Let me explain a scenario,

I have to find out the number of sessions during the interval '07/01/2009 11:00:00' to '07/01/2009 11:59:59'.

If I have to find the number of sessions present at any instant time say '07/01/2009 11:25:25', i can easily find out using the following Query,

select count(log_session_id) from session where '07/01/2009 11:25:25' between start_ts and end_ts ;

But, I have to find the number of sessions present during the "HOUR INTERVAL" '07/01/2009 11:00:00' to '07/01/2009 11:59:59'.

I tried using wildcards in timestamp '07/01/2009 11:%:%" but in vain. I tries googling / searching archives without any success either.

I feel this is a general requirement and this topic should have already been discussed.

Could someone help me solve this please ?  Any lead would do, like some special postgres-function or any other means.

Many Thanks,
../rssrik

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