> i've heard that long running transaction(updatable) affect performance.

Only indirectly - long running transactions prevent garbage collecting.
(in the whole database, not only in one table)

> what can be said long running transaction
> i mean how long is long and with how many users and how it affects ,
> will my select queries get slow down ?(how much) ?

Depends on what is happening in the database - if you make 10 updates
a day, then transaction opened for a week will not degrade performance.
(It can affect only concurrency, i.e. row updated in one tansaction can't
be updated by concurrent transactions until the change is committed/rolled back)

Ivan

>
> can transaction - open for 16 hour with 15 users be said long running and
> does it affect performance ?
>
> does it affects performance of transactions aginst that perticular tables
> (on which
> transaction is running) or
> does it affect performance for whole database for all the tables