Table lock while adding a column and clients are logged in

hello pg fans,

we have an application that communicates via ODBC directly to the postgres database.

if i'm trying to add an additional column to a table in pgadmin while clients are logged in, pgadmin hangs. only if all
cientsare logged out it returns to the normal state. according to our consultant of the application this behavior
doesn'tappear in oracle or sql-server. 

how can i avoid this?

cheers sven
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