How to give file read permission to user postgres ?

Hi, all !

I try to use the "COPY" instruction in a postgresql database to import a csv file into a table :

COPY table FROM '/home/XXXXXX/YYYYYYY/test.csv' WITH CSV;

(XXXXXXX is the domain name, YYYYYYYY is a directory)

but I get "permission denied", even if the permissions of file test.csv are -rw-r--r-- (world readable ?)

I tried to add user "postgres" in XXXXXXX group, www-data group, but "permission denied" again.

Does anybody knows ho to give reading permission to the user postgres to certain files/directories ?

Thanks for any hint !

Fred