probelm with alter table add constraint......

I have an issue with the add foreign key constraint which goes for waiting
and locks other queries as well.

ALTER TABLE ONLY holding_positions ADD CONSTRAINT
holding_positions_stock_id_fkey FOREIGN KEY (stock_id)
REFERENCES stocks (stock_id) MATCH SIMPLE
ON UPDATE NO ACTION ON DELETE NO ACTION;

The holding_positions table has no data in it as yet.

ps aux | grep postgres
postgres 5855 0.8 1.6 346436 271452 ? Ss 16:34 0:07
postgres: abc stocks 192.100.100.111(60308) SELECT waiting
postgres 6134 0.0 0.0 346008 4184 ? Ss 16:44 0:00
postgres: xyz stocks 192.100.100.222(34604) ALTER TABLE waiting

Any suggestions would be appreciated.

Roopa