ALTER TABLE ADD COLUMN takes forever

Hello Experts,

I am running a query on a table - ALTER TABLE mytable ADD COLUMN deposited
DOUBLE PRECISION DEFAULT 0.0;

this query just runs forever. Few more details about this table -
Total row count - 300000 (a small table of 200MB)
Columns - 42
Indexes - 15
Foreign key constraint - 14
Table reference - 11

When i check for the query in pg_stat_activity the wait even is *LOCK* and
wait_event_type is *RELATION*.
Also when i check in pg_locks the granted status is '*f*'. However there
are no pid's blocking this transaction.

This has just 500 dead tuples, so no problems there as well

I am a *DBA* and not good at development.
Unable to understand what would be causing this? Could you guys help me
what should be my next approach or solutions on this .

Regards,
Devendra Yadav