The following query blocks the postgres server when executed
ALTER TABLE "image_data" ADD COLUMN "user_status" varchar(30) NULL
engine version 10.6
# of rows 1.5 mils
I have a somewhat odd problem with my production database server (I am unable to reproduce on my local server). As part of the deployment process I wanted to add a new column with a default value of null. When I ran the above query (which was produced by dajngo's migration process), It failed and in the process locked the database and blocked other queries (non-locking selects) from executing. Since it was a production environment, I rolled back after a few minutes (approximately 10). What I find most unusual is that this same query executes within less than a second on my local db replicate (which has the exact same data as the production).
Edit: When I run this transaction, it blocks the access to the database for all other select queries - which I think (please correct me if I'm wrong) implies that the database is in fact blocked by this transaction.