PostgreSQL Select statement produces errors in datagrip but not pgAdmin4

I would guess you had open connections / transactions while you dropped the db initially and the DROP is stalled somehow.

You can get more info on databases with statistics views. For instance:

SELECT * FROM pg_stat_database WHERE datname = 'mixmobi_master';

In case you find open backend processes, you might want to shut them down like this:

SELECT pg_terminate_backend(procpid)
FROM   pg_stat_activity
WHERE  datname = 'mixmobi_master';

More about that in the manual here.

Find a safe method to drop a database under load at this related answer:

• Force drop db while others may be connected

If your function truly uses SELECT * then I would suggest switching to using an explicit list of fields and then separate out the amendment of the table and the function.

ADDING COLUMNS: Alter the table first, then change the function once the table is complete.

DELETING COLUMNS: Change the function first and then the table. If the table is large and under heavy load I'd still expect problems.