Re: Different Lock Behavior With Create and Drop Foreign Key

Laurenz Albe <laurenz.albe@cybertec.at> writes:
> On Fri, 2020-04-10 at 01:40 +0000, Virendra Kumar wrote:
>> [ $subject ]

> That is because foreign keys are implemented with system triggers, some of which
> are defined on the target table.
> Now CREATE TRIGGER does not require an ACCESS EXCLUSIVE lock, but DROP TRIGGER does.

Yeah.  The documentation could be clearer about this though.  The relevant
bit on the ALTER TABLE page is

    Addition of a foreign key constraint requires a SHARE ROW EXCLUSIVE
    lock on the referenced table, in addition to the lock on the table
    receiving the constraint.

which, at least to my eyes, isn't very clear that SHARE ROW EXCLUSIVE
is the lock level used for *both* tables.

            regards, tom lane