From: Tom Lane <tgl@sss.pgh.pa.us>
To: Laurenz Albe <laurenz.albe@cybertec.at>
cc: Virendra Kumar <viru_7683@yahoo.com>,
        Pgsql-general General <pgsql-general@postgresql.org>
Subject: Re: Different Lock Behavior With Create and Drop Foreign Key
In-reply-to: <3a4e7019df93301b82a98ddbfa73d31170d03a45.camel@cybertec.at>
References: <834603375.3470346.1586482852542.ref@mail.yahoo.com>
 <834603375.3470346.1586482852542@mail.yahoo.com>
 <3a4e7019df93301b82a98ddbfa73d31170d03a45.camel@cybertec.at>
Comments: In-reply-to Laurenz Albe <laurenz.albe@cybertec.at>
	message dated "Fri, 10 Apr 2020 07:27:37 +0200"
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Date: Fri, 10 Apr 2020 12:13:59 -0400
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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 T=
RIGGER 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