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  1. Doc: clarify locking requirements for ALTER TABLE ADD FOREIGN KEY.

  1. Different Lock Behavior With Create and Drop Foreign Key

    Virendra Kumar <viru_7683@yahoo.com> — 2020-04-10T01:40:52Z

    Hi Team,
Here is test case.----create table testx
(
    v_code character varying(32),
    client_id bigint
);
alter table testx add constraint testx_pkey primary key (v_code);

create table testy
(
    dxid bigint,
    v_code character varying(32)
);
alter table testy add constraint testy_pkey primary key (dxid);create index on testy (v_code);

Let's begin two session, in session1 I am going begin a transaction and run select on parent table:
Session1----begin;select * from testx;

On another session let's call session2, I am running create FK on second table
Session2--- alter table testy add constraint testy_fkey foreign key (v_code) references testx(v_code); <--This works.alter table testy drop constraint testy_fkey; <--Hangs


Regards,Virendra Kumar

  2. Re: Different Lock Behavior With Create and Drop Foreign Key

    Laurenz Albe <laurenz.albe@cybertec.at> — 2020-04-10T05:27:37Z

    On Fri, 2020-04-10 at 01:40 +0000, Virendra Kumar wrote:
> Here is test case.
> ----
> create table testx
> (
>     v_code character varying(32),
>     client_id bigint
> );
> alter table testx add constraint testx_pkey primary key (v_code);
> 
> create table testy
> (
>     dxid bigint,
>     v_code character varying(32)
> );
> alter table testy add constraint testy_pkey primary key (dxid);
> create index on testy (v_code);
> 
> Let's begin two session, in session1 I am going begin a transaction and run select on parent table:
> 
> Session1
> ----
> begin;
> select * from testx;
> 
> 
> On another session let's call session2, I am running create FK on second table
> 
> Session2
> ---
> alter table testy add constraint testy_fkey foreign key (v_code) references testx(v_code); <--This works.
> alter table testy drop constraint testy_fkey; <--Hangs

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.

Session 1 holds an ACCESS SHARE lock on the table, which conflicts only with ACCESS EXCLUSIVE.

So creating the foreign key works, but dropping it hangs when the triggers are dropped.

Yours,
Laurenz Albe
-- 
Cybertec | https://www.cybertec-postgresql.com

  3. Re: Different Lock Behavior With Create and Drop Foreign Key

    Tom Lane <tgl@sss.pgh.pa.us> — 2020-04-10T16:13:59Z

    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