Re: Different Lock Behavior With Create and Drop Foreign Key

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
-- 
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