Re: foreign key locks

Andres Freund wrote:

> No, I was thinking about an update without triggers present.
>
> T0: CREATE TABLE tbl(id serial pk, name text unique, data text);
> T1: BEGIN; -- read committed
> T1: UPDATE tbl SET name = 'foo' WHERE name = 'blarg'; /* key update of row id = 1 */
> T2: BEGIN; -- read committed
> T2: UPDATE tbl SET name = 'blarg', data = 'blarg' WHERE id = 1; /* no key update, waiting */
> T1: COMMIT;
> T2: /* UPDATE follows to updated row, due to the changed name its a key update now */
>
> Does that make sense?

So I guess your question is "is T2 now holding a TupleLockExclusive
lock?" To answer it, I turned your example into a isolationtester spec:

setup
{
CREATE TABLE tbl(id serial primary key, name text unique, data text);
INSERT INTO tbl VALUES (1, 'blarg', 'no data');
}

teardown
{
DROP TABLE tbl;
}

session "s1"
step "s1b" { BEGIN; }
step "s1u" { UPDATE tbl SET name = 'foo' WHERE name = 'blarg'; }
step "s1c" { COMMIT; }

session "s2"
step "s2b" { BEGIN; }
step "s2u" { UPDATE tbl SET name = 'blarg', data = 'blarg' WHERE id = 1; }
step "s2c" { COMMIT; }

session "s3"
step "s3l" { SELECT * FROM tbl FOR KEY SHARE; }

permutation "s1b" "s1u" "s2b" "s2u" "s1c" "s3l" "s2c"

And the results:
Parsed test spec with 3 sessions

starting permutation: s1b s1u s2b s2u s1c s3l s2c
step s1b: BEGIN;
step s1u: UPDATE tbl SET name = 'foo' WHERE name = 'blarg';
step s2b: BEGIN;
step s2u: UPDATE tbl SET name = 'blarg', data = 'blarg' WHERE id = 1; <waiting ...>
step s1c: COMMIT;
step s2u: <... completed>
step s3l: SELECT * FROM tbl FOR KEY SHARE; <waiting ...>
step s2c: COMMIT;
step s3l: <... completed>
id name data

1 blarg blarg

So session 3 is correctly waiting for session 2 to finish before being
ablt to grab its FOR KEY SHARE lock, indicating that session 2 is
holding a FOR UPDATE lock. Good.

If I change session 1 to update the data column instead of name, session
3 no longer needs to wait for session 2, meaning session 2 now only
grabs a FOR NO KEY UPDATE lock. Also good.

--
Álvaro Herrera http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services