CREATE TABLE testsnap(t int);
CREATE TABLE testsnap2 ();
INSERT INTO testsnap VALUES(1),(1);


T1: 
BEGIN TRANSACTION ISOLATION LEVEL SERIALIZABLE;
SELECT count(*) FROM testsnap2; -- serializable snapshot is frozen

T2:
DELETE FROM testsnap;
ALTER TABLE testsnap ADD CONSTRAINT t unique(t);

T1:
SELECT count(*) FROM testsnap;
SELECT t1.* FROM testsnap t1 LEFT JOIN testsnap t2 USING(t);
explain analyze SELECT t1.* FROM testsnap t1 LEFT JOIN testsnap t2 USING(t);