Parsed test spec with 2 sessions

starting permutation: r1 r2 w1 w2 c1 c2
step r1: SELECT COALESCE(MAX(invoice_number) + 1, 1) FROM invoice WHERE year = 2016;
coalesce
--------
       3
(1 row)

step r2: SELECT COALESCE(MAX(invoice_number) + 1, 1) FROM invoice WHERE year = 2016;
coalesce
--------
       3
(1 row)

step w1: INSERT INTO invoice VALUES (2016, 3);
step w2: INSERT INTO invoice VALUES (2016, 3); <waiting ...>
step c1: COMMIT;
step w2: <... completed>
ERROR:  could not serialize access due to read/write dependencies among transactions
step c2: COMMIT;

starting permutation: r1 w1 w2 c1 c2
step r1: SELECT COALESCE(MAX(invoice_number) + 1, 1) FROM invoice WHERE year = 2016;
coalesce
--------
       3
(1 row)

step w1: INSERT INTO invoice VALUES (2016, 3);
step w2: INSERT INTO invoice VALUES (2016, 3); <waiting ...>
step c1: COMMIT;
step w2: <... completed>
ERROR:  duplicate key value violates unique constraint "invoice_pkey"
step c2: COMMIT;

starting permutation: r2 w1 w2 c1 c2
step r2: SELECT COALESCE(MAX(invoice_number) + 1, 1) FROM invoice WHERE year = 2016;
coalesce
--------
       3
(1 row)

step w1: INSERT INTO invoice VALUES (2016, 3);
step w2: INSERT INTO invoice VALUES (2016, 3); <waiting ...>
step c1: COMMIT;
step w2: <... completed>
ERROR:  duplicate key value violates unique constraint "invoice_pkey"
step c2: COMMIT;