Unexpected serialization error

Hi guys,

I had no luck over at general slack channel. So I'm beginning to treat this
as a bug. Here's a test case that unexpectedly fails at last insert. I
expect it not to fail because the rows that the two transactions act on are
unrelated to one another.

CREATE TABLE t1 (
class integer NOT NULL
);

CREATE INDEX ON t1 (class);

CREATE TABLE t2 (
class integer NOT NULL
);

CREATE INDEX ON t2 (class);

ALTER SYSTEM SET enable_seqscan TO off;

-- Session 1
BEGIN TRANSACTION ISOLATION LEVEL SERIALIZABLE;
SELECT * FROM t2 WHERE class=1;

-- Session 2
BEGIN TRANSACTION ISOLATION LEVEL SERIALIZABLE;
SELECT * FROM t1 WHERE class=2;
INSERT INTO t2 VALUES(2);
COMMIT;

-- Session 1
INSERT INTO t1 VALUES(1);
COMMIT;

Postgres version is PostgreSQL 12.5 on x86_64-pc-linux-musl, compiled by
gcc (Alpine 9.3.0) 9.3.0, 64-bit.

-- Lule