ERROR: found unexpected null value in index

Hi everyone,

Consider the following statement sequence:

CREATE TABLE t0(c0 TEXT);
INSERT INTO t0(c0) VALUES('b'), ('a');
ANALYZE;
INSERT INTO t0(c0) VALUES (NULL);
UPDATE t0 SET c0 = 'a';
CREATE INDEX i0 ON t0(c0);
SELECT * FROM t0 WHERE 'baaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa'
> t0.c0; -- unexpected: ERROR: found unexpected null value in index
"i0"

The SELECT can result in "ERROR: found unexpected null value in index
"i0"". I could reproduce this error only when some actions on other
databases are performed. The error is rather difficult to reproduce,
and small changes to the above statements cause it to no longer be
reproducible on my machine.

I've attached a Java program that runs the above statement sequence on
distinct databases using 32 threads, which, on my machine, reproduces
the error instantly. It is also possible to reproduce the error using
2 threads, which takes multiple minutes on my machine.

My Postgres version is Ubuntu 11.4-1.pgdg19.04+1.

Best,
Manuel