Unexpected results when using system information functions #16026

CrateDB version

CrateDB 5.8.0-SNAPSHOT built 0dcd1a0

CrateDB setup information

Manual build following the instructions here: https://github.com/crate/crate/blob/master/devs/docs/basics.rst

Problem description

Consider the test case below. It is unexpected that the last query returns -1, because the result of the second query is NULL, which means that the WHERE clause in the last query should be evaluated to NULL and thus the row should be filtered.

I also observed similar issues in CURRENT_SETTING and PG_GET_FUNCTION_RESULT

Steps to Reproduce

DROP TABLE IF EXISTS t0;

CREATE TABLE t0(c1 INTEGER);
INSERT INTO t0(c1) VALUES (-1);
REFRESH TABLE t0;

SELECT * FROM t0; -- -1
SELECT (NOT ((PG_ENCODING_TO_CHAR(t0.c1))<(false))) FROM t0; -- NULL
SELECT * FROM t0 WHERE (NOT ((PG_ENCODING_TO_CHAR(t0.c1))<(false)));
-- Expected: empty table
-- Actual: -1

Actual Result

-1

Expected Result

empty table