Missing SELECT results when filtering conditional on IS NULL on an indexed column along with any other condition

[davidfiala]

Describe the bug

In queries/updates on rows where you conditionally select on a column with an index against NULL /plus/ one more condition, the results will be empty.

Conditions required for bug:

• WHERE clause with at least 2 conditions AND'd together
• You must be checking for IS NULL or IS NOT NULL on an indexed column.
• The indexed condition must come before some other unrelated condition.
  • The unrelated check can be as simple as true or 1=1, but it must be present.
  • The order is critical to reproduce this bug, as shown in the example below we demonstrate it only happens in one direction.

To Reproduce

https://oguimbal.github.io/pg-mem-playground/

CREATE TABLE "public"."foo" (apple integer,banana integer,charlie integer);
CREATE INDEX "idx1" on "public"."foo" ("apple"); -- when this index is removed, all queries return results
INSERT INTO "foo" (apple, banana, charlie) values (10, 20, 30);
SELECT * FROM FOO;
SELECT * FROM FOO WHERE TRUE;
SELECT * FROM FOO WHERE TRUE AND TRUE;
SELECT * FROM FOO WHERE APPLE = 10 AND TRUE;
SELECT * FROM FOO WHERE APPLE IS NOT NULL;
SELECT * FROM FOO WHERE TRUE AND APPLE IS NOT NULL;
SELECT * FROM FOO WHERE APPLE IS NOT NULL AND TRUE; -- **bug** Actual is empty, but expected results

-- just to prove it's both 'IS NULL' and 'IS NOT NULL' we flip it around too:
UPDATE FOO SET APPLE = NULL;
SELECT * FROM FOO;
SELECT * FROM FOO WHERE TRUE;
SELECT * FROM FOO WHERE TRUE AND TRUE;
SELECT * FROM FOO WHERE APPLE IS NULL;
SELECT * FROM FOO WHERE TRUE AND APPLE IS NULL;
SELECT * FROM FOO WHERE APPLE IS NULL AND TRUE; -- **bug** Actual is empty, but expected results

NOTE: When you comment out the index, all queries work.

pg-mem version

2.8.1

[abdul-mattee-pikessoft]

Still facing this issue with 2.8.1.