BUG #18643: EXPLAIN estimated rows mismatch

The following bug has been logged on the website:

Bug reference: 18643
Logged by: ming wei tan
Email address: mingwei(dot)tc(at)gmail(dot)com
PostgreSQL version: 12.20
Operating system: Debian 12.20-1.pgdg120+1 (Docker)
Description:

Given predicate A and B, it is expected that size (SELECT X where A) <=
size (SELECT X WHERE A or B)

However, `EXPLAIN SELECT t2.c0 FROM t2 WHERE t2.c0 IN (t2.c0)` returns
rows=2537
QUERY PLAN
------------------------------------------------------
Seq Scan on t2 (cost=0.00..35.50 rows=2537 width=4)
Filter: (c0 IS NOT NULL)

While, `EXPLAIN SELECT t2.c0 FROM t2 WHERE (t2.c0 IN (t2.c0)) OR (t2.c0 >
4)` returns rows=858
QUERY PLAN
-----------------------------------------------------
Seq Scan on t2 (cost=0.00..48.25 rows=858 width=4)
Filter: ((c0 = c0) OR (c0 > 4))

Running on docker image postgres:12

Test cases:

DROP DATABASE IF EXISTS database4;
CREATE DATABASE database4 WITH ENCODING 'UTF8' TEMPLATE template0;
\c database4;
CREATE TABLE t2(c0 int);
INSERT INTO t2(c0) VALUES(1);
INSERT INTO t2(c0) VALUES(2);
EXPLAIN SELECT t2.c0 FROM t2 WHERE t2.c0 IN (t2.c0);
EXPLAIN SELECT t2.c0 FROM t2 WHERE (t2.c0 IN (t2.c0)) OR (t2.c0 > 4);

DROP DATABASE IF EXISTS database4;
CREATE DATABASE database4 WITH ENCODING 'UTF8' TEMPLATE template0;
\c database4;
CREATE TABLE t2(c0 int);
INSERT INTO t2(c0) VALUES(1);
INSERT INTO t2(c0) VALUES(2);
EXPLAIN SELECT t2.c0 FROM t2 WHERE (t2.c0)=(t2.c0);
EXPLAIN SELECT t2.c0 FROM t2 WHERE ((t2.c0)=(t2.c0) OR (t2.c0 > 4);