The following bug has been logged on the website:
Bug reference: 19571
Logged by: cl hl
Email address: 2320415112@qq.com
PostgreSQL version: 17.10
Operating system: Linux LAPTOP-2SQAVLB0 6.6.87.2-microsoft-standard-
Description:
## Description
This issue concerns the Boolean identity `P OR FALSE = P`. The equivalence
remains valid under SQL three-valued logic, including when `P` evaluates to
NULL. When `P` is an `IN` subquery, PostgreSQL nevertheless optimizes the
two forms differently.
### Expected Behaviour
PostgreSQL should simplify `P OR FALSE` early enough that both forms are
planned identically. In particular, the redundant constant should not
prevent an `IN` subquery from being pulled up into a semijoin.
### Actual Behaviour
The plain `IN` predicate becomes a `Hash Semi Join`. Wrapping the same
predicate in `OR FALSE` leaves it as a hashed `SubPlan` attached to an outer
sequential scan. In the standalone case, execution time increases from 7.143
ms to 13.810 ms, approximately 1.93x.
The generated pair 837 shows a much larger order-stable instance: median
time increases from 2.750 ms to 668.339 ms, approximately 243.9x.
## How to repeat
```sql
DROP TABLE IF EXISTS identity_outer;
DROP TABLE IF EXISTS identity_inner;
CREATE TABLE identity_outer (v INTEGER NOT NULL);
CREATE TABLE identity_inner (v INTEGER NOT NULL);
INSERT INTO identity_outer
SELECT g FROM generate_series(90001, 91000) AS g;
INSERT INTO identity_inner
SELECT g FROM generate_series(1, 100000) AS g;
ANALYZE identity_outer;
ANALYZE identity_inner;
-- Original form P.
EXPLAIN (ANALYZE, BUFFERS, VERBOSE, SETTINGS, TIMING OFF)
SELECT COUNT(*)
FROM identity_outer AS o
WHERE o.v IN (
SELECT i.v FROM identity_inner AS i
);
-- Equivalent form P OR FALSE.
EXPLAIN (ANALYZE, BUFFERS, VERBOSE, SETTINGS, TIMING OFF)
SELECT COUNT(*)
FROM identity_outer AS o
WHERE (
o.v IN (SELECT i.v FROM identity_inner AS i)
) OR FALSE;
```
Both queries return 1,000. Characteristic plans and measured times are:
```text
P:
Hash Semi Join
Execution Time: 7.143 ms
P OR FALSE:
Seq Scan on identity_outer
Filter: ANY (... hashed SubPlan 1 ...)
Execution Time: 13.810 ms
```
