BUG #14107: Major query planner bug regarding subqueries and indices

The following bug has been logged on the website:

Bug reference: 14107
Logged by: Mathias Kunter
Email address: mathiaskunter(at)gmail(dot)com
PostgreSQL version: 9.5.0
Operating system: Windows 7
Description:

The query planner doesn't use an index although it could, causing an
unneccessary sequential table scan. Step by step instructions to reproduce
the problem are given below.

Step 1 - just create a simple test table with an indexed id column:

CREATE TABLE test (id serial NOT NULL, CONSTRAINT pkey PRIMARY KEY (id));

Step 2 - note that the index is used for the following query as expected:

EXPLAIN SELECT * FROM test WHERE id = 1 OR id IN (2);
QUERY PLAN
-------------------------------------------------------------------------
Bitmap Heap Scan on test (cost=8.33..13.67 rows=2 width=4)
Recheck Cond: ((id = 1) OR (id = 2))
-> BitmapOr (cost=8.33..8.33 rows=2 width=0)
-> Bitmap Index Scan on pkey (cost=0.00..4.16 rows=1 width=0)
Index Cond: (id = 1)
-> Bitmap Index Scan on pkey (cost=0.00..4.16 rows=1 width=0)
Index Cond: (id = 2)

Step 3 - note that the index is NOT used for the following query:

EXPLAIN SELECT * FROM test WHERE id = 1 OR id IN (SELECT id FROM test WHERE
id = 2);
QUERY PLAN
-------------------------------------------------------------------------------------
Seq Scan on test (cost=8.17..56.42 rows=1275 width=4)
Filter: ((id = 1) OR (hashed SubPlan 1))
SubPlan 1
-> Index Only Scan using pkey on test test_1 (cost=0.16..8.17 rows=1
width=4)
Index Cond: (id = 2)