Pg-Mem returns incomplete data when index has a where clauses that doesn't capture it

[rswheeldon]

Describe the bug

If I have a partial index (for performance) but look for values which don't use that index, postgres still finds them but pg-mem doesn't.

To Reproduce

First set up a table and an index - this all works fine.

postgres=# create table foo(x integer, y integer, z integer);
CREATE TABLE
postgres=# create index foox1 on foo(y) where x = 1;
CREATE INDEX
postgres=# insert into foo values (1,2,3), (1,1,1), (2,3,5);
INSERT 0 3
postgres=# select * from foo;
 x | y | z 
---+---+---
 1 | 2 | 3
 1 | 1 | 1
 2 | 3 | 5
(3 rows)

postgres=# select * from foo where y = 1;
 x | y | z 
---+---+---
 1 | 1 | 1
(1 row)

Now add an additional row which falls outsides the index:

insert into foo values (4,1,4);
select * from foo where y = 1;

The index can't be used, hence why the postgres plan is this:

postgres=# explain select * from foo where y = 1;
                      QUERY PLAN                      
------------------------------------------------------
 Seq Scan on foo  (cost=0.00..35.50 rows=10 width=12)
   Filter: (y = 1)
(2 rows)

But (presumably) pg-mem still tries to use the index which is incomplete and thus returns the wrong data, namely:

pg-mem version

3.0.5 and current playground