Incorrect column metadata returned by ResultSetMetaData#isNullable for columns that contain null values filled from a left join

[jie-d]

Describe the issue
ResultSetMetaData#isNullable returns ResultSetMetaData.columnNoNulls for columns that contain null values filled from a left join.
We had previously filed this bug when we first discovered the issue however that did not lead anywhere. We are able to provide an easier repro this time, see To Reproduce below.

Driver Version?
42.2.14

Java Version?
8.0.275.open

OS Version?
Windows 10

PostgreSQL Version?
12.5

To Reproduce
Create table schedules that contains a NOT NULL integer-type column schedule_id:

CREATE TABLE schedules (
    schedule_id         integer NOT NULL
);

Info on the actual data of schedule_id:
All values of column schedule_id stored in the table schedules are positive integers.

Execute query:

SELECT
         *
FROM (
        SELECT
            -1              s_id 
        ) s
        LEFT JOIN schedules
           ON s_id = schedule_id

Result of the query:

----------------------------
s_id       |    schedule_id
 -1        |       Null
----------------------------

Expected behavior
Calling ResultSetMetaData#isNullable on the object returned from PreparedStatement#getMetadata or Statement#executeQuery#getMetaData for this query suggests that the column schedule_id is nullable given that the column can be filled with null values from the left join.

Actual behavior
ResultSetMetaData#isNullable returns ResultSetMetaData.columnNoNulls in the above case.

[davecramer]

Unfortunately we don't get this information from the backend so it's impossible to fix.

[jie-d]

Hi Dave, can you elaborate a bit on the info needed from the backend here? Thanks!

[davecramer]

we see the table metadata not the result set metadata. So we think it is not nullable