[FLINK-13503] Add contract in `LookupableTableSource` to specify the behavior when lookupKeys contains null

• Type: Technical Debt
• Status: Open
• Priority: Not a Priority
• Resolution: Unresolved
• Affects Version/s: 1.9.0, 1.10.0
• Component/s: Connectors / JDBC, Table SQL / API
• Labels: auto-deprioritized-major auto-deprioritized-minor auto-unassigned pull-request-available
• Assignee: Unassigned
• Reporter: Jing Zhang
• Created: 31/Jul/19 06:14
• Updated: 15/Jul/22 22:37

Description

I think we should add contract in `LookupableTableSource` to specify expected behavior when the lookupKeys contains null value.
For example, one input record of eval method is (null,1) which means to look up data in (a,b) columns which key satisfy the requirement. there are at least three possibility here.

• to ignore null value, that is, in the above example, only looks `b = 1`
• to lookup `is value`, that is, in the above example, only looks `a is null and b = 1`
• to return empty records, that is, in the above example, only looks `a = null and b = 1`

In fact, there are different behavior in current code.
For example, in Jdbc connector,
The query template in `JdbcLookUpFunction` like:
SELECT c, d, e, f from T where a = ? and b = ?

If pass (null, 1) to `eval` method, it will generate the following query:
SELECT c, d, e, f from T where a = null and b = 1
Which always outputs empty records.
BTW, Is this behavior reasonable?

and the `InMemoryLookupableTableSource` behaviors like point 2 in the above list.
some private connector in Blink behaviors like point 1

Issue Links

• Is contained by: FLINK-16780 improve Flink lookup join
• links to: GitHub Pull Request #9335