[CALCITE-4597] Allow RelNodes to have an empty row type (zero fields)

• Type: Bug
• Status: Open
• Priority: Major
• Resolution: Unresolved
• Assignee: Unassigned
• Reporter: Julian Hyde
• Created: 05/May/21 23:37
• Updated: 19/Oct/21 13:33

Description

Add an option, EmptyRowTypePolicy, to allow creation of {{RelNode}}s whose row type is empty. That is, contains zero fields.

There are three values:

• FORBIDDEN - Calcite prevents empty row type. (For example, planner and RelBuilder throw if they see one.) Rules must not produce empty row types. Rules can assume that they will not encounter empty row types.
• DISCOURAGED - Empty row types are discouraged. (Planner and RelBuilder will not throw if they see one.) Rules must not fail if they encounter an empty row type. Rules should not produce empty row types (with reasonable exceptions, such as if the input has an empty row type).
• ALLOWED - Empty row types are OK. All rules should handle {{RelNode}}s with empty row types, and it's OK if they generate {{RelNode}}s with empty row types.

The current policy is effectively DISCOURAGED. We try not to create empty RelNodes, but we don't check, and they crop up occasionally.

After this change, and for a few releases, the policy will be DISCOURAGED by default, but we will run tests in all three modes. All rules must run in all modes.

At some point in the future, we will change the default policy to ALLOWED. All rules must continue to run in all modes.

Issue Links

• relates to: CALCITE-4596 RelFieldTrimmer#trimFields fails if values row type is empty record
• relates to: CALCITE-4743 Convert rel to sql fail, when meeting aggregate of no-group and no-aggcall.
• relates to: CALCITE-4334 LITERAL_AGG, an aggregate function that returns a constant value
• relates to: CALCITE-4594 Interpreter returns wrong result when Values has zero fields
• relates to: CALCITE-4634 Improve AggregateProjectPullUpConstantsRule to remove all constant keys