Julian Hyde created CALCITE-4597:
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Summary: Allow RelNodes to have an empty row type (zero fields)
Key: CALCITE-4597
URL: https://issues.apache.org/jira/browse/CALCITE-4597
Project: Calcite
Issue Type: Bug
Reporter: Julian Hyde
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.
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