[CALCITE-3969] Method RelTrait.apply(Mappings.Mapping) throws exception when mapping doesn't cover collation or distribution keys

• Type: Bug
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Fix Version/s: 1.23.0
• Component/s: core
• Labels: pull-request-available
• Assignee: Roman Kondakov
• Reporter: Roman Kondakov
• Created: 03/May/20 15:16
• Updated: 02/Sep/20 19:25
• Resolved: 11/May/20 18:44

Description

Let's consider we have an input (id, name) ordered by id (i.e. collation == [0]). If we have a Project("name") on the top of this input and we apply project's mapping on the collation, we'll end up with exception:

java.lang.NullPointerException: at index 0
	at com.google.common.collect.ObjectArrays.checkElementNotNull(ObjectArrays.java:239)
	at com.google.common.collect.ObjectArrays.checkElementsNotNull(ObjectArrays.java:230)
	at com.google.common.collect.ObjectArrays.checkElementsNotNull(ObjectArrays.java:225)
	at com.google.common.collect.ImmutableList.construct(ImmutableList.java:281)
	at com.google.common.collect.ImmutableList.copyOf(ImmutableList.java:239)
	at org.apache.calcite.rel.RelCollations.of(RelCollations.java:69)
	at org.apache.calcite.rex.RexUtil.apply(RexUtil.java:1271)
	at org.apache.calcite.rel.RelCollationImpl.apply(RelCollationImpl.java:122)
	at org.apache.calcite.rel.RelCollationImpl.apply(RelCollationImpl.java:40)

This happens because the collation field id is not a part of the mapping. The same problem is with distribution trait when distribution keys are not covered by the mapping.

Calcite should handle such situations gracefully. If it is not possible to deduce the collation/distribution after the mapping application, we should return:

• EMPTY collation for RelCollation trait.
• RANDOM_DISTRIBUTED distribution for RelDistribution trait.

Issue Links

• is related to: CALCITE-3889 Add apply(Mappings.Mapping) to RelTrait and RelTraitSet
• relates to: CALCITE-4218 Clarify RelMetadataQuery#getDistribution nullability
• links to: PR 1959