Fix ClassCastException in PPL multisearch on indexes with @timestamp alias field

[gingeekrishna]

Summary

Fixes #5533.

When @timestamp is defined as a field-type alias in the index mapping (e.g. "@timestamp": {"type": "alias", "path": "timestamp"}), running a | multisearch command on that index throws:

java.lang.ClassCastException: class org.apache.calcite.rel.RelCompositeTrait
    cannot be cast to class org.apache.calcite.rel.RelCollation

Root cause

reIndexCollations() (CalciteLogicalIndexScan) and pushDownSort() (AbstractCalciteIndexScan) both called RelTraitSet.plus() to set the collation trait on a scan node. plus() composes traits — when the trait set already contains a RelCollation, it merges the old and new collations into a RelCompositeTrait. Calcite's RelTraitSet.getCollation() then does an unchecked cast (RelCollation) getTrait(...) which fails at runtime for RelCompositeTrait.

The @timestamp alias path specifically triggers this because wrapProjectForAliasFields() adds a LogicalProject on top of each sub-scan during multisearch plan building. Calcite's push-down rules later push this project back into the scan via pushDownProject() → reIndexCollations(), which uses plus() to remap an existing sort collation — producing the bad composite trait.

Fix

Use RelTraitSet.replace() in both locations instead of plus(). replace() substitutes the collation trait in-place regardless of what was already there, which is the correct semantics for "this scan is sorted by these columns".

Files changed (3):

• opensearch/.../AbstractCalciteIndexScan.java — pushDownSort(): plus → replace
• opensearch/.../CalciteLogicalIndexScan.java — reIndexCollations(): plus → replace
• integ-test/.../CalciteMultisearchCommandIT.java — regression IT against TEST_INDEX_ALIAS (which maps @timestamp as a field-type alias to original_date)

Test plan

• Added testMultisearchWithTimestampAliasFieldDoesNotThrow IT — runs a multisearch against an index where @timestamp is a field-type alias; this would have crashed with ClassCastException before this fix
• All existing CalciteMultisearchCommandIT tests pass
• OpenSearchIndexScanOptimizationTest and related unit tests pass (.replace() is correct semantics here — a scan's collation is always a full replacement, never a union of multiple sort orders)

Checklist

• New functionality includes testing
• New functionality has been documented
• Commits are signed and DCO verification will pass

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[Copilot]

Pull request overview

This PR fixes a Calcite planning/runtime ClassCastException triggered by | multisearch on indices where @timestamp is a field-type alias. The fix corrects collation trait handling on scan nodes so the collation trait is replaced (not composed) during sort/project pushdown.

Changes:

• Replace RelTraitSet.plus(RelCollations.of(...)) with RelTraitSet.replace(RelCollations.of(...)) in sort pushdown to avoid creating RelCompositeTrait.
• Replace RelTraitSet.plus(...) with replace(...) when reindexing collations after project pushdown.
• Add an integration regression test covering multisearch against an index with an alias-mapped @timestamp.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
opensearch/src/main/java/org/opensearch/sql/opensearch/storage/scan/CalciteLogicalIndexScan.java	Uses RelTraitSet.replace() when reindexing collations after project pushdown.
opensearch/src/main/java/org/opensearch/sql/opensearch/storage/scan/AbstractCalciteIndexScan.java	Uses RelTraitSet.replace() when setting collations during sort pushdown.
integ-test/src/test/java/org/opensearch/sql/calcite/remote/CalciteMultisearchCommandIT.java	Adds regression IT for multisearch on an index where @timestamp is an alias field.

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[mengweieric]

@gingeekrishna thanks for taking care of this issue, could you please check CI failures?