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

Fixes #5533.

When `@timestamp` is defined as a field-type alias in the index mapping,
multisearch queries threw:

  ClassCastException: RelCompositeTrait cannot be cast to RelCollation

Root cause: `reIndexCollations()` in `CalciteLogicalIndexScan` and
`pushDownSort()` in `AbstractCalciteIndexScan` both called
`RelTraitSet.plus()` to update the collation trait on a scan node.
`plus()` *composes* traits — if 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 project on top of each sub-scan
which is later pushed back down via `pushDownProject()`.
`pushDownProject()` calls `reIndexCollations()` to remap field indices
inside an existing collation — but re-using `plus()` here composes the
existing sort collation with the re-indexed one, producing the bad
composite.

Fix: use `RelTraitSet.replace()` in both locations.  `replace()`
substitutes the collation trait in-place regardless of what was there
before, which is the correct semantics for "this scan is now sorted by
these columns".

Added a regression IT (`testMultisearchWithTimestampAliasFieldDoesNotThrow`)
that runs a multisearch against `TEST_INDEX_ALIAS`, whose mapping defines
`@timestamp` as an alias for `original_date`.

Signed-off-by: Radhakrishnan Pachyappan <gingeekrishna@gmail.com>

Author: gingeekrishna

integ-test/src/test/java/org/opensearch/sql/calcite/remote/CalciteMultisearchCommandIT.java

@@ -12,6 +12,7 @@
 import static org.opensearch.sql.util.MatcherUtils.schema;
 import static org.opensearch.sql.util.MatcherUtils.verifyDataRows;
 import static org.opensearch.sql.util.MatcherUtils.verifySchema;
+import static org.opensearch.sql.util.MatcherUtils.verifySchemaInOrder;
 
 import java.io.IOException;
 import org.json.JSONObject;
@@ -31,6 +32,7 @@ public void init() throws Exception {
     loadIndex(Index.TIME_TEST_DATA);
     loadIndex(Index.TIME_TEST_DATA2);
     loadIndex(Index.LOCATIONS_TYPE_CONFLICT);
+    loadIndex(Index.DATA_TYPE_ALIAS);
   }
 
   @Test
@@ -462,4 +464,38 @@ public void testMultisearchTypeConflictWithStats() {
             .getMessage()
             .contains("Unable to process column 'age' due to incompatible types:"));
   }
+
+  /**
+   * Regression test for GitHub issue #5533. When {@code @timestamp} is defined as a field-type
+   * alias in the index mapping, multisearch used to throw:
+   *
+   * <pre>ClassCastException: RelCompositeTrait cannot be cast to RelCollation</pre>
+   *
+   * <p>Root cause: {@code reIndexCollations()} and {@code pushDownSort()} both used
+   * {@code RelTraitSet.plus()} which composes collation traits into a {@link
+   * org.apache.calcite.rel.RelCompositeTrait} when a collation is already present. Calcite's
+   * {@code RelTraitSet.getCollation()} then fails with a ClassCastException. Fixed by using
+   * {@code RelTraitSet.replace()} instead to always replace the collation trait.
+   */
+  @Test
+  public void testMultisearchWithTimestampAliasFieldDoesNotThrow() throws IOException {
+    // TEST_INDEX_ALIAS has @timestamp defined as an alias field pointing to original_date.
+    // Running multisearch on such an index used to crash with ClassCastException.
+    JSONObject result =
+        executeQuery(
+            String.format(
+                "| multisearch "
+                    + "[search source=%s | where original_col > 1 | fields original_col,"
+                    + " @timestamp] "
+                    + "[search source=%s | where original_col = 1 | fields original_col,"
+                    + " @timestamp]",
+                TEST_INDEX_ALIAS, TEST_INDEX_ALIAS));
+
+    verifySchemaInOrder(
+        result,
+        schema("original_col", null, "int"),
+        schema("@timestamp", null, "timestamp"));
+    // 2 rows from original_col > 1, 1 row from original_col = 1
+    assertEquals(3, result.getInt("total"));
+  }
 }

opensearch/src/main/java/org/opensearch/sql/opensearch/storage/scan/AbstractCalciteIndexScan.java

@@ -323,7 +323,7 @@ && isAnyCollationNameInAggregators(collationNames)) {
         // aggregators.
         return null;
       }
-      RelTraitSet traitsWithCollations = getTraitSet().plus(RelCollations.of(collations));
+      RelTraitSet traitsWithCollations = getTraitSet().replace(RelCollations.of(collations));
       PushDownContext pushDownContextWithoutSort = this.pushDownContext.cloneWithoutSort();
       AbstractAction<?> action;
       Object digest;

opensearch/src/main/java/org/opensearch/sql/opensearch/storage/scan/CalciteLogicalIndexScan.java

@@ -321,7 +321,7 @@ private RelTraitSet reIndexCollations(List<Integer> selectedColumns) {
                   collation ->
                       collation.withFieldIndex(selectedColumns.indexOf(collation.getFieldIndex())))
               .collect(Collectors.toList());
-      newTraitSet = getTraitSet().plus(RelCollations.of(newCollations));
+      newTraitSet = getTraitSet().replace(RelCollations.of(newCollations));
     } else {
       newTraitSet = getTraitSet();
     }