Implement reverse performance optimization

core/src/main/java/org/opensearch/sql/calcite/CalciteRelNodeVisitor.java

@@ -47,9 +47,14 @@
 import org.apache.calcite.adapter.enumerable.RexToLixTranslator;
 import org.apache.calcite.plan.RelOptTable;
 import org.apache.calcite.plan.ViewExpanders;
+import org.apache.calcite.rel.BiRel;
+import org.apache.calcite.rel.RelCollation;
 import org.apache.calcite.rel.RelNode;
 import org.apache.calcite.rel.core.Aggregate;
 import org.apache.calcite.rel.core.JoinRelType;
+import org.apache.calcite.rel.core.SetOp;
+import org.apache.calcite.rel.core.Uncollect;
+import org.apache.calcite.rel.logical.LogicalProject;
 import org.apache.calcite.rel.logical.LogicalValues;
 import org.apache.calcite.rel.type.RelDataType;
 import org.apache.calcite.rel.type.RelDataTypeFamily;
@@ -681,25 +686,162 @@ public RelNode visitHead(Head node, CalcitePlanContext context) {
     return context.relBuilder.peek();
   }
 
-  private static final String REVERSE_ROW_NUM = "__reverse_row_num__";
+  /**
+   * Backtrack through the RelNode tree to find the first Sort node with non-empty collation. Stops
+   * at blocking operators that break ordering:
+   *
+   * <ul>
+   *   <li>Aggregate - aggregation destroys input ordering
+   *   <li>BiRel - covers Join, Correlate, and other binary relations
+   *   <li>SetOp - covers Union, Intersect, Except
+   *   <li>Uncollect - unnesting operation that may change ordering
+   *   <li>Project with window functions (RexOver) - ordering determined by window's ORDER BY
+   * </ul>
+   *
+   * @param node the starting RelNode to backtrack from
+   * @return the collation found, or null if no sort or blocking operator encountered
+   */
+  private RelCollation backtrackForCollation(RelNode node) {
+    while (node != null) {
+      // Check for blocking operators that destroy collation
+      // BiRel covers Join, Correlate, and other binary relations
+      // SetOp covers Union, Intersect, Except
+      // Uncollect unnests arrays/multisets which may change ordering
+      if (node instanceof Aggregate
+          || node instanceof BiRel
+          || node instanceof SetOp
+          || node instanceof Uncollect) {
+        return null;
+      }
+
+      // Project with window functions has ordering determined by the window's ORDER BY clause
+      // We should not destroy its output order by inserting a reversed sort
+      if (node instanceof LogicalProject && ((LogicalProject) node).containsOver()) {
+        return null;
+      }
+
+      // Check for Sort node with collation
+      if (node instanceof org.apache.calcite.rel.core.Sort) {
+        org.apache.calcite.rel.core.Sort sort = (org.apache.calcite.rel.core.Sort) node;
+        if (sort.getCollation() != null && !sort.getCollation().getFieldCollations().isEmpty()) {
+          return sort.getCollation();
+        }
+      }
+
+      // Continue to child node
+      if (node.getInputs().isEmpty()) {
+        break;
+      }
+      node = node.getInput(0);
+    }
+    return null;
+  }
+
+  /**
+   * Insert a reversed sort node after finding the original sort in the tree. This rebuilds the tree
+   * with the reversed sort inserted right after the original sort.
+   *
+   * @param root the root of the tree to rebuild
+   * @param reversedCollation the reversed collation to insert
+   * @param context the Calcite plan context
+   * @return the rebuilt tree with reversed sort inserted
+   */
+  private RelNode insertReversedSortInTree(
+      RelNode root, RelCollation reversedCollation, CalcitePlanContext context) {
+    return root.accept(
+        new org.apache.calcite.rel.RelHomogeneousShuttle() {
+          boolean sortFound = false;
+
+          @Override
+          public RelNode visit(RelNode other) {
+            if (!sortFound && other instanceof org.apache.calcite.rel.core.Sort) {
+              org.apache.calcite.rel.core.Sort sort = (org.apache.calcite.rel.core.Sort) other;
+              // Treat a Sort with fetch or offset as a barrier (limit node).
+              // Place the reversed sort above the barrier to preserve limit semantics,
+              // rather than inserting below the downstream collation Sort.
+              if (sort.fetch != null || sort.offset != null) {
+                sortFound = true;
+                RelNode visitedBarrier = super.visit(other);
+                return org.apache.calcite.rel.logical.LogicalSort.create(
+                    visitedBarrier, reversedCollation, null, null);
+              }
+              // Found a collation Sort - insert reversed sort on top of it
+              if (sort.getCollation() != null
+                  && !sort.getCollation().getFieldCollations().isEmpty()) {
+                sortFound = true;
+                RelNode visitedSort = super.visit(other);
+                return org.apache.calcite.rel.logical.LogicalSort.create(
+                    visitedSort, reversedCollation, null, null);
+              }
+            }
+            // For all other nodes, continue traversal
+            return super.visit(other);
+          }
+        });
+  }
 
   @Override
   public RelNode visitReverse(
       org.opensearch.sql.ast.tree.Reverse node, CalcitePlanContext context) {
     visitChildren(node, context);
-    // Add ROW_NUMBER() column
-    RexNode rowNumber =
-        context
-            .relBuilder
-            .aggregateCall(SqlStdOperatorTable.ROW_NUMBER)
-            .over()
-            .rowsTo(RexWindowBounds.CURRENT_ROW)
-            .as(REVERSE_ROW_NUM);
-    context.relBuilder.projectPlus(rowNumber);
-    // Sort by row number descending
-    context.relBuilder.sort(context.relBuilder.desc(context.relBuilder.field(REVERSE_ROW_NUM)));
-    // Remove row number column
-    context.relBuilder.projectExcept(context.relBuilder.field(REVERSE_ROW_NUM));
+
+    // Check if there's an existing sort to reverse
+    List<RelCollation> collations =
+        context.relBuilder.getCluster().getMetadataQuery().collations(context.relBuilder.peek());
+    RelCollation collation = collations != null && !collations.isEmpty() ? collations.get(0) : null;
+
+    if (collation != null && !collation.getFieldCollations().isEmpty()) {
+      // If there's an existing sort, reverse its direction
+      RelCollation reversedCollation = PlanUtils.reverseCollation(collation);
+      RelNode currentNode = context.relBuilder.peek();
+      if (currentNode instanceof org.apache.calcite.rel.core.Sort) {
+        org.apache.calcite.rel.core.Sort existingSort =
+            (org.apache.calcite.rel.core.Sort) currentNode;
+        if (existingSort.getCollation() != null
+            && !existingSort.getCollation().getFieldCollations().isEmpty()
+            && existingSort.fetch == null
+            && existingSort.offset == null) {
+          // Pure collation sort (no fetch/offset) - replace in-place to avoid consecutive
+          // sorts. Calcite's physical optimizer merges consecutive LogicalSort nodes and may
+          // discard the reversed direction. Replacing in-place avoids this issue.
+          RelCollation reversedFromSort = PlanUtils.reverseCollation(existingSort.getCollation());
+          RelNode replacedSort =
+              org.apache.calcite.rel.logical.LogicalSort.create(
+                  existingSort.getInput(), reversedFromSort, null, null);
+          PlanUtils.replaceTop(context.relBuilder, replacedSort);
+        } else {
+          // Sort with fetch/offset (limit) or fetch-only Sort - add a separate reversed
+          // sort on top so the "limit then reverse" semantics are preserved.
+          context.relBuilder.sort(reversedCollation);
+        }
+      } else {
+        context.relBuilder.sort(reversedCollation);
+      }
+    } else {
+      // Collation not found on current node - try backtracking
+      RelNode currentNode = context.relBuilder.peek();
+      RelCollation backtrackCollation = backtrackForCollation(currentNode);
+
+      if (backtrackCollation != null && !backtrackCollation.getFieldCollations().isEmpty()) {
+        // Found collation through backtracking - rebuild tree with reversed sort
+        RelCollation reversedCollation = PlanUtils.reverseCollation(backtrackCollation);
+        RelNode rebuiltTree = insertReversedSortInTree(currentNode, reversedCollation, context);
+        // Replace the current node in the builder with the rebuilt tree
+        context.relBuilder.build(); // Pop the current node
+        context.relBuilder.push(rebuiltTree); // Push the rebuilt tree
+      } else {
+        // Check if @timestamp field exists in the row type
+        List<String> fieldNames = context.relBuilder.peek().getRowType().getFieldNames();
+        if (fieldNames.contains(OpenSearchConstants.IMPLICIT_FIELD_TIMESTAMP)) {
+          // If @timestamp exists, sort by it in descending order
+          context.relBuilder.sort(
+              context.relBuilder.desc(
+                  context.relBuilder.field(OpenSearchConstants.IMPLICIT_FIELD_TIMESTAMP)));
+        }
+        // If neither collation nor @timestamp exists, ignore the reverse command (no-op)
+      }
+    }
+
     return context.relBuilder.peek();
   }
 

core/src/main/java/org/opensearch/sql/calcite/utils/PlanUtils.java

@@ -28,6 +28,9 @@
 import org.apache.calcite.plan.RelOptRuleCall;
 import org.apache.calcite.plan.RelOptTable;
 import org.apache.calcite.plan.volcano.VolcanoPlanner;
+import org.apache.calcite.rel.RelCollation;
+import org.apache.calcite.rel.RelCollations;
+import org.apache.calcite.rel.RelFieldCollation;
 import org.apache.calcite.rel.RelHomogeneousShuttle;
 import org.apache.calcite.rel.RelNode;
 import org.apache.calcite.rel.RelShuttle;
@@ -593,6 +596,37 @@ public Void visitCorrelVariable(RexCorrelVariable correlVar) {
     }
   }
 
+  /**
+   * Reverses the direction of a RelCollation.
+   *
+   * @param original The original collation to reverse
+   * @return A new RelCollation with reversed directions
+   */
+  public static RelCollation reverseCollation(RelCollation original) {
+    if (original == null || original.getFieldCollations().isEmpty()) {
+      return original;
+    }
+
+    List<RelFieldCollation> reversedFields = new ArrayList<>();
+    for (RelFieldCollation field : original.getFieldCollations()) {
+      RelFieldCollation.Direction reversedDirection = field.direction.reverse();
+
+      // Handle null direction properly - reverse it as well
+      RelFieldCollation.NullDirection reversedNullDirection =
+          field.nullDirection == RelFieldCollation.NullDirection.FIRST
+              ? RelFieldCollation.NullDirection.LAST
+              : field.nullDirection == RelFieldCollation.NullDirection.LAST
+                  ? RelFieldCollation.NullDirection.FIRST
+                  : field.nullDirection;
+
+      RelFieldCollation reversedField =
+          new RelFieldCollation(field.getFieldIndex(), reversedDirection, reversedNullDirection);
+      reversedFields.add(reversedField);
+    }
+
+    return RelCollations.of(reversedFields);
+  }
+
   /** Adds a rel node to the top of the stack while preserving the field names and aliases. */
   static void replaceTop(RelBuilder relBuilder, RelNode relNode) {
     try {