Improve error message for unsupported window functions in eventstats/streamstats

api/src/test/java/org/opensearch/sql/api/UnifiedQueryPlannerTest.java

@@ -171,7 +171,8 @@ public void unsupportedWindowFunctionIsRethrownAsSemanticCheckException() {
     givenInvalidQuery("source = catalog.employees | eventstats percent_rank()")
         .assertErrorType(SemanticCheckException.class)
         .assertCauseType(CalciteUnsupportedException.class)
-        .assertErrorMessageContains("Unexpected window function: percent_rank");
+        .assertErrorMessageContains(
+            "Window function 'percent_rank' is not supported in eventstats/streamstats");
   }
 
   @Test

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

@@ -95,6 +95,7 @@
 import org.opensearch.sql.exception.CalciteUnsupportedException;
 import org.opensearch.sql.exception.ExpressionEvaluationException;
 import org.opensearch.sql.exception.SemanticCheckException;
+import org.opensearch.sql.executor.QueryType;
 import org.opensearch.sql.expression.function.BuiltinFunctionName;
 import org.opensearch.sql.expression.function.CoercionUtils;
 import org.opensearch.sql.expression.function.PPLBuiltinOperators;
@@ -763,6 +764,17 @@ public RexNode visitWindowFunction(WindowFunction node, CalcitePlanContext conte
             .toList();
     List<RexNode> orderKeys = translateOrderKeys(node.getSortList(), context);
     return BuiltinFunctionName.ofWindowFunction(funcName)
+        // rank/dense_rank were added to WINDOW_FUNC_MAPPING to support SQL's RANK()/DENSE_RANK()
+        // OVER (...), but PPL only ever reaches this visitor through eventstats/streamstats
+        // (there's no other PPL syntax that builds a WindowFunction node), which don't support
+        // them - PPL has no ORDER BY syntax for eventstats/streamstats, so ranking would be
+        // meaningless there. Excluding them here for PPL keeps that restriction despite the
+        // shared mapping, without needing a PPL-specific grammar change.
+        .filter(
+            functionName ->
+                !(context.queryType == QueryType.PPL
+                    && (functionName == BuiltinFunctionName.RANK
+                        || functionName == BuiltinFunctionName.DENSE_RANK)))
         .map(
             functionName -> {
               RexNode field = arguments.isEmpty() ? null : arguments.getFirst();
@@ -808,7 +820,14 @@ public RexNode visitWindowFunction(WindowFunction node, CalcitePlanContext conte
                       node.getWindowFrame());
             })
         .orElseThrow(
-            () -> new CalciteUnsupportedException("Unexpected window function: " + funcName));
+            () ->
+                new CalciteUnsupportedException(
+                    "Window function '"
+                        + funcName
+                        + "' is not supported in eventstats/streamstats."
+                        + " Supported functions: avg, count, dc, distinct_count, earliest,"
+                        + " latest, max, min, row_number, stddev_pop, stddev_samp, sum,"
+                        + " var_pop, var_samp."));
   }
 
   private List<RexNode> translateOrderKeys(

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

@@ -329,7 +329,22 @@ public void testUnsupportedWindowFunctions() {
                   executeQuery(
                       String.format(
                           "source=%s | eventstats %s(age)", TEST_INDEX_STATE_COUNTRY, u)));
-      verifyErrorMessageContains(e, "Unexpected window function: " + u);
+      verifyErrorMessageContains(e, "is not supported in eventstats/streamstats");
+    }
+  }
+
+  @Test
+  public void testRankingWindowFunctionsUnsupportedInEventstats() {
+    for (String func : List.of("rank", "dense_rank")) {
+      Throwable e =
+          assertThrowsWithReplace(
+              UnsupportedOperationException.class,
+              () ->
+                  executeQuery(
+                      String.format(
+                          "source=%s | eventstats %s() by state", TEST_INDEX_STATE_COUNTRY, func)));
+      verifyErrorMessageContains(
+          e, "Window function '" + func + "' is not supported in eventstats/streamstats");
     }
   }
 

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

@@ -846,7 +846,23 @@ public void testUnsupportedWindowFunctions() {
                   executeQuery(
                       String.format(
                           "source=%s | streamstats %s(age)", TEST_INDEX_STATE_COUNTRY, u)));
-      verifyErrorMessageContains(e, "Unexpected window function: " + u);
+      verifyErrorMessageContains(e, "is not supported in eventstats/streamstats");
+    }
+  }
+
+  @Test
+  public void testRankingWindowFunctionsUnsupportedInStreamstats() {
+    for (String func : List.of("rank", "dense_rank")) {
+      Throwable e =
+          assertThrowsWithReplace(
+              UnsupportedOperationException.class,
+              () ->
+                  executeQuery(
+                      String.format(
+                          "source=%s | streamstats %s() by state",
+                          TEST_INDEX_STATE_COUNTRY, func)));
+      verifyErrorMessageContains(
+          e, "Window function '" + func + "' is not supported in eventstats/streamstats");
     }
   }