Allow mismatched sort-by field types if there are no docs to sort

docs/changelog/102779.yaml

@@ -0,0 +1,5 @@
+pr: 102779
+summary: Allow mismatched sort-by field types if there are no docs to sort
+area: Search
+type: bug
+issues: []

server/src/internalClusterTest/java/org/elasticsearch/search/sort/FieldSortIT.java

@@ -150,7 +150,7 @@ public void testIssue8226() {
         );
     }
 
-    public void testIssue6614() throws ExecutionException, InterruptedException {
+    public void testIssue6614() throws InterruptedException {
         List<IndexRequestBuilder> builders = new ArrayList<>();
         boolean strictTimeBasedIndices = randomBoolean();
         final int numIndices = randomIntBetween(2, 25); // at most 25 days in the month
@@ -2137,4 +2137,19 @@ public void testSortMixedFieldTypes() {
         }
     }
 
+    public void testSortMixedFieldTypesWithNoDocsForOneType() {
+        assertAcked(prepareCreate("index_long").setMapping("foo", "type=long").get());
+        assertAcked(prepareCreate("index_other").setMapping("bar", "type=keyword").get());
+        assertAcked(prepareCreate("index_double").setMapping("foo", "type=double").get());
+
+        prepareIndex("index_long").setId("1").setSource("foo", "123").get();
+        prepareIndex("index_long").setId("2").setSource("foo", "124").get();
+        prepareIndex("index_other").setId("1").setSource("bar", "124").get();
+        refresh();
+
+        assertNoFailures(
+            prepareSearch("index_long", "index_double", "index_other").addSort(new FieldSortBuilder("foo").unmappedType("boolean"))
+                .setSize(10)
+        );
+    }
 }

server/src/main/java/org/elasticsearch/action/search/SearchPhaseController.java

@@ -241,8 +241,7 @@ static TopDocs mergeTopDocs(Collection<TopDocs> results, int topN, int from) {
             final TopFieldGroups[] shardTopDocs = results.toArray(new TopFieldGroups[numShards]);
             mergedTopDocs = TopFieldGroups.merge(sort, from, topN, shardTopDocs, false);
         } else if (topDocs instanceof TopFieldDocs firstTopDocs) {
-            checkSameSortTypes(results, firstTopDocs.fields);
-            final Sort sort = new Sort(firstTopDocs.fields);
+            final Sort sort = checkSameSortTypes(results, firstTopDocs.fields);
             final TopFieldDocs[] shardTopDocs = results.toArray(new TopFieldDocs[numShards]);
             mergedTopDocs = TopDocs.merge(sort, from, topN, shardTopDocs);
         } else {
@@ -252,19 +251,26 @@ static TopDocs mergeTopDocs(Collection<TopDocs> results, int topN, int from) {
         return mergedTopDocs;
     }
 
-    private static void checkSameSortTypes(Collection<TopDocs> results, SortField[] firstSortFields) {
-        if (results.size() < 2) return;
+    private static Sort checkSameSortTypes(Collection<TopDocs> results, SortField[] firstSortFields) {
+        Sort sort = new Sort(firstSortFields);
+        if (results.size() < 2) return sort;
 
-        SortField.Type[] firstTypes = new SortField.Type[firstSortFields.length];
+        SortField.Type[] firstTypes = null;
         boolean isFirstResult = true;
         for (TopDocs topDocs : results) {
+            // We don't actually merge in empty score docs, so ignore potentially mismatched types if there are no docs
+            if (topDocs.scoreDocs == null || topDocs.scoreDocs.length == 0) {
+                continue;
+            }
             SortField[] curSortFields = ((TopFieldDocs) topDocs).fields;
             if (isFirstResult) {
+                sort = new Sort(curSortFields);
+                firstTypes = new SortField.Type[curSortFields.length];
                 for (int i = 0; i < curSortFields.length; i++) {
-                    firstTypes[i] = getType(firstSortFields[i]);
+                    firstTypes[i] = getType(curSortFields[i]);
                     if (firstTypes[i] == SortField.Type.CUSTOM) {
                         // for custom types that we can't resolve, we can't do the check
-                        return;
+                        return sort;
                     }
                 }
                 isFirstResult = false;
@@ -274,7 +280,7 @@ private static void checkSameSortTypes(Collection<TopDocs> results, SortField[]
                     if (curType != firstTypes[i]) {
                         if (curType == SortField.Type.CUSTOM) {
                             // for custom types that we can't resolve, we can't do the check
-                            return;
+                            return sort;
                         }
                         throw new IllegalArgumentException(
                             "Can't sort on field ["
@@ -289,6 +295,7 @@ private static void checkSameSortTypes(Collection<TopDocs> results, SortField[]
                 }
             }
         }
+        return sort;
     }
 
     private static SortField.Type getType(SortField sortField) {