[Backport 2.x] FIX: UOE While building Exists query for nested search_as_you_type field

CHANGELOG.md

@@ -53,6 +53,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 ### Fixed
 - Fix bulk API ignores ingest pipeline for upsert ([#12883](https://github.com/opensearch-project/OpenSearch/pull/12883))
 - Fix issue with feature flags where default value may not be honored ([#12849](https://github.com/opensearch-project/OpenSearch/pull/12849))
+- Fix UOE While building Exists query for nested search_as_you_type field ([#12048](https://github.com/opensearch-project/OpenSearch/pull/12048))
 - Enabled mockTelemetryPlugin for IT and fixed OOM issues ([#13054](https://github.com/opensearch-project/OpenSearch/pull/13054))
 - Fix implement mark() and markSupported() in class FilterStreamInput ([#13098](https://github.com/opensearch-project/OpenSearch/pull/13098))
 - Fix snapshot _status API to return correct status for partial snapshots ([#13260](https://github.com/opensearch-project/OpenSearch/pull/13260))

modules/mapper-extras/src/test/java/org/opensearch/index/mapper/SearchAsYouTypeFieldMapperTests.java

@@ -47,6 +47,7 @@
 import org.apache.lucene.search.DisjunctionMaxQuery;
 import org.apache.lucene.search.MatchNoDocsQuery;
 import org.apache.lucene.search.MultiPhraseQuery;
+import org.apache.lucene.search.NormsFieldExistsQuery;
 import org.apache.lucene.search.Query;
 import org.apache.lucene.search.SynonymQuery;
 import org.apache.lucene.search.TermQuery;
@@ -68,6 +69,7 @@
 import org.opensearch.index.query.MatchPhraseQueryBuilder;
 import org.opensearch.index.query.MultiMatchQueryBuilder;
 import org.opensearch.index.query.QueryShardContext;
+import org.opensearch.index.query.QueryStringQueryBuilder;
 import org.opensearch.plugins.Plugin;
 
 import java.io.IOException;
@@ -541,6 +543,31 @@ public void testMatchPhrase() throws IOException {
         }
     }
 
+    public void testNestedExistsQuery() throws IOException {
+        MapperService mapperService = createMapperService(mapping(b -> {
+            b.startObject("field");
+            {
+                b.field("type", "object");
+                b.startObject("properties");
+                {
+                    b.startObject("nested_field");
+                    {
+                        b.field("type", "search_as_you_type");
+                    }
+                    b.endObject();
+                }
+                b.endObject();
+            }
+            b.endObject();
+        }));
+        QueryShardContext queryShardContext = createQueryShardContext(mapperService);
+        Query actual = new QueryStringQueryBuilder("field:*").toQuery(queryShardContext);
+        Query expected = new ConstantScoreQuery(
+            new BooleanQuery.Builder().add(new NormsFieldExistsQuery("field.nested_field"), BooleanClause.Occur.SHOULD).build()
+        );
+        assertEquals(expected, actual);
+    }
+
     private static BooleanQuery buildBoolPrefixQuery(String shingleFieldName, String prefixFieldName, List<String> terms) {
         final BooleanQuery.Builder builder = new BooleanQuery.Builder();
         for (int i = 0; i < terms.size() - 1; i++) {

server/src/main/java/org/opensearch/index/query/ExistsQueryBuilder.java

@@ -201,6 +201,11 @@ private static Query newObjectFieldExistsQuery(QueryShardContext context, String
         BooleanQuery.Builder booleanQuery = new BooleanQuery.Builder();
         Collection<String> fields = context.simpleMatchToIndexNames(objField + ".*");
         for (String field : fields) {
+            int dotPos = field.lastIndexOf('.');
+            if (dotPos > 0 && field.charAt(dotPos + 1) == '_') {
+                // This is a subfield (e.g. prefix) of a complex field type. Skip it.
+                continue;
+            }
             Query existsQuery = context.getMapperService().fieldType(field).existsQuery(context);
             booleanQuery.add(existsQuery, Occur.SHOULD);
         }

test/framework/src/main/java/org/opensearch/index/mapper/MapperServiceTestCase.java

@@ -266,6 +266,9 @@ protected QueryShardContext createQueryShardContext(MapperService mapperService)
         when(queryShardContext.getSearchQuoteAnalyzer(any())).thenCallRealMethod();
         when(queryShardContext.getSearchAnalyzer(any())).thenCallRealMethod();
         when(queryShardContext.getIndexSettings()).thenReturn(mapperService.getIndexSettings());
+        when(queryShardContext.getObjectMapper(anyString())).thenAnswer(
+            inv -> mapperService.getObjectMapper(inv.getArguments()[0].toString())
+        );
         when(queryShardContext.simpleMatchToIndexNames(any())).thenAnswer(
             inv -> mapperService.simpleMatchToFullName(inv.getArguments()[0].toString())
         );