tests pass, I think

Signed-off-by: Anthony Leong <aj.leong623@gmail.com>

Author: ajleong623

server/src/main/java/org/opensearch/search/aggregations/bucket/missing/MissingAggregator.java

@@ -123,17 +123,17 @@ protected boolean tryPrecomputeAggregationForLeaf(LeafReaderContext ctx) throws
             return false;
         }
 
-        // There is a chance that when the fieldName does not exist, there could be a missing
-        // parameter. We must check that case separately.
+        // When fieldname does not exist, we cannot collect through the precomputation.
         if (fieldName == null || weight == null) {
-            // we do not collect any documents through the missing aggregation when the missing parameter
-            // is up.
-            if (valuesSourceConfig != null && valuesSourceConfig.missing() != null) {
-                return true;
-            }
             return false;
         }
 
+        // we do not collect any documents through the missing aggregation when the missing parameter
+        // is up.
+        if (valuesSourceConfig != null && valuesSourceConfig.missing() != null) {
+            return true;
+        }
+
         // The optimization could only be used if there are no deleted documents and the top-level
         // query matches all documents in the segment.
         if (weight.count(ctx) == 0) {

server/src/test/java/org/opensearch/search/aggregations/bucket/missing/MissingAggregatorTests.java

@@ -452,7 +452,7 @@ public void testUnmappedWithMissingParam() throws IOException {
 
         final MissingAggregationBuilder builder = new MissingAggregationBuilder("_name").field("unknown_field").missing(randomLong());
 
-        // Having the missing parameter will make the missing aggregator not responsible for any documents, so it will short circuit
+        // Because the field is unmapped, the fieldname will not existm so we cannot collect through precomputation optimization.
         testCase(newMatchAllQuery(), builder, writer -> {
             for (int i = 0; i < numDocs; i++) {
                 final long randomLong = randomLong();
@@ -466,7 +466,7 @@ public void testUnmappedWithMissingParam() throws IOException {
         }, internalMissing -> {
             assertEquals(0, internalMissing.getDocCount());
             assertFalse(AggregationInspectionHelper.hasValue(internalMissing));
-        }, singleton(aggFieldType), 0);
+        }, singleton(aggFieldType), numDocs);
 
         testCase(newMatchAllQuery(), builder, writer -> {
             for (int i = 0; i < numDocs; i++) {
@@ -475,7 +475,7 @@ public void testUnmappedWithMissingParam() throws IOException {
         }, internalMissing -> {
             assertEquals(0, internalMissing.getDocCount());
             assertFalse(AggregationInspectionHelper.hasValue(internalMissing));
-        }, singleton(aggFieldType), 0);
+        }, singleton(aggFieldType), numDocs);
     }
 
     public void testMissingParam() throws IOException {