Less jank in after-key parsing for unmapped fields (#86359) (#97282)

Resolves #85928

The after-key parsing is pretty weird, and there are probably more bugs there. I did not take the opportunity to refactor the whole thing, but we should. This fixes the immediate problem by treating after keys as bytes refs when we don't have a field but think we want a keyword. We were already doing that if the user asked for a missing bucket, this just extends the behavior in the case that we don't.

Long term, the terms Composite source (and probably other Composite sources) should have specializations for unmapped fields. That's the direction we want to take aggs in general.

docs/changelog/97282.yaml

@@ -0,0 +1,6 @@
+pr: 97282
+summary: Less jank in after-key parsing for unmapped fields
+area: Aggregations
+type: bug
+issues:
+ - 85928

server/src/main/java/org/elasticsearch/search/aggregations/bucket/composite/GlobalOrdinalValuesSource.java

@@ -105,7 +105,7 @@ void setAfter(Comparable<?> value) {
         if (missingBucket && value == null) {
             afterValue = null;
             afterValueGlobalOrd = MISSING_VALUE_FLAG;
-        } else if (value.getClass() == String.class || (missingBucket && fieldType == null)) {
+        } else if (value.getClass() == String.class || (fieldType == null)) {
             // the value might be not string if this field is missing in this shard but present in other shards
             // and doesn't have a string type
             afterValue = format.parseBytesRef(value.toString());

server/src/test/java/org/elasticsearch/search/aggregations/bucket/composite/CompositeAggregatorTests.java

@@ -151,13 +151,16 @@ public void testUnmappedFieldWithTerms() throws Exception {
                 createDocument("keyword", "c")
             )
         );
+
+        // Only aggregate on unmapped field, no missing bucket => no results
         testSearchCase(
             Arrays.asList(new MatchAllDocsQuery(), new DocValuesFieldExistsQuery("keyword")),
             dataset,
             () -> new CompositeAggregationBuilder("name", Arrays.asList(new TermsValuesSourceBuilder("unmapped").field("unmapped"))),
             (result) -> { assertEquals(0, result.getBuckets().size()); }
         );
 
+        // Only aggregate on unmapped field, missing bucket => one null bucket with all values
         testSearchCase(
             Arrays.asList(new MatchAllDocsQuery(), new DocValuesFieldExistsQuery("keyword")),
             dataset,
@@ -173,6 +176,7 @@ public void testUnmappedFieldWithTerms() throws Exception {
             }
         );
 
+        // Only aggregate on the unmapped field, after key for that field is set as `null` => no results
         testSearchCase(
             Arrays.asList(new MatchAllDocsQuery(), new DocValuesFieldExistsQuery("keyword")),
             dataset,
@@ -183,6 +187,7 @@ public void testUnmappedFieldWithTerms() throws Exception {
             (result) -> { assertEquals(0, result.getBuckets().size()); }
         );
 
+        // Mapped field first, then unmapped, no missing bucket => no results
         testSearchCase(
             Arrays.asList(new MatchAllDocsQuery(), new DocValuesFieldExistsQuery("keyword")),
             dataset,
@@ -196,6 +201,7 @@ public void testUnmappedFieldWithTerms() throws Exception {
             (result) -> { assertEquals(0, result.getBuckets().size()); }
         );
 
+        // Mapped + unmapped, include missing => 3 buckets
         testSearchCase(
             Arrays.asList(new MatchAllDocsQuery(), new DocValuesFieldExistsQuery("keyword")),
             dataset,
@@ -217,6 +223,91 @@ public void testUnmappedFieldWithTerms() throws Exception {
                 assertEquals(1L, result.getBuckets().get(2).getDocCount());
             }
         );
+
+        // Unmapped field, keyword after key, unmapped sorts after, include unmapped => 1 bucket
+        testSearchCase(
+            Arrays.asList(new MatchAllDocsQuery(), new DocValuesFieldExistsQuery("keyword")),
+            dataset,
+            () -> new CompositeAggregationBuilder(
+                "name",
+                Arrays.asList(
+                    new TermsValuesSourceBuilder("unmapped").field("unmapped").missingBucket(true).missingOrder(MissingOrder.LAST)
+                )
+            ).aggregateAfter(Collections.singletonMap("unmapped", "cat")),
+            (InternalComposite result) -> {
+                assertEquals(1, result.getBuckets().size());
+                assertEquals("{unmapped=null}", result.afterKey().toString());
+                assertEquals("{unmapped=null}", result.getBuckets().get(0).getKeyAsString());
+                assertEquals(5L, result.getBuckets().get(0).getDocCount());
+            }
+        );
+
+        // Unmapped field, keyword after key, unmapped sorts before, include unmapped => 0 buckets
+        testSearchCase(
+            Arrays.asList(new MatchAllDocsQuery(), new DocValuesFieldExistsQuery("keyword")),
+            dataset,
+            () -> new CompositeAggregationBuilder(
+                "name",
+                Arrays.asList(
+                    new TermsValuesSourceBuilder("unmapped").field("unmapped").missingBucket(true).missingOrder(MissingOrder.FIRST)
+                )
+            ).aggregateAfter(Collections.singletonMap("unmapped", "cat")),
+            (InternalComposite result) -> { assertEquals(0, result.getBuckets().size()); }
+        );
+
+        // Unmapped field, number after key, unmapped sorts after, include unmapped => 1 bucket
+        testSearchCase(
+            Arrays.asList(new MatchAllDocsQuery(), new DocValuesFieldExistsQuery("keyword")),
+            dataset,
+            () -> new CompositeAggregationBuilder(
+                "name",
+                Arrays.asList(
+                    new TermsValuesSourceBuilder("unmapped").field("unmapped").missingBucket(true).missingOrder(MissingOrder.LAST)
+                )
+            ).aggregateAfter(Collections.singletonMap("unmapped", 42)),
+            (InternalComposite result) -> {
+                assertEquals(1, result.getBuckets().size());
+                assertEquals("{unmapped=null}", result.afterKey().toString());
+                assertEquals("{unmapped=null}", result.getBuckets().get(0).getKeyAsString());
+                assertEquals(5L, result.getBuckets().get(0).getDocCount());
+            }
+        );
+
+        // Unmapped field, number after key, unmapped sorts before, include unmapped => 0 buckets
+        testSearchCase(
+            Arrays.asList(new MatchAllDocsQuery(), new DocValuesFieldExistsQuery("keyword")),
+            dataset,
+            () -> new CompositeAggregationBuilder(
+                "name",
+                Arrays.asList(
+                    new TermsValuesSourceBuilder("unmapped").field("unmapped").missingBucket(true).missingOrder(MissingOrder.FIRST)
+                )
+            ).aggregateAfter(Collections.singletonMap("unmapped", 42)),
+            (InternalComposite result) -> { assertEquals(0, result.getBuckets().size()); }
+        );
+
+    }
+
+    public void testUnmappedTermsLongAfter() throws Exception {
+        final List<Map<String, List<Object>>> dataset = new ArrayList<>();
+        dataset.addAll(
+            Arrays.asList(
+                createDocument("keyword", "a"),
+                createDocument("keyword", "c"),
+                createDocument("keyword", "a"),
+                createDocument("keyword", "d"),
+                createDocument("keyword", "c")
+            )
+        );
+
+        // Unmapped field, number after key, no missing bucket => 0 buckets
+        testSearchCase(
+            Arrays.asList(new MatchAllDocsQuery(), new DocValuesFieldExistsQuery("keyword")),
+            dataset,
+            () -> new CompositeAggregationBuilder("name", Arrays.asList(new TermsValuesSourceBuilder("unmapped").field("unmapped")))
+                .aggregateAfter(Collections.singletonMap("unmapped", 42)),
+            (InternalComposite result) -> { assertEquals(0, result.getBuckets().size()); }
+        );
     }
 
     public void testUnmappedFieldWithGeopoint() throws Exception {