feat: optimized aggregation pipeline to use indexes when possible

Author: jmarkerink

core/src/main/java/de/bwaldvogel/mongo/backend/aggregation/Aggregation.java

@@ -189,6 +189,15 @@ public static Aggregation fromPipeline(List<Document> pipeline, DatabaseResolver
     }
 
     private List<Document> runStages() {
+        // Only apply index optimization if there are no variables to inject
+        // Variables need to be added to documents before $match can evaluate them
+        if (!hasVariables()) {
+            return stages.stream()
+                .filter(stage -> stage instanceof MatchStage)
+                .findFirst()
+                .map(match -> runStages(collection.handleQueryAsStream(((MatchStage) match).getQuery())))
+                .orElseGet(() -> runStages(collection.queryAllAsStream()));
+        }
         return runStages(collection.queryAllAsStream());
     }
 

core/src/main/java/de/bwaldvogel/mongo/backend/aggregation/stage/MatchStage.java

@@ -24,4 +24,8 @@ public String name() {
     public Stream<Document> apply(Stream<Document> stream) {
         return stream.filter(document -> queryMatcher.matches(document, query));
     }
+
+    public Document getQuery() {
+        return query;
+    }
 }