feat: optimized aggregation pipeline to use indexes when possible

Author: jmarkerink

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

@@ -189,11 +189,16 @@ public static Aggregation fromPipeline(List<Document> pipeline, DatabaseResolver
     }
 
     private List<Document> runStages() {
-        return stages.stream()
-            .filter(stage -> stage instanceof MatchStage)
-            .findFirst()
-            .map(match -> runStages(collection.handleQueryAsStream(((MatchStage) match).getQuery())))
-            .orElseGet(() -> runStages(collection.queryAllAsStream()));
+        // 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());
     }
 
     public List<Document> runStages(Stream<Document> stream) {