Make `relation.exists?` more performant when using eager loading

`relation.exists?` just wants to know if there is a result or not, does
not need the exact records matched. Therefore, an intermediate SELECT
query for eager loading is not necessary.

1 files changed, 4 insertions, 3 deletions

diff --git a/activerecord/lib/active_record/relation/finder_methods.rb b/activerecord/lib/active_record/relation/finder_methods.rb
index 50d0f14b98..12ac89f5ad 100644
--- a/activerecord/lib/active_record/relation/finder_methods.rb
+++ b/activerecord/lib/active_record/relation/finder_methods.rb
@@ -313,7 +313,7 @@ module ActiveRecord
       return false if !conditions || limit_value == 0
 
       if eager_loading?
-        relation = apply_join_dependency(construct_join_dependency(eager_loading: false))
+        relation = apply_join_dependency(eager_loading: false)
         return relation.exists?(conditions)
       end
 
@@ -396,10 +396,11 @@ module ActiveRecord
         )
       end
 
-      def apply_join_dependency(join_dependency = construct_join_dependency)
+      def apply_join_dependency(join_dependency = nil, eager_loading: true)
+        join_dependency ||= construct_join_dependency(eager_loading: eager_loading)
         relation = except(:includes, :eager_load, :preload).joins!(join_dependency)
 
-        if using_limitable_reflections?(join_dependency.reflections)
+        if !eager_loading || using_limitable_reflections?(join_dependency.reflections)
           relation
         else
           if has_limit_or_offset?