Skip query cache for in_batches and friends

The `find_each`, `find_in_batches` and `in_batches` APIs usually operate
on large numbers of records, where it's preferable not to load them all
into memory at once.

If the query cache is enabled, it will hold onto the query results until
the end of the execution context (request/job), which means the memory
used is still proportional to the total number of records. These queries
are typically not repeated, so the query cache isn't desirable here.

1 files changed, 25 insertions, 13 deletions

diff --git a/activerecord/lib/active_record/relation.rb b/activerecord/lib/active_record/relation.rb
index 52f5d5f3e3..76cf47a3ed 100644
--- a/activerecord/lib/active_record/relation.rb
+++ b/activerecord/lib/active_record/relation.rb
@@ -6,7 +6,7 @@ module ActiveRecord
                             :extending, :unscope]
 
     SINGLE_VALUE_METHODS = [:limit, :offset, :lock, :readonly, :reordering,
-                            :reverse_order, :distinct, :create_with]
+                            :reverse_order, :distinct, :create_with, :skip_query_cache]
     CLAUSE_METHODS = [:where, :having, :from]
     INVALID_METHODS_FOR_DELETE_ALL = [:limit, :distinct, :offset, :group, :having]
 
@@ -657,20 +657,32 @@ module ActiveRecord
       end
 
       def exec_queries(&block)
-        @records = eager_loading? ? find_with_associations.freeze : @klass.find_by_sql(arel, bound_attributes, &block).freeze
-
-        preload = preload_values
-        preload += includes_values unless eager_loading?
-        preloader = nil
-        preload.each do |associations|
-          preloader ||= build_preloader
-          preloader.preload @records, associations
-        end
+        skip_query_cache_if_necessary do
+          @records = eager_loading? ? find_with_associations.freeze : @klass.find_by_sql(arel, bound_attributes, &block).freeze
+
+          preload = preload_values
+          preload += includes_values unless eager_loading?
+          preloader = nil
+          preload.each do |associations|
+            preloader ||= build_preloader
+            preloader.preload @records, associations
+          end
 
-        @records.each(&:readonly!) if readonly_value
+          @records.each(&:readonly!) if readonly_value
 
-        @loaded = true
-        @records
+          @loaded = true
+          @records
+        end
+      end
+
+      def skip_query_cache_if_necessary
+        if skip_query_cache_value
+          uncached do
+            yield
+          end
+        else
+          yield
+        end
       end
 
       def build_preloader