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.

5 files changed, 37 insertions, 19 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
diff --git a/activerecord/lib/active_record/relation/batches.rb b/activerecord/lib/active_record/relation/batches.rb
index ee1f25ec84..c7e4f8a88a 100644
--- a/activerecord/lib/active_record/relation/batches.rb
+++ b/activerecord/lib/active_record/relation/batches.rb
@@ -209,6 +209,7 @@ module ActiveRecord
 
       relation = relation.reorder(batch_order).limit(batch_limit)
       relation = apply_limits(relation, start, finish)
+      relation.skip_query_cache! # Retaining the results in the query cache would undermine the point of batching
       batch_relation = relation
 
       loop do
diff --git a/activerecord/lib/active_record/relation/calculations.rb b/activerecord/lib/active_record/relation/calculations.rb
index 8a54f8f2c3..aaba6c71f2 100644
--- a/activerecord/lib/active_record/relation/calculations.rb
+++ b/activerecord/lib/active_record/relation/calculations.rb
@@ -184,7 +184,7 @@ module ActiveRecord
         relation.select_values = column_names.map { |cn|
           @klass.has_attribute?(cn) || @klass.attribute_alias?(cn) ? arel_attribute(cn) : cn
         }
-        result = klass.connection.select_all(relation.arel, nil, bound_attributes)
+        result = skip_query_cache_if_necessary { klass.connection.select_all(relation.arel, nil, bound_attributes) }
         result.cast_values(klass.attribute_types)
       end
     end
@@ -260,7 +260,7 @@ module ActiveRecord
           query_builder = relation.arel
         end
 
-        result = @klass.connection.select_all(query_builder, nil, bound_attributes)
+        result = skip_query_cache_if_necessary { @klass.connection.select_all(query_builder, nil, bound_attributes) }
         row    = result.first
         value  = row && row.values.first
         type   = result.column_types.fetch(column_alias) do
@@ -311,7 +311,7 @@ module ActiveRecord
         relation.group_values  = group_fields
         relation.select_values = select_values
 
-        calculated_data = @klass.connection.select_all(relation.arel, nil, relation.bound_attributes)
+        calculated_data = skip_query_cache_if_necessary { @klass.connection.select_all(relation.arel, nil, relation.bound_attributes) }
 
         if association
           key_ids     = calculated_data.collect { |row| row[group_aliases.first] }
diff --git a/activerecord/lib/active_record/relation/finder_methods.rb b/activerecord/lib/active_record/relation/finder_methods.rb
index eee0f36f63..ac0b4f597e 100644
--- a/activerecord/lib/active_record/relation/finder_methods.rb
+++ b/activerecord/lib/active_record/relation/finder_methods.rb
@@ -315,7 +315,7 @@ module ActiveRecord
 
       relation = construct_relation_for_exists(relation, conditions)
 
-      connection.select_value(relation.arel, "#{name} Exists", relation.bound_attributes) ? true : false
+      skip_query_cache_if_necessary { connection.select_value(relation.arel, "#{name} Exists", relation.bound_attributes) } ? true : false
     rescue ::RangeError
       false
     end
@@ -376,7 +376,7 @@ module ActiveRecord
           if ActiveRecord::NullRelation === relation
             []
           else
-            rows = connection.select_all(relation.arel, "SQL", relation.bound_attributes)
+            rows = skip_query_cache_if_necessary { connection.select_all(relation.arel, "SQL", relation.bound_attributes) }
             join_dependency.instantiate(rows, aliases)
           end
         end
@@ -424,7 +424,7 @@ module ActiveRecord
 
         relation = relation.except(:select).select(values).distinct!
 
-        id_rows = @klass.connection.select_all(relation.arel, "SQL", relation.bound_attributes)
+        id_rows = skip_query_cache_if_necessary { @klass.connection.select_all(relation.arel, "SQL", relation.bound_attributes) }
         id_rows.map { |row| row[primary_key] }
       end
 
diff --git a/activerecord/lib/active_record/relation/query_methods.rb b/activerecord/lib/active_record/relation/query_methods.rb
index 9da8f96337..79495ead91 100644
--- a/activerecord/lib/active_record/relation/query_methods.rb
+++ b/activerecord/lib/active_record/relation/query_methods.rb
@@ -913,6 +913,11 @@ module ActiveRecord
       self
     end
 
+    def skip_query_cache! # :nodoc:
+      self.skip_query_cache_value = true
+      self
+    end
+
     # Returns the Arel object associated with the relation.
     def arel # :nodoc:
       @arel ||= build_arel