Add `config.active_record.warn_on_records_fetched_greater_than` option

When set to an integer, a warning will be logged whenever a result set
larger than the specified size is returned by a query. Fixes #16463

The warning is outputed a module which is prepended in an initializer,
so there will be no performance impact if
`config.active_record.warn_on_records_fetched_greater_than` is not set.

1 files changed, 50 insertions, 0 deletions

diff --git a/activerecord/lib/active_record/relation/record_fetch_warning.rb b/activerecord/lib/active_record/relation/record_fetch_warning.rb
new file mode 100644
index 0000000000..0d31f73ddd
--- /dev/null
+++ b/activerecord/lib/active_record/relation/record_fetch_warning.rb
@@ -0,0 +1,50 @@
+module ActiveRecord
+  class Relation
+    module RecordFetchWarning
+      # When this module is prepended to ActiveRecord::Relation and
+      # `config.active_record.warn_on_records_fetched_greater_than` is
+      # set to an integer, if the number of records a query returns is
+      # greater than the value of `warn_on_records_fetched_greater_than`,
+      # a warning is logged. This allows for the dection of queries that
+      # return a large number of records, which could cause memory
+      # bloat.
+      #
+      # In most cases, fetching large number of records can be performed
+      # efficiently using the ActiveRecord::Batches methods.
+      # See active_record/lib/relation/batches.rb for more information.
+      def exec_queries
+        QueryRegistry.reset
+
+        super.tap do
+          if logger && warn_on_records_fetched_greater_than
+            if @records.length > warn_on_records_fetched_greater_than
+              logger.warn "Query fetched #{@records.size} #{@klass} records: #{QueryRegistry.queries.join(";")}"
+            end
+          end
+        end
+      end
+
+      ActiveSupport::Notifications.subscribe("sql.active_record") do |*args|
+        payload = args.last
+
+        QueryRegistry.queries << payload[:sql]
+      end
+
+      class QueryRegistry # :nodoc:
+        extend ActiveSupport::PerThreadRegistry
+
+        attr_accessor :queries
+
+        def initialize
+          reset
+        end
+
+        def reset
+          @queries = []
+        end
+      end
+    end
+  end
+end
+
+ActiveRecord::Relation.prepend ActiveRecord::Relation::RecordFetchWarning