Merge pull request #16421 from tsukasaoishi/prevant_manycall_showtables

Tables existence check query is executed in large quantities

activerecord/lib/active_record/connection_adapters/schema_cache.rb

@@ -19,6 +19,7 @@ module ActiveRecord
 
       # A cached lookup for table existence.
       def table_exists?(name)
+        prepare_tables if @tables.empty?
         return @tables[name] if @tables.key? name
 
         @tables[name] = connection.table_exists?(name)
@@ -82,6 +83,12 @@ module ActiveRecord
       def marshal_load(array)
         @version, @columns, @columns_hash, @primary_keys, @tables = array
       end
+
+      private
+
+        def prepare_tables
+          connection.tables.each { |table| @tables[table] = true }
+        end
     end
   end
 end