Merge pull request #37614 from jhawthorn/transform_values

Use transform_values to build hash_rows

activerecord/lib/active_record/result.rb

@@ -145,8 +145,18 @@ def hash_rows
             # used as keys in ActiveRecord::Base's @attributes hash
             columns = @columns.map(&:-@)
             length  = columns.length
+            template = nil
 
             @rows.map { |row|
+              if template
+                # We use transform_values to build subsequent rows from the
+                # hash of the first row. This is faster because we avoid any
+                # reallocs and in Ruby 2.7+ avoid hashing entirely.
+                index = -1
+                template.transform_values do
+                  row[index += 1]
+                end
+              else
                 # In the past we used Hash[columns.zip(row)]
                 #  though elegant, the verbose way is much more efficient
                 #  both time and memory wise cause it avoids a big array allocation
@@ -159,7 +169,12 @@ def hash_rows
                   index += 1
                 end
 
+                # It's possible to select the same column twice, in which case
+                # we can't use a template
+                template = hash if hash.length == length
+
                 hash
+              end
             }
           end
       end