1 files changed, 72 insertions, 0 deletions
diff --git a/activerecord/lib/active_record/connection_adapters/mysql/explain_pretty_printer.rb b/activerecord/lib/active_record/connection_adapters/mysql/explain_pretty_printer.rb
new file mode 100644
index 0000000000..20c3c83664
--- /dev/null
+++ b/activerecord/lib/active_record/connection_adapters/mysql/explain_pretty_printer.rb
@@ -0,0 +1,72 @@
+# frozen_string_literal: true
+
+module ActiveRecord
+  module ConnectionAdapters
+    module MySQL
+      class ExplainPrettyPrinter # :nodoc:
+        # Pretty prints the result of an EXPLAIN in a way that resembles the output of the
+        # MySQL shell:
+        #
+        #   +----+-------------+-------+-------+---------------+---------+---------+-------+------+-------------+
+        #   | id | select_type | table | type  | possible_keys | key     | key_len | ref   | rows | Extra       |
+        #   +----+-------------+-------+-------+---------------+---------+---------+-------+------+-------------+
+        #   |  1 | SIMPLE      | users | const | PRIMARY       | PRIMARY | 4       | const |    1 |             |
+        #   |  1 | SIMPLE      | posts | ALL   | NULL          | NULL    | NULL    | NULL  |    1 | Using where |
+        #   +----+-------------+-------+-------+---------------+---------+---------+-------+------+-------------+
+        #   2 rows in set (0.00 sec)
+        #
+        # This is an exercise in Ruby hyperrealism :).
+        def pp(result, elapsed)
+          widths    = compute_column_widths(result)
+          separator = build_separator(widths)
+
+          pp = []
+
+          pp << separator
+          pp << build_cells(result.columns, widths)
+          pp << separator
+
+          result.rows.each do |row|
+            pp << build_cells(row, widths)
+          end
+
+          pp << separator
+          pp << build_footer(result.rows.length, elapsed)
+
+          pp.join("\n") + "\n"
+        end
+
+        private
+
+          def compute_column_widths(result)
+            [].tap do |widths|
+              result.columns.each_with_index do |column, i|
+                cells_in_column = [column] + result.rows.map { |r| r[i].nil? ? "NULL" : r[i].to_s }
+                widths << cells_in_column.map(&:length).max
+              end
+            end
+          end
+
+          def build_separator(widths)
+            padding = 1
+            "+" + widths.map { |w| "-" * (w + (padding * 2)) }.join("+") + "+"
+          end
+
+          def build_cells(items, widths)
+            cells = []
+            items.each_with_index do |item, i|
+              item = "NULL" if item.nil?
+              justifier = item.is_a?(Numeric) ? "rjust" : "ljust"
+              cells << item.to_s.send(justifier, widths[i])
+            end
+            "| " + cells.join(" | ") + " |"
+          end
+
+          def build_footer(nrows, elapsed)
+            rows_label = nrows == 1 ? "row" : "rows"
+            "#{nrows} #{rows_label} in set (%.2f sec)" % elapsed
+          end
+      end
+    end
+  end
+end