Merge pull request #206 from cturner/master

Updated MySQL JDBC adapter to support Rails 3.2 explain feature

lib/arjdbc/mysql/adapter.rb

@@ -466,6 +466,87 @@ def initialize(*args)
         configure_connection
       end
 
+      ## EXPLAIN support lifted from the mysql2 gem with slight modifications
+      ## to work in the JDBC adapter gem.
+      def supports_explain?
+        true
+      end
+
+      def explain(arel, binds = [])
+        sql     = "EXPLAIN #{to_sql(arel, binds.dup)}"
+        start   = Time.now.to_f
+        raw_result  = execute(sql, "EXPLAIN")
+        ar_result = ActiveRecord::Result.new(raw_result[0].keys, raw_result)
+        elapsed = Time.now.to_f - start
+        ExplainPrettyPrinter.new.pp(ar_result, elapsed)
+      end
+
+      class ExplainPrettyPrinter # :nodoc:
+        # Pretty prints the result of a 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.values, 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 do |col|
+              cells_in_column = [col] + result.rows.map {|r| r[col].nil? ? 'NULL' : r[col].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
+
       def jdbc_connection_class(spec)
         ::ArJdbc::MySQL.jdbc_connection_class
       end