Move `explain` into `AbstractMysqlAdapter`

Common methods in both mysql adapters are should be added to
`AbstractMysqlAdapter`, but some methods had been added to
`Mysql2Adapter`. (8744632f, 0306f82e, #14359)

Some methods already moved from `Mysql2Adapter` to
`AbstractMysqlAdapter`. (#17601, #17998)

Common methods in both mysql adapters are remaining only the `explain`
method in `Mysql2Adapter`.

4 files changed, 113 insertions, 78 deletions

diff --git a/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb b/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb
index c084431588..ea46512660 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb
+++ b/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb
@@ -272,6 +272,10 @@ module ActiveRecord
         version[0] >= 5
       end
 
+      def supports_explain?
+        true
+      end
+
       def supports_indexes_in_create?
         true
       end
@@ -365,6 +369,80 @@ module ActiveRecord
       # DATABASE STATEMENTS ======================================
       #++
 
+      def explain(arel, binds = [])
+        sql     = "EXPLAIN #{to_sql(arel, binds)}"
+        start   = Time.now
+        result  = exec_query(sql, 'EXPLAIN', binds)
+        elapsed = Time.now - start
+
+        ExplainPrettyPrinter.new.pp(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, 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
+
       def clear_cache!
         super
         reload_type_map
diff --git a/activerecord/lib/active_record/connection_adapters/mysql2_adapter.rb b/activerecord/lib/active_record/connection_adapters/mysql2_adapter.rb
index 21631be25c..77ab30e5d7 100644
--- a/activerecord/lib/active_record/connection_adapters/mysql2_adapter.rb
+++ b/activerecord/lib/active_record/connection_adapters/mysql2_adapter.rb
@@ -37,10 +37,6 @@ module ActiveRecord
         configure_connection
       end
 
-      def supports_explain?
-        true
-      end
-
       # HELPER METHODS ===========================================
 
       def each_hash(result) # :nodoc:
@@ -95,80 +91,6 @@ module ActiveRecord
       # DATABASE STATEMENTS ======================================
       #++
 
-      def explain(arel, binds = [])
-        sql     = "EXPLAIN #{to_sql(arel, binds.dup)}"
-        start   = Time.now
-        result  = exec_query(sql, 'EXPLAIN', binds)
-        elapsed = Time.now - start
-
-        ExplainPrettyPrinter.new.pp(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, 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
-
       # FIXME: re-enable the following once a "better" query_cache solution is in core
       #
       # The overrides below perform much better than the originals in AbstractAdapter
diff --git a/activerecord/lib/active_record/connection_adapters/mysql_adapter.rb b/activerecord/lib/active_record/connection_adapters/mysql_adapter.rb
index 64985ee933..23875f3f4e 100644
--- a/activerecord/lib/active_record/connection_adapters/mysql_adapter.rb
+++ b/activerecord/lib/active_record/connection_adapters/mysql_adapter.rb
@@ -184,6 +184,14 @@ module ActiveRecord
       # DATABASE STATEMENTS ======================================
       #++
 
+      def select_all(arel, name = nil, binds = [])
+        if ExplainRegistry.collect? && prepared_statements
+          unprepared_statement { super }
+        else
+          super
+        end
+      end
+
       def select_rows(sql, name = nil, binds = [])
         @connection.query_with_result = true
         rows = exec_query(sql, name, binds).rows
diff --git a/activerecord/test/cases/adapters/mysql/explain_test.rb b/activerecord/test/cases/adapters/mysql/explain_test.rb
new file mode 100644
index 0000000000..f75386abb9
--- /dev/null
+++ b/activerecord/test/cases/adapters/mysql/explain_test.rb
@@ -0,0 +1,27 @@
+require "cases/helper"
+require 'models/developer'
+require 'models/computer'
+
+module ActiveRecord
+  module ConnectionAdapters
+    class MysqlAdapter
+      class ExplainTest < ActiveRecord::TestCase
+        fixtures :developers
+
+        def test_explain_for_one_query
+          explain = Developer.where(:id => 1).explain
+          assert_match %(EXPLAIN for: SELECT `developers`.* FROM `developers` WHERE `developers`.`id` = 1), explain
+          assert_match %r(developers |.* const), explain
+        end
+
+        def test_explain_with_eager_loading
+          explain = Developer.where(:id => 1).includes(:audit_logs).explain
+          assert_match %(EXPLAIN for: SELECT `developers`.* FROM `developers` WHERE `developers`.`id` = 1), explain
+          assert_match %r(developers |.* const), explain
+          assert_match %(EXPLAIN for: SELECT `audit_logs`.* FROM `audit_logs` WHERE `audit_logs`.`developer_id` = 1), explain
+          assert_match %r(audit_logs |.* ALL), explain
+        end
+      end
+    end
+  end
+end