Merge pull request #3683 from christos/schema_introspection_speedup

Only used detailed schema introspection when doing a schema dump.
Conflicts:

	activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb

activerecord/lib/active_record/connection_adapters/mysql2_adapter.rb

@@ -572,6 +572,13 @@ def show_variable(name)
 
       # Returns a table's primary key and belonging sequence.
       def pk_and_sequence_for(table)
+        execute_and_free("DESCRIBE #{quote_table_name(table)}", 'SCHEMA') do |result|
+          keys = each_hash(result).select { |row| row[:Key] == 'PRI' }.map { |row| row[:Field] }
+          keys.length == 1 ? [keys.first, nil] : nil
+        end
+      end
+
+      def detailed_pk_and_sequence_for(table) #:nodoc:
         keys = []
         sql = <<-SQL
           SELECT t.constraint_type, k.column_name

activerecord/lib/active_record/connection_adapters/mysql_adapter.rb

@@ -755,6 +755,13 @@ def show_variable(name)
 
       # Returns a table's primary key and belonging sequence.
       def pk_and_sequence_for(table) #:nodoc:
+        execute_and_free("DESCRIBE #{quote_table_name(table)}", 'SCHEMA') do |result|
+          keys = each_hash(result).select { |row| row[:Key] == 'PRI' }.map { |row| row[:Field] }
+          keys.length == 1 ? [keys.first, nil] : nil
+        end
+      end
+
+      def detailed_pk_and_sequence_for(table) #:nodoc:
         keys = []
         sql = <<-SQL
           SELECT t.constraint_type, k.column_name

activerecord/lib/active_record/schema_dumper.rb

@@ -86,7 +86,9 @@ def table(table, stream)
           tbl = StringIO.new
 
           # first dump primary key column
-          if @connection.respond_to?(:pk_and_sequence_for)
+          if @connection.respond_to?(:detailed_pk_and_sequence_for)
+            pk, _ = @connection.detailed_pk_and_sequence_for(table)
+          elsif @connection.respond_to?(:pk_and_sequence_for)
             pk, _ = @connection.pk_and_sequence_for(table)
           elsif @connection.respond_to?(:primary_key)
             pk = @connection.primary_key(table)