Use `show index from`. We could fix `pk_and_sequence_for` method's performance problem (GH #3678)

1 files changed, 2 insertions, 18 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 3e043992e9..f72d2974c8 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb
+++ b/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb
@@ -576,24 +576,8 @@ module ActiveRecord
 
       # 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)
-        sql = <<-SQL
-          SELECT t.constraint_type, k.column_name
-          FROM information_schema.table_constraints t
-          JOIN information_schema.key_column_usage k
-          USING (constraint_name, table_schema, table_name)
-          WHERE t.table_schema = DATABASE()
-            AND t.table_name   = '#{table}'
-        SQL
-
-        execute_and_free(sql, 'SCHEMA') do |result|
-          keys = each_hash(result).select { |row| row[:constraint_type] == 'PRIMARY KEY' }.map { |row| row[:column_name] }
+        execute_and_free("SHOW INDEX FROM #{quote_table_name(table)} WHERE Key_name = 'PRIMARY'", 'SCHEMA') do |result|
+          keys = each_hash(result).map { |row| row[:Column_name] }
           keys.length == 1 ? [keys.first, nil] : nil
         end
       end