MDEV-8212 alter table - failing to ADD PRIMARY KEY IF NOT EXISTS when…

… existing index of same as column name.

        The default name for the primary key is rather 'PRIMARY' instead of the indexed column name.

mysql-test/r/alter_table.result

@@ -1488,6 +1488,17 @@ t2	CREATE TABLE `t2` (
 ) ENGINE=MyISAM DEFAULT CHARSET=latin1
 DROP TABLE t2;
 DROP TABLE t1;
+CREATE TABLE t1 (
+`transaction_id` int(11) NOT NULL DEFAULT '0', 
+KEY `transaction_id` (`transaction_id`));
+ALTER TABLE t1 DROP KEY IF EXISTS transaction_id, ADD PRIMARY KEY IF NOT EXISTS (transaction_id);
+SHOW CREATE TABLE t1;
+Table	Create Table
+t1	CREATE TABLE `t1` (
+  `transaction_id` int(11) NOT NULL DEFAULT '0',
+  PRIMARY KEY (`transaction_id`)
+) ENGINE=MyISAM DEFAULT CHARSET=latin1
+DROP TABLE t1;
 # Bug#11748057 (formerly known as 34972): ALTER TABLE statement doesn't
 #                                         identify correct column name.
 #

mysql-test/t/alter_table.test

@@ -1326,6 +1326,14 @@ SHOW CREATE TABLE t2;
 DROP TABLE t2;
 DROP TABLE t1;
 
+CREATE TABLE t1 (
+    `transaction_id` int(11) NOT NULL DEFAULT '0', 
+    KEY `transaction_id` (`transaction_id`));
+ALTER TABLE t1 DROP KEY IF EXISTS transaction_id, ADD PRIMARY KEY IF NOT EXISTS (transaction_id);
+SHOW CREATE TABLE t1;
+
+DROP TABLE t1;
+
 --echo # Bug#11748057 (formerly known as 34972): ALTER TABLE statement doesn't
 --echo #                                         identify correct column name.
 --echo #

sql/sql_table.cc

@@ -5859,12 +5859,17 @@ handle_if_exists_options(THD *thd, TABLE *table, Alter_info *alter_info)
       /* let us check the name of the first key part. */
       if ((keyname= key->name.str) == NULL)
       {
-        List_iterator<Key_part_spec> part_it(key->columns);
-        Key_part_spec *kp;
-        if ((kp= part_it++))
-          keyname= kp->field_name.str;
-        if (keyname == NULL)
-          continue;
+        if (key->type == Key::PRIMARY)
+          keyname= primary_key_name;
+        else
+        {
+          List_iterator<Key_part_spec> part_it(key->columns);
+          Key_part_spec *kp;
+          if ((kp= part_it++))
+            keyname= kp->field_name.str;
+          if (keyname == NULL)
+            continue;
+        }
       }
       if (key->type != Key::FOREIGN_KEY)
       {