diff --git a/lib/mysqlschema.php b/lib/mysqlschema.php
index c3d3501c74..435ba4e3a9 100644
--- a/lib/mysqlschema.php
+++ b/lib/mysqlschema.php
@@ -298,6 +298,16 @@ class MysqlSchema extends Schema
         return "{$tableName}_{$columnName}_idx";
     }
 
+    /**
+     * MySQL doesn't take 'DROP CONSTRAINT', need to treat primary keys as
+     * if they were indexes here, but can use 'PRIMARY KEY' special name.
+     *
+     * @param array $phrase
+     */
+    function appendAlterDropPrimary(array &$phrase)
+    {
+        $phrase[] = 'DROP PRIMARY KEY';
+    }
 
     /**
      * MySQL doesn't take 'DROP CONSTRAINT', need to treat unique keys as
diff --git a/lib/schema.php b/lib/schema.php
index 2e27955881..aad705a533 100644
--- a/lib/schema.php
+++ b/lib/schema.php
@@ -579,6 +579,10 @@ class Schema
             $this->appendAlterDropUnique($phrase, $keyName);
         }
 
+        if (isset($old['primary key']) && (!isset($def['primary key']) || $def['primary key'] != $old['primary key'])) {
+            $this->appendAlterDropPrimary($phrase);
+        }
+
         foreach ($fields['add'] as $columnName) {
             $this->appendAlterAddColumn($phrase, $columnName,
                     $def['fields'][$columnName]);
@@ -594,6 +598,10 @@ class Schema
             $this->appendAlterDropColumn($phrase, $columnName);
         }
 
+        if (isset($def['primary key']) && (!isset($old['primary key']) || $old['primary key'] != $def['primary key'])) {
+            $this->appendAlterAddPrimary($phrase, $def['primary key']);
+        }
+
         foreach ($uniques['mod'] + $uniques['add'] as $keyName) {
             $this->appendAlterAddUnique($phrase, $keyName, $def['unique keys'][$keyName]);
         }
@@ -713,6 +721,19 @@ class Schema
         $phrase[] = implode(' ', $sql);
     }
 
+    function appendAlterAddPrimary(array &$phrase, array $def)
+    {
+        $sql = array();
+        $sql[] = 'ADD';
+        $this->appendPrimaryKeyDef($sql, $def);
+        $phrase[] = implode(' ', $sql);
+    }
+
+    function appendAlterDropPrimary(array &$phrase)
+    {
+        $phrase[] = 'DROP CONSTRAINT PRIMARY KEY';
+    }
+
     function appendAlterDropUnique(array &$phrase, $keyName)
     {
         $phrase[] = 'DROP CONSTRAINT ' . $keyName;