6.x core.php hook_schema_alter(&$schema)
7.x system.api.php hook_schema_alter(&$schema)

Perform alterations to existing database schemas.

When a module modifies the database structure of another module (by changing, adding or removing fields, keys or indexes), it should implement hook_schema_alter() to update the default $schema to take its changes into account.

See hook_schema() for details on the schema definition structure.


$schema: Nested array describing the schemas for all modules.

Related topics

1 invocation of hook_schema_alter()
drupal_get_complete_schema in includes/bootstrap.inc
Gets the whole database schema.


modules/system/system.api.php, line 3294
Hooks provided by Drupal core and the System module.


function hook_schema_alter(&$schema) {
  // Add field to existing schema.
  $schema['users']['fields']['timezone_id'] = array(
    'type' => 'int',
    'not null' => TRUE,
    'default' => 0,
    'description' => 'Per-user timezone configuration.',


ranavaibhav’s picture

Example of how to use this hook -

rudiedirkx’s picture

That's a D6 example. It's NOT the code for D7. See the db_add_field.

leonli’s picture

When drupal_get_schema() is called it goes through this sequence:
1) hook_schema()
2)_drupal_schema_initialize() (this DROPs the "description" field.)
3) hook_schema_alter() /* "description" field here will not be dropped. */

This creates inconsistency in the resulting $schema.

Should it be done in this sequence:
1) hook_schema()
2) hook_schema_alter()

SebCorbin’s picture

Note that this hook does not actually change the schema in your database, you'll have to use db_drop_field or db_add_field in a hook_install/hook_uninstall in order to add a field to another table.

n0rtan’s picture

This hook is used to alter the schema when using the drupal_get_schema