Index: modules/field/field.api.php
===================================================================
RCS file: /cvs/drupal/drupal/modules/field/field.api.php,v
retrieving revision 1.61
diff -u -p -r1.61 field.api.php
--- modules/field/field.api.php	13 Jan 2010 04:37:03 -0000	1.61
+++ modules/field/field.api.php	22 Jan 2010 07:05:04 -0000
@@ -78,6 +78,7 @@ function hook_field_extra_fields() {
  * @see hook_field_info().
  * @see hook_field_info_alter().
  * @see hook_field_schema().
+ * @see hook_field_schema_alter().
  * @see hook_field_load().
  * @see hook_field_validate().
  * @see hook_field_presave().
@@ -239,6 +240,17 @@ function hook_field_schema($field) {
 }
 
 /**
+ * Allows modules to alter the schema for a field.
+ *
+ * @param $schema
+ *   The schema definition as returned by hook_field_schema().
+ * @param $field
+ *   The field definition.
+ */
+function hook_schema_alter($schema, $field) {
+}
+
+/**
  * Defines custom load behavior for this module's field types.
  *
  * Unlike most other field hooks, this hook operates on multiple objects. The
Index: modules/field/field.crud.inc
===================================================================
RCS file: /cvs/drupal/drupal/modules/field/field.crud.inc,v
retrieving revision 1.46
diff -u -p -r1.46 field.crud.inc
--- modules/field/field.crud.inc	13 Jan 2010 14:06:46 -0000	1.46
+++ modules/field/field.crud.inc	22 Jan 2010 07:05:05 -0000
@@ -301,7 +301,7 @@ function field_create_field($field) {
   // Collect storage information.
   $schema = (array) module_invoke($field['module'], 'field_schema', $field);
   $schema += array('columns' => array(), 'indexes' => array());
-  // 'columns' are hardcoded in the field type.
+  drupal_alter('field_schema', $schema, $field);
   $field['columns'] = $schema['columns'];
   // 'indexes' can be both hardcoded in the field type, and specified in the
   // incoming $field definition.