Inwe deferred hook_field_schema_alter() to a follow-up issue, but then never opened the follow-up issue. I still think we need it - for optimizing queries both in default SQL storage, and in non-SQL storage.
Barry's argument on that issue was that we don't have hook_field_schema_alter() because there's no hook_schema_alter() - except that we do: http://api.drupal.org/api/function/hook_schema_alter/6
For the reasons we have hook_schema_alter() - mainly site-specific tweaks, we should allow the same for field storage.
Passed on all environments.
PASSED: [[SimpleTest]]: [MySQL] 34,047 pass(es).
FAILED: [[SimpleTest]]: [MySQL] Unable to apply patch 691932_16-d7.patch. Unable to apply patch. See the log in the details link for more information.
FAILED: [[SimpleTest]]: [MySQL] 33,945 pass(es), 0 fail(s), and 778 exception(es).
PASSED: [[SimpleTest]]: [MySQL] 34,078 pass(es).