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field_sql_storage_schema() exposes the schema of dynamic field data tables.
That's a *lot* of data in drupal_get_schema(), and computing it from configuration data on sites with lots of fields takes some CPU too.
Discussing with various people (@effulgentsia, @chx, @fago), it seems no-one sees the interest of exposing the schema for those tables. We don't ever use drupal_write_record() on those.
@DamZ pointed that ctools D7 uses the 'foreign keys' schema entries to auto-generate relationships - but it reads those from each individual field's hook_field_schema() rather than from drupal_schema()
PASSED: [[SimpleTest]]: [MySQL] 55,757 pass(es). View
FAILED: [[SimpleTest]]: [MySQL] 53,770 pass(es), 5 fail(s), and 1 exception(s). View