Clean up after Field API bulk deletion operations.
Field API provides functions for deleting data attached to individual entities as well as deleting entire fields or field instances in a single operation.
Deleting field data items for an entity with field_attach_delete() involves three separate operations:
entity. The hook for each field type receives the entity and the specific field being deleted. A file field module might use this hook to delete uploaded files from the filesystem.
data from the primary field storage. The hook implementation receives the entity being deleted and deletes data for all of the entity's bundle's fields.
modules that implement it. Each hook implementation receives the entity being deleted and can operate on whichever subset of the entity's bundle's fields it chooses to.
When a field, bundle, or field instance is deleted, it is not practical to invoke these hooks immediately on every affected entity in a single page request; there could be thousands or millions of them. Instead, the appropriate field data items, instances, and/or fields are marked as deleted so that subsequent load or query operations will not return them. Later, a separate process cleans up, or "purges", the marked-as-deleted data by going through the three-step process described above and, finally, removing deleted field and instance records.
Purging field data is made somewhat tricky by the fact that, while field_attach_delete() has a complete entity to pass to the various deletion hooks, the Field API purge process only has the field data it has previously stored. It cannot reconstruct complete original entities to pass to the deletion hooks. It is even possible that the original entity to which some Field API data was attached has been itself deleted before the field purge operation takes place.
Field API resolves this problem by using "pseudo-entities" during purge operations. A pseudo-entity contains only the information from the original entity that Field API knows about: entity type, id, revision id, and bundle. It also contains the field data for whichever field instance is currently being purged. For example, suppose that the node type 'story' used to contain a field called 'subtitle' but the field was deleted. If node 37 was a story with a subtitle, the pseudo-entity passed to the purge hooks would look something like this:
$entity = stdClass Object( [nid] => 37, [vid] => 37, [type] => 'story', [subtitle] => array(  => array( 'value' => 'subtitle text', ), ), );
See Field API for information about the other parts of the Field API.
||Purges a batch of deleted Field API data, instances, or fields.|
||Purges the field data for a single field on a single pseudo-entity.|
||Purges a field record from the database.|
||Purges a field instance record from the database.|