- Fix a problem with EXPLAIN multi_table UPDATE:
= Do use multi_update object, because multi_update::prepare() does
various setup, e.g. it disables index-only for the tables to be updated.
= Protect multi_update::prepare() from being invoked multiple times.
If the query has subqueries, they may try to invoke it, for some reason.
- Make EXPLAIN UPDATE/DELETE work inside SPs
- Return correct error code from mysql_delete()
- EXPLAIN <multi-DELETE> will create a multi_delete object (as it
affects the optimization). select_result will be only used for
producing EXPLAIN output.
Single table UPDATE/DELETE
- Correctly print type=SIMPLE vs type=PRIMARY
- Handle UPDATE/DELETE of mergeable VIEWs: we get the
VIEW's select as the first subquery.
(MySQL 5.6 doesn't print it because it finds that the
subquery is not attached to any select)
