EXPLAIN INSERT ... SELECT tried to use SELECT's execution path. This
caused a collection of problems:
- SELECT_DESCRIBE flag was not put into select_lex->options, which
means it was not in JOIN::select_options either (except for the first
member of the UNION).
- This caused UNION members to be executed. They would attempt to write
join output rows to the output.
- (Actual cause of the crash) second join sibling would call
result->send_eof() when finished execution. Then,
Explain_query::print_explain would attempt to write to query output
again, and cause an assertion due to non-empty query output.
[EXPLAIN] INSERT .. SELECT creates a select_insert object.
select_insert calls handler->start_bulk_insert() during
initialization.
For MyISAM/Aria this requires that a matching call to
handler->end_bulk_insert() call is made.
Regular INSERT .. SELECT accomplishes this by calling either
select_result->send_eof() or select_result->abort_result_set().
EXPLAIN INSERT ... SELECT didn't call either, which resulted in
improper de-initializaiton of handler object. Make it call
abort_result_set(), which invokes handler->end_bulk_insert().
- 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)
