- It turns out, there are statements that will call lex_start(thd->lex)
after parsing has been finished. lex_start() will set lex->explain=NULL,
which will lose the pointer to already allocated Explain_plan object.
- To get rid of this, switch to lazy creation of lex->explain. Now, it is
created only when we get a part ot query plan.
Update the SHOW EXPLAIN code to work with the
new architecture (part#1):
Before, SHOW EXPLAIN operated on real query plan structures,
which meant it had to check when SELECTs are created/deleted.
SELECTs would call apc_target->enable() when they got a query
plan and disable() when their query plan was deleted.
Now, Explain data structure becomes available at once (and we
call apc_target->enable()) and then it stays until it is deleted
(when that happens, we call apc_target->disable()).
- Don't save UNION's EXPLAIN data if optimization failed with
an error. We could end up saving incomplete plan, which will
cause a crash when we attempt to print it.
- Generate correct contents of `Extra` column for UPDATEs/DELETEs that use quick selects
- UPDATEs with used_key_is_modified=true will show "Using buffer"
