MDEV-305: SHOW EXPLAIN: ref returned by SHOW EXPLAIN is different from the normal EXPLAIN ('const' vs empty string)
- The problem was that create_ref_for_key() would act differently, depending on 
  whether we're running EXPLAIN or the actual query.
- As the first step, fixed the EXPLAIN printout not to depend on actions in create_ref_for_key().

MDEV-275: SHOW EXPLAIN: server crashes in JOIN::print_explain with IN subquery and aggregate function
- Don't try to produce plans after JOIN::cleanup() has been called, because:
   = JOIN::cleanup leaves data structures in partially-cleaned state
   = Walking them is hazardous (see this bug), and has funny effects
     (See previous commits, "Using join cache" may or may not be shown)
   = Changing data structures to be persisted may cause unwanted side effects
- The consequence is that SHOW EXPLAIN will show "Query plan already deleted" when e.g. 
  reading data after filesort.

- It turns out, there is a case where the join is degenerate, but 
  join->table_count!= && join->tables_list!=NULL. Need to also check 
  if join->zero_result_cause!=NULL, too.

- There is a slight problem: The code sets
    zero_result_cause= "no matching row in const table"
  when NOT running EXPLAIN. The result is that SHOW EXPLAIN will show this line while 
  regular EXPLAIN will not.

 select tables optimized away
- Take into account, that for some degenerate joins instead of "join->table_count=0"
  the code sets "join->tables_list=0".

- Make SHOW EXPLAIN code handle degenerate subquery cases (No tables, impossible where, etc)

- Fix the bug: SHOW EXPLAIN may hit a case where a join is partially 
  optimized.
- Change JOIN::optimized to use enum instead of numeric constants

- Make all functions that produce parts of EXPLAIN output take 
  explain_flags as parameter, instead of looking into thd->lex->describe

- Support SHOW EXPLAIN for selects that have "Using temporary; Using filesort".

Make SHOW EXPLAIN work for queries that do "Using temporary" and/or "Using filesort"
- Patch#1: Don't lose "Using temporary/filesort" in the SHOW EXPLAIN output.

The code to re-enable checkpointing after UNLOCK TABLES was lost in the 5.5
merge, so re-add it back in.

5.5 version. for cmake, not autotools.

mtr should do it as well, to avoid differences in test output. 

This fixes sys_vars.secure_file_priv on FreeBSD9.0 buildbot.

is gone.

The problem was increment of aborted_threads variable due to thd->killed which was set when threadpool connection was terminated .  The fix is not to set thd->killed anymore, there is no real reason for doing it..

Added a test that checks that status variable aborted_clients does not grow for ordinary disconnects, and that successful KILL increments this variable.

MDEV-214 lp:967242 Wrong result with JOIN, AND in ON condition, multi-part key, GROUP BY, subquery and OR in WHERE

The problem was in the code (update_const_equal_items()) which marked
index parts constant independently of the place where the equality was used.
In the test suite it marked t2_1.c part constant despite the fact that
it connected by OR with other expression.

Solution is to mark constant only top equalities connected with AND.

Fix is to set  maybe_null  flag for Item_func_last_day.

unnecessary date->string->date conversion

Create an Item_cache based on item's cmp_type, not result_type in 
subselect_engine.

Use result_field in Item_cache_temporal::cache_value(),
just like all other Item_cache*::cache_value() do.

  Points and lines should disappear if we got negative D.
  To make it work properly inside the GEOMETRYCOLLECTION,
  we add the empty operation there.

bug #986977 Assertion `!cur_p->event' failed in Gcalc_scan_iterator::arrange_event(int, int).
  The double->inernal coord conversion produced -0 (minus zero) on some data.
  That minus-zero produces invalid comparison results when compared agains plus-zero.
  So we fixed the gcalc_set_double() to avoid it.

per-file comments:
  mysql-test/r/gis-precise.result
        result updated.
  mysql-test/t/gis-precise.test
        tests for #977021 and #986977 added.
  sql/gcalc_slicescan.cc
        bug #986977. The gcalc_set_double fixed to not produce minus-zero.
  sql/item_geofunc.cc
        bug #977021. Add the NOOP for the disappearing features.

MDEV-216 lp:976104 - Assertion `0' failed in my_message_sql on UPDATE IGNORE, or unknown error on release build

Don't send_error at the end of mysql_multi_update() if select failed.
The error, if there was any, was already sent by mysql_select