Analysis:
The reason for the inefficent plan was that Item_subselect::is_expensive()
didn't detect the special case when a subquery was optimized, but had no
join plan because it either has no table, or its tables have been optimized
away, or the optimizer detected that the result set is empty.
  
Solution:
Identify the special cases above in the Item_subselect::is_expensive(),
and consider such degenerate subqueries inexpensive.

This bug was introduced by the patch for WL#3220.
If the memory allocated for the tree to store unique elements
to be counted is not big enough to include all of them then
an external file is used to store the elements.
The unique elements are guaranteed not to be nulls. So, when 
reading them from the file we don't have to care about the null
flags of the read values. However, we should remove the flag 
at the very beginning of the process. If we don't do it and
if the last value written into the record buffer for the field
whose distinct values needs to be counted happens to be null,
then all values read from the file  are considered to be nulls
and are not counted in.
The fix does not remove a possible null flag for the read values.
Rather it just counts the values in the same way it was done
before WL #3220.

Extend plugin auth api to support up to 512 bytes in the user names.
Use the API versioning to support old auth plugins too!

          The get_mbr() method shouldn't return the error, rather an invalid MBR
          in this case.

Fixed printing column_get finction.

In some cases, when using views the optimizer incorrectly determined
possible join orders for queries with nested outer and inner joins.
This could lead to invalid execution plans for such queries.

        The Geometry::get_mbr() function can return an error on
        a bad data. We have to check for that and act respectively.

- Set MI_INFO::external_ref for MyISAM tables that are parts of myisamMRG table.

./mtr --suite=main,plugins
will work on all branches.

MDEV-4293 Valgrind warnings (Conditional jump or move depends on uninitialised value) in remove_eq_conds on time functions with NULL argument

val_int() is expected to return 0 for NULL's

(in INFORMATION_SCHEMA).

  
The reason is the limitation of ReadConsole() API, it returns error if handle to redirected  input  is used.
  
 The fix is to use a  handle returned by CreateFile("CONIN$",...), rather than GetStdHandle(STD_HANDLE_INPUT) to get the true console handle.

The output of mysqlbinlog (with "-v --base64-output=DECODE-ROWS" flags) can not always be read or parsed correctly
when string columns contain single-quotes or backslash characters.

The fix for this bug is to escape single-quote and backslash characters on output, so that the result is both more
readable and more easily parse-able.

Note that this is just for comments, so it doesn't affect the replication.

sql/log_event.cc:
  Escape \ and ' properly for mysqlbin user comments.

        Need to check if the number of points is 0 for the polygon.

        Forgotten DBUG_ASSERT should be replaced with the 'return error'.

Item_default_value inherited form Item_field so should create temporary table field similary.

    Additional fixes for possible overflows in length-related
    calculations in 'spatial' implementations.
    Checks added to the ::get_data_size() methods.
    max_n_points decreased to occupy less 2G size. An
    object of that size is practically inoperable anyway.

Item_func_make_set wasn't taking into account the first argument when
calculating maybe_null.

sql/item_strfunc.cc:
  rewrite Item_func_make_set, removing separate storage of the first argument
sql/item_strfunc.h:
  rewrite Item_func_make_set, removing separate storage of the first argument

  
with decimals=NOT_FIXED_DEC it is possible to have 'decimals' larger
than 'max_length', it's not an error for temporal functions.
  
But when Item_func_numhybrid converts the value to DECIMAL_RESULT,
it must limit 'decimals' to be a valid scale of a decimal number.

AVG() returns a double, its max_length is reasonably
limited by a double number length, even if the argument
is many Kbytes long.

Flip the switch and create Item_cache based on the argument's cmp_type, not argument's result_type().
Fix subselect_engine to calculate cmp_type correctly

sql/item_subselect.h:
  mdev:4284

MDEV-4281 Assertion `maybe_null && item->null_value' fails in make_sortkey on CASE with different return types, GROUP_CONCAT, GROUP BY

Fix Item::get_date() to mark the item NULL when returning an error.

Incorrect NULL value handling in Item_func_conv_charset fixed.

mysql-test/r/keywords.result:
  Test that option works as table/column/variable
mysql-test/suite/funcs_1/r/storedproc.result:
  OPTION is now a valid identifier
mysql-test/suite/funcs_1/t/storedproc.test:
  OPTION is now a valid identifier
mysql-test/t/keywords.test:
  Test that option works as table/column/variable
sql/sql_yacc.yy:
  OPTION is now a valid identifier

  Adding an include file which checks whether long names are supported

get_datetime_value() should not double-cache its own Item_cache_temporal items,
but it *should* cache other Item_cache items, such as Item_cache_str.

sql/item.h:
  shortcut, to avoid going through the switch in Item::cmp_type()
sql/item_cmpfunc.cc:
  even if the item is Item_cache_str - it still needs to be converted and cached.
sql/item_timefunc.h:
  all descendants of Item_temporal_func always have cmp_type==TIME_RESULT.
  Even Item_date_add_interval, that might have field_type == MYSQL_TYPE_STRING.