MDEV-5446: Assertion `!table || (!table->read_set || bitmap_is_set(table->read_set, field_index))' fails on EXPLAIN EXTENDED with VALUES function

field_index should be set correctly for null field created by Item_insert_value::fix_fields().

(as it's done in 10.0)

Item_default_value::arg can be NULL so walk() should take it into consideration.

Item_default_value::arg can be NULL so walk() should take it into consideration.

MDEV-5740: Assertion `!derived->first_select()->exclude_from_table_unique_test || derived->outer_select()-> exclude_from_table_unique_test' failed on 2nd execution of PS with derived_merge

Do not check tables of executed units.
Debug info about stages of derived tables execution added.

The calls of the function remove_eq_conds() may change the and/or structure
of the where conditions. So JOIN::equal_cond should be updated for non-recursive
calls of remove_eq_conds(). 

(assert added by MySQL verified that strnxfrm can only *increase*
the string length if from == to, and the latter is a random
decision made by individual items and String::realloc).

Fix it by avoiding the memcpy in the first place.

Reason for the bug was an optimization for higher connect speed where we moved when global status was updated,
but forgot to update states when slave thread dies.
Fixed by adding thd->add_status_to_global() before deleting slave thread's thd.


mysys/my_delete.c:
  Added missing newline
sql/mysqld.cc:
  Use add_status_to_global()
sql/slave.cc:
  Added missing add_status_to_global()
sql/sql_class.cc:
  Use add_status_to_global()
sql/sql_class.h:
  Simplify adding local status to global by adding add_status_to_global()

update_used_tables for the the where condition to update cached
indicators of constant subexpressions. It should be done before further
possible simplification of the where condition.

This change caused simplification of the executed where conditions 
in many test cases.

MDEV-4556 Server crashes in SEL_ARG::rb_insert with index_merge+index_merge_sort_union, FORCE INDEX
- merge_same_index_scans() may put the same SEL_ARG tree in multiple result plans.
  make it call incr_refs() on the SEL_ARG trees that it does key_or() on, because 
  key_or(sel_arg_tree_1, sel_arg_tree_2) call may invalidate SEL_ARG trees pointed 
  by sel_arg_tree_1 and sel_arg_tree_2.
    

In function ‘void* memset(void*, int, size_t)’,
    inlined from ‘void Lifo_buffer::set_buffer_space(uchar*, uchar*)’ at sql_lifo_buffer.h:70:5,
    inlined from ‘int DsMrr_impl::dsmrr_init(handler*, RANGE_SEQ_IF*, void*, uint, uint, HANDLER_BUFFER*)’ at multi_range_read.cc:895:62:
/usr/include/i386-linux-gnu/bits/string3.h:82:32: error: call to ‘__warn_memset_zero_len’ declared with attribute warning: memset used with constant zero length parameter; this could be due to transposed parameters [-Werror]

It was intentional that the buffer is set to zero length there.

(gdb) p dbug_print_table_row(table)
  $33 = "SUBQUERY2_t1(col_int_key,col_varchar_nokey)=(7,c)"

- Item_direct_view_ref didn't clear its pointer to item_equal in ::cleanup.
- Some Item_direct_view_ref objects have statement lifetime (i.e. they
  survive across multiple EXECUTE commands). Item_equal objects live only for
  the duration of one EXECUTE. This caused Item_direct_view_ref to have a stale pointer,
  which could cause all sorts of effects. (In this bug's testcase it was pointing to
  the wrong Item_equal, causing wrong query result)
- Fixed by doing what Item_field::cleanup() does - don't keep item_equal pointer value.
- There is no testcase because the only testcase I've got is highly fragile (e.g. the
  bug will not show up if @@datadir is of the wrong length).

Fixed so the MYSQL_TYPE_GEOMETRY is treated as BLOB.

The result is EMPTY for a buffer(line, -1), but we still
need one FALSE operation to be stored in the condition.
And we actually add it but forgot to alloc memory to store it.

- The problem was that JOIN::prepare() tried to set TABLE::maybe_null
  for a table in join. Non-merged semi-join tables 1) are present as 
  join's base tables on second EXECUTE, but 2) do not yet have a TABLE 
  object.
  Worked around the problem by putting mixed_implicit_grouping into JOIN
  object, and then passing it to JTBM tables in setup_jtbm_semi_joins().

(just to have the test in the tree when we merge the upstream fix)

Don't ignore the error from st_select_lex_unit::optimize()

as it was supposed to be in bug#11747970, don't fake an error.
(this kill can be useful for other bugs too)

Pre-MDL versions had direct relationship between LOCK_open and
LOCK_global_system_variables, e.g.:
  intern_sys_var_ptr // locks LOCK_global_system_variable
  mysql_sys_var_char
  create_options_are_valid
  ha_innobase::create
  handler::ha_create
  ha_create_table
  rea_create_table
  mysql_create_table_no_lock // locks LOCK_open
  mysql_create_table

With MDL this relationship was removed, but mutex order was still
recorded. In fact there is indirect relationship between LOCK_open
and LOCK_global_system_variables via rwlocks in reverse order.

Removed LOCK_open and LOCK_global_system_variables order recording,
instead assert that LOCK_open is never held in intern_sys_var_ptr().

This solves only one of many problems detected with MDEV-5089.

- There was List<N>::nth_element() but it didn't work because linker removed it. 
- Now, removal by linker is prevented for important values of T, and function is renamed. 

(This is attempt at fix #2) (re-commit with fixed typo)
- Moved the testcase from partition_test to partition_innodb.test where it can really work.
- Made ordered index scans over ha_partition tables to satisfy ROR property for 
  the case where underlying table uses extended keys.