- Moving Item_xxx_field declarations after Item_sum_xxx declarations,
  so Item_xxx_field constructors can be defined directly in item_sum.h
  rather than item_sum.cc. This removes some duplicate code, e.g.
  initialization of the following members at constructor time:
  name, decimals, max_length, unsigned_flag, field, maybe_null.
- Adding Item_sum_field as a common parent for Item_avg_field and
  Item_variance_field
- Deriving Item_sum_field directly from Item rather that Item_result_field,
  as Item_sum_field descendants do not need anything from Item_result_field.
- Removing hybrid infrastructure from Item_avg_field,
  adding Item_avg_field_decimal and Item_avg_field_double instead,
  as desired result type is already known at constructor time
  (not only at fix_fields time). This simplifies the code.
- Changing Item_avg_field_decimal::val_int() to call val_int_from_decimal()
  instead of doing { return (longlong) rint(val_real()); }
  This is the fix itself.

infrastructure, though in fact they always return REAL result.
Removing hybrid type artifacts.

- MDEV-8875 Wrong metadata for MAX(CAST(time_column AS DATETIME))
- MDEV-8873 Wrong field type or metadata for LEAST(int_column,string_column)
- MDEV-8912 Wrong metadata or type for @c:=string_or_blob_field

Adding Item_hybrid_func as a common parent for Item_func_hybrid_field_type,
Item_func_min_max, Item_func_user_var. This removes some duplicate code.

backport mysql parser fixes
0034963fbf199696792491bcb79d5f0731c98804
5948561812bc691bd0c13cf518a3fe77d9daf920

The problem was that GROUP BY code created Item_field objects
that referred to fields in the temp. tables used for GROUP BY.

Item_ref and set_items_ref_array() call caused pointers to temp.
table fields to occur in many places.

This patch introduces Item_temptable_field, which can handle
item->print() calls made after the underlying table is freed.

MDEV-8901: InnoDB: Punch hole is incorrecty done also to log files causing assertion and database corruption

Analysis: Problem is that punch hole does not know the actual page size
of the page and does the page belong to an data file or to a log file.

Fix: Pass down the file type and page size to os layer to be used
when trim is called. Also fix unsafe null pointer access to
actual write_size.

MDEV-7817 ALTER TABLE {ADD|DROP} INDEX IF [NOT] EXISTS does not get written into binlog if nothing to do.
Just log the ALTER statement even if there's nothing to do.

in case of STRING_RESULT to switch(cmp_type()).

The assumption is that the engine should not need to
evaluate HAVING on the table->record[0] - the engine either
can evaluate HAVING internally before writing it to the
table->record[0], or it should leave it to the server,
that will evaluate HAVING(table->record[0]).

Similarly the engine should not need to evaluate ORDER
on the table->record[0]. Either it returns the data already
sorted, or the server will sort the table.

instead pass the whole query down and let the engine return
unsupported parts back

down to the point where all tables are already known
(and subqueries converted to joins, if needed)

into a Pushdown_query object

This task is to allow storage engines that can execute GROUP BY or
summary queries efficiently to intercept a full query or sub query from
MariaDB and deliver the result either to the client or to a temporary
table for further processing.

- Added code in sql_select.cc to intercept GROUP BY queries.
  Creation of group_by_handler is done after all optimizations to allow
  storage engine to benefit of an optimized WHERE clause and suggested
  indexes to use.
- Added group by handler to sequence engine and a group_by test suite as
  a way to test the new interface.
- Intercept EXPLAIN with a message "Storage engine handles GROUP BY"

libmysqld/CMakeLists.txt:
  Added new group_by_handler files
sql/CMakeLists.txt:
  Added new group_by_handler files
sql/group_by_handler.cc:
  Implementation of group_by_handler functions
sql/group_by_handler.h:
  Definition of group_by_handler class
sql/handler.h:
  Added handlerton function to create a group_by_handler, if the storage
  engine can intercept the query.
sql/item_cmpfunc.cc:
  Allow one to evaluate item_equal any time.
sql/sql_select.cc:
  Added code to intercept GROUP BY queries
  - If all tables are from the same storage engine and the query is
    using sum functions, call create_group_by() to check if the storage
    engine can intercept the query.
    - If yes:
       - create a temporary table to hold a GROUP_BY row or result
       - In do_select() intercept normal query execution by instead
         calling the group_by_handler to get the result
       - Intercept EXPLAIN
sql/sql_select.h:
  Added handling of group_by_handler
  Added caching of the original join tab (needed for cleanup after
  group_by handler)
storage/sequence/mysql-test/sequence/group_by.result:
  Test group_by_handler interface
storage/sequence/mysql-test/sequence/group_by.test:
  Test group_by_handler interface
storage/sequence/sequence.cc:
  Added simple group_by_engine for handling COUNT(*) and
  SUM(primary_key).  This was done as a test of the group_by_handler
  interface

Fixed compiler warnings

sql/sql_view.cc:
Ensure that merged_for_insert is properly reset
(Left of an old patch)

if we didn't write the CREATE TEMPORARY TABLE statement.

- Enable old code from one of my older changesets that didn't make into 10.0
- Fix test cased that failed as they expected DROP TEMPORARY TABLE in the log.

Remove debug printout that was accidentally left in place

- Turning get_mm_tree_for_const() from a static function into
  a protected method in Item.
- Adding a new class Item_bool_func2_with_rev, for the functions and operators
  that have a reverse function and can use the range optimizer for
  to optimize "value OP field" as "field REV_OP value". Deriving
  Item_bool_rowready_func2 and Item_funt_spatial_rel from the new class.
- Removing Item_bool_func2::have_rev_func().

- Item_bool_func::get_func_mm_tree()
- Item_bool_func::get_mm_parts()
- Item_bool_func::get_ne_mm_tree()

from Item_bool_func::get_mm_leaf() into Field_xxx::can_optimize_range().
This reduces the total amount of virtual calls. Also, it's a prerequisite
change for the pluggable data types.

MDEV-8523: InnoDB: Assertion failure in file buf0buf.cc line 5963 (Failing assertion: key_version == 0 || key_version >= bpage->key_version)

Item_ref::val_bool_result().

Check if the engine is supported/allowed before replicating the
statement.