dict_col_t::DROPPED: Magic value for dict_col_t::ind

row_metadata_to_tuple(): Convert a metadata record to a data tuple,
based on the new info_bits of the metadata record.

btr_cur_pessimistic_update(): Invoke row_metadata_to_tuple() if needed.

dict_index_t::instant_add_field(): Append fields at the end, if any.
The fields for dropped columns are preserved.

dict_table_t::instant_column(): Renamed from instant_add_column().
Add the parameter col_map so that columns can be reordered.
FIXME: Also support ADD/DROP/reorder/rename of VIRTUAL COLUMN.

ha_innobase_inplace_ctx::prepare_instant(): Preserve any instantly
dropped columns in instant_table and its first index.
FIXME: Support instant ALTER TABLE even if hidden FTS_DOC_ID exists.

innobase_instant_try(): Simplify the logic for detecting dropped
or reordered columns. If any drop or reorder is needed, construct
an update vector with the metadata blob.

dtype_t::metadata_blob_init(): Initialize the metadata BLOB data type.

dict_table_t::find(): Find an old column based on a new column number.

rec_convert_dtuple_to_metadata_comp(): Convert an alter metadata tuple
into a record.

row_rec_to_index_entry_impl(): Add the template parameter mblob
and the optional parameter info_bits for specifying the desired new
info bits. For the metadata tuple, allow conversion between the original
format (ADD COLUMN only) and the generic format (with hidden BLOB).

row_upd_index_replace_metadata(): Apply an update vector to an
alter_metadata tuple.

innobase_add_one_virtual(), innobase_insert_sys_virtual(): Change
the return type to bool, and invoke my_error() when detecting an error.

innodb_insert_sys_columns(): Insert a record into SYS_COLUMNS.
Refactored from innobase_add_one_virtual() and innobase_instant_add_col().

innobase_instant_add_col(): Replace the parameter dfield with type.

innobase_instant_drop_cols(): Drop matching columns from SYS_COLUMNS
and all columns from SYS_VIRTUAL.

innobase_instant_try(): Only invoke innobase_instant_drop_cols() when
there are other changes than adding columns last.

innodb_update_cols(): Renamed from innodb_update_n_cols().

innobase_instant_try(): If any columns were dropped or
reordered, delete any SYS_COLUMNS records for the following
columns, and insert SYS_COLUMNS records for all subsequent
stored columns as well as for all virtual columns.

commit_try_norebuild(): If ctx->is_instant(), let the virtual
columns be added or dropped by innobase_instant_try().

There is no need to omit dropped columns in rec_get_offsets() and
related functions. Dropped columns will have to be dealt with
at a higher level when converting between table rows and
clustered index records.

This is a too specific function, and there is only one caller.

Simplify the code in btr_cur_instant_init_clust_index(),
and make it a part of btr_cur_instant_init_metadata().

Remove some unnecessary computations and conditions.

FIXME: Instant add/drop/reorder operations on stored columns
cannot yet be combined with operations on virtual columns.

innobase_instant_drop_col(): Remove the parameter for the table name.
The InnoDB data dictionary could be out of sync with the .frm file,
because renaming columns between upper and lower case may not be
reflected by the InnoDB dictionary.

innobase_op_instant_try(): Remember the number of the last stored
column, and support dropping multiple columns at a time.
FIXME: adjust SYS_COLUMNS.POS for the pre-existing (not added)
virtual columns.

dict_col_t::add_to_core(): Convert an instantly added column
to a "core column". The old name remove_instant() could
be mistaken to refer to "instant DROP COLUMN".

dict_col_t::is_added(): The new name for is_instant().

- Optimised the remove_instant if there is no dropped column present.

Clarify some comments about accessing an externally stored column
on which a spatial index has been defined. Add a TODO comment that
we should actually write the minimum bounding rectangle (MBR) to
the undo log record, so that we can avoid fetching BLOBs and recomputing
MBR.

row_build_spatial_index_key(): Split from row_build_index_entry_low().

can handle for dropped columns too.
- Restricted the drop column with FTS_DOC_ID and
hidden fts index removal for instant operation.

Do not exclude instantly dropped columns from the size of the record.
This has to be handled at the upper level, by row_build() and the
to-be-written replacement of row_build_index_entry(), called
row_build_clust_index_entry().

Hopefully only a small number of functions will have to directly
access those clustered index record fields that can be reordered
or instantly dropped.

Until row_build() is taking care of reordering the fields in the
clustered index record, we cannot really support anything else
than instant DROP COLUMN of the last columns of a table.

rec_info_bits_valid(): Remove. The only case when this would fail
is when the record is the generic ALTER TABLE metadata record.

rec_is_alter_metadata(): Check if a record is the metadata record
for instant ALTER TABLE (other than ADD COLUMN). NOTE: This function
must not be invoked on node pointer records, because the delete-mark
flag in those records may be set (it is garbage), and then a debug
assertion could fail because index->is_instant() does not necessarily
hold.

rec_offs_validate(): FIXME: Temporarily disabled an assertion
that does not hold if rec_is_alter_metadata(). Because we do not know
here if a ROW_FORMAT=REDUNDANT record is for the leaf page, we cannot
invoke rec_is_alter_metadata() here.