Allow ADD COLUMN anywhere in a table, not only adding as the
last column.

Allow instant DROP COLUMN and instant changing the order of columns.

The added columns will always be added last in clustered index records.
In new records, instantly dropped columns will be stored as NULL or
empty when possible.

Information about dropped and reordered columns will be written in
a metadata BLOB (mblob), which is stored before the first 'user' field
in the hidden metadata record at the start of the clustered index.
The presence of mblob is indicated by setting the delete-mark flag in
the metadata record.

The metadata BLOB stores the number of clustered index fields,
followed by an array of column information for each field.
For dropped columns, we store the NOT NULL flag, the fixed length,
and for variable-length columns, whether the maximum length exceeded
255 bytes. For non-dropped columns, we store the column position.

Unlike with MDEV-11369, when a table becomes empty, it cannot
be converted back to the canonical format. The reason for this is
that other threads may hold cached objects such as
row_prebuilt_t::ins_node that could refer to dropped or reordered
index fields.

For instant DROP COLUMN and ROW_FORMAT=COMPACT or ROW_FORMAT=DYNAMIC,
we must store the n_core_null_bytes in the root page, so that the
chain of node pointer records can be followed in order to reach the
leftmost leaf page where the metadata record is located.
If the mblob is present, we will zero-initialize the strings
"infimum" and "supremum" in the root page, and use the last byte of
"supremum" for storing the number of null bytes (which are allocated
but useless on node pointer pages). This is necessary for
btr_cur_instant_init_metadata() to be able to navigate to the mblob.

If the PRIMARY KEY contains any variable-length column and some
nullable columns were instantly dropped, the dict_index_t::n_nullable
in the data dictionary could be smaller than it actually is in the
non-leaf pages. Because of this, the non-leaf pages could use more
bytes for the null flags than the data dictionary expects, and we
could be reading the lengths of the variable-length columns from the
wrong offset, and thus reading the child page number from wrong place.
This is the result of two design mistakes that involve unnecessary
storage of data: First, it is nonsense to store any data fields for
the leftmost node pointer records, because the comparisons would be
resolved by the MIN_REC_FLAG alone. Second, there cannot be any null
fields in the clustered index node pointer fields, but we nevertheless
reserve space for all the null flags.

Limitations (future work):

MDEV-17459 Allow instant ALTER TABLE even if FULLTEXT INDEX exists
MDEV-17468 Avoid table rebuild on operations on generated columns
MDEV-17494 Refuse ALGORITHM=INSTANT when the row size is too large

btr_page_reorganize_low(): Preserve any metadata in the root page.
Call lock_move_reorganize_page() only after restoring the "infimum"
and "supremum" records, to avoid a memcmp() assertion failure.

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

dict_col_t::clear_instant(): Renamed from dict_col_t::remove_instant().
Do not assert that the column was instantly added, because we
sometimes call this unconditionally for all columns.
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(): Rename from dict_col_t::is_instant().

dtype_t::metadata_blob_init(): Initialize the mblob data type.

dtuple_t::is_metadata(), dtuple_t::is_alter_metadata(),
upd_t::is_metadata(), upd_t::is_alter_metadata(): Check if info_bits
refer to a metadata record.

dict_table_t::instant: Metadata about dropped or reordered columns.

dict_table_t::prepare_instant(): Prepare
ha_innobase_inplace_ctx::instant_table for instant ALTER TABLE.
innobase_instant_try() will pass this to dict_table_t::instant_column().
On rollback, dict_table_t::rollback_instant() will be called.

dict_table_t::instant_column(): Renamed from instant_add_column().
Add the parameter col_map so that columns can be reordered.
Copy and adjust v_cols[] as well.

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

dict_table_t::serialise_columns(), dict_table_t::deserialise_columns():
Convert the mblob.

dict_index_t::instant_metadata(): Create the metadata record
for instant ALTER TABLE. Invoke dict_table_t::serialise_columns().

dict_index_t::reconstruct_fields(): Invoked by
dict_table_t::deserialise_columns().

dict_index_t::clear_instant_alter(): Move the fields for the
dropped columns to the end, and sort the surviving index fields
in ascending order of column position.

ha_innobase::check_if_supported_inplace_alter(): Do not allow
adding a FTS_DOC_ID column if a hidden FTS_DOC_ID column exists
due to FULLTEXT INDEX. (This always required ALGORITHM=COPY.)

instant_alter_column_possible(): Add a parameter for InnoDB table,
to check for additional conditions, such as the maximum number of
index fields.

ha_innobase_inplace_ctx::first_alter_pos: The first column whose position
is affected by instant ADD, DROP, or changing the order of columns.

innobase_build_col_map(): Skip added virtual columns.

prepare_inplace_add_virtual(): Correctly compute num_to_add_vcol.
Remove some unnecessary code. Note that the call to
innodb_base_col_setup() should be executed later.

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

innobase_instant_try(): Fill in a zero default value for the
hidden column FTS_DOC_ID (to reduce the work needed in MDEV-17459).
If any columns were dropped or reordered (or added not last),
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. If any virtual column is dropped, rewrite
all virtual column metadata. Use a shortcut only for adding
virtual columns. This is because innobase_drop_virtual_try()
assumes that the dropped virtual columns still exist in ctx->old_table.

innodb_update_cols(): Renamed from innodb_update_n_cols().

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_add_virtual_try(), innobase_drop_virtual_try(): Let
the caller invoke innodb_update_cols().

innobase_rename_column_try(): Skip dropped columns.

commit_cache_norebuild(): Update table->fts->doc_col.

dict_mem_table_col_rename_low(): Skip dropped columns.

trx_undo_rec_get_partial_row(): Skip dropped columns.

trx_undo_update_rec_get_update(): Handle the metadata BLOB correctly.

trx_undo_page_report_modify(): Avoid out-of-bounds access to record fields.
Log metadata records consistently.
Apparently, the first fields of a clustered index may be updated
in an update_undo vector when the index is ID_IND of SYS_FOREIGN,
as part of renaming the table during ALTER TABLE. Normally, updates of
the PRIMARY KEY should be logged as delete-mark and an insert.

row_undo_mod_parse_undo_rec(), row_purge_parse_undo_rec():
Use trx_undo_metadata.

row_undo_mod_clust_low(): On metadata rollback, roll back the root page too.

row_undo_mod_clust(): Relax an assertion. The delete-mark flag was
repurposed for ALTER TABLE metadata records.

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).
Add the optional parameter "pad" to determine whether the tuple should
be padded to the index fields (on ALTER TABLE it should), or whether
it should remain at its original size (on rollback).

row_build_index_entry_low(): Clean up the code, removing
redundant variables and conditions. For instantly dropped columns,
generate a dummy value that is NULL, the empty string, or a
fixed length of NUL bytes, depending on the type of the dropped column.

row_upd_clust_rec_by_insert_inherit_func(): On the update of PRIMARY KEY
of a record that contained a dropped column whose value was stored
externally, we will be inserting a dummy NULL or empty string value
to the field of the dropped column. The externally stored column would
eventually be dropped when purge removes the delete-marked record for
the old PRIMARY KEY value.

btr_index_rec_validate(): Recognize the metadata record.

btr_discard_only_page_on_level(): Preserve the generic instant
ALTER TABLE metadata.

btr_set_instant(): Replaces page_set_instant(). This sets a clustered
index root page to the appropriate format, or upgrades from
the MDEV-11369 instant ADD COLUMN to generic ALTER TABLE format.

btr_cur_instant_init_low(): Read and validate the metadata BLOB page
before reconstructing the dictionary information based on it.

btr_cur_instant_init_metadata(): Do not read any lengths from the
metadata record header before reading the BLOB. At this point, we
would not actually know how many nullable fields the metadata record
contains.

btr_cur_instant_root_init(): Initialize n_core_null_bytes in one
of two possible ways.

btr_cur_trim(): Handle the mblob record.

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.
Invoke dtuple_convert_big_rec() for metadata records if the record is
too large, or if the mblob is not yet marked as externally stored.

btr_cur_optimistic_delete_func(), btr_cur_pessimistic_delete():
When the last user record is deleted, do not delete the
generic instant ALTER TABLE metadata record. Only delete
MDEV-11369 instant ADD COLUMN metadata records.

btr_cur_optimistic_insert(): Avoid unnecessary computation of rec_size.

btr_pcur_store_position(): Allow a logically empty page to contain
a metadata record for generic ALTER TABLE.

REC_INFO_DEFAULT_ROW_ADD: Renamed from REC_INFO_DEFAULT_ROW.
This is for the old instant ADD COLUMN (MDEV-11369) only.

REC_INFO_DEFAULT_ROW_ALTER: The more generic metadata record,
with additional information for dropped or reordered columns.

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_is_add_metadata(): Check if a record is MDEV-11369 ADD COLUMN metadata
record (not more generic instant ALTER TABLE).

rec_get_converted_size_comp_prefix_low(): Assume that the metadata
field will be stored externally. In dtuple_convert_big_rec() during
the rec_get_converted_size() call, it would not be there yet.

rec_get_converted_size_comp(): Replace status,fields,n_fields with tuple.

rec_init_offsets_comp_ordinary(), rec_get_converted_size_comp_prefix_low(),
rec_convert_dtuple_to_rec_comp(): Add template<bool mblob = false>.
With mblob=true, process a record with a metadata BLOB.

rec_copy_prefix_to_buf(): Assert that no fields beyond the key and
system columns are being copied. Exclude the metadata BLOB field.

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

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

row_log_allocate(): Replace dict_index_t::is_instant()
with a more appropriate condition that ignores dict_table_t::instant.
Only a table on which the MDEV-11369 ADD COLUMN was performed
can "lose its instantness" when it becomes empty. After
instant DROP COLUMN or reordering columns, we cannot simply
convert the table to the canonical format, because the data
dictionary cache and all possibly existing references to it
from other client connection threads would have to be adjusted.

row_quiesce_write_index_fields(): Do not crash when the table contains
an instantly dropped column.

Thanks to Thirunarayanan Balathandayuthapani for discussing the design
and implementing an initial prototype of this.
Thanks to Matthias Leich for testing.