FM / private

/usr/include/mysql/server/private

Name	Size	Mode	Actions
atomic/	-	0755	rm
data/	-	0755	rm
providers/	-	0755	rm
aligned.h	1136	0644	edit dl rm
aria_backup.h	1787	0644	edit dl rm
assume_aligned.h	2350	0644	edit dl rm
authors.h	10194	0644	edit dl rm
backup.h	1703	0644	edit dl rm
bounded_queue.h	6093	0644	edit dl rm
client_settings.h	1935	0644	edit dl rm
compat56.h	2280	0644	edit dl rm
config.h	14351	0644	edit dl rm
contributors.h	4931	0644	edit dl rm
create_options.h	4193	0644	edit dl rm
create_tmp_table.h	2808	0644	edit dl rm
cset_narrowing.h	3968	0644	edit dl rm
custom_conf.h	1082	0644	edit dl rm
datadict.h	1700	0644	edit dl rm
ddl_log.h	12807	0644	edit dl rm
debug.h	1234	0644	edit dl rm
debug_sync.h	2046	0644	edit dl rm
derived_handler.h	2379	0644	edit dl rm
derror.h	980	0644	edit dl rm
des_key_file.h	1236	0644	edit dl rm
discover.h	1570	0644	edit dl rm
dur_prop.h	1082	0644	edit dl rm
embedded_priv.h	1733	0644	edit dl rm
events.h	4704	0644	edit dl rm
event_data_objects.h	4187	0644	edit dl rm
event_db_repository.h	3648	0644	edit dl rm
event_parse_data.h	2899	0644	edit dl rm
event_queue.h	3438	0644	edit dl rm
event_scheduler.h	3290	0644	edit dl rm
field.h	222414	0644	edit dl rm
field_comp.h	1174	0644	edit dl rm
filesort.h	7283	0644	edit dl rm
filesort_utils.h	8195	0644	edit dl rm
ft_global.h	3113	0644	edit dl rm
gcalc_slicescan.h	17330	0644	edit dl rm
gcalc_tools.h	11900	0644	edit dl rm
grant.h	2758	0644	edit dl rm
group_by_handler.h	3534	0644	edit dl rm
gstream.h	2437	0644	edit dl rm
handler.h	199857	0644	edit dl rm
handle_connections_win.h	884	0644	edit dl rm
hash.h	4452	0644	edit dl rm
hash_filo.h	5599	0644	edit dl rm
ha_handler_stats.h	2335	0644	edit dl rm
ha_partition.h	64696	0644	edit dl rm
ha_sequence.h	6245	0644	edit dl rm
heap.h	9487	0644	edit dl rm
hostname.h	5419	0644	edit dl rm
ilist.h	7237	0644	edit dl rm
init.h	852	0644	edit dl rm
innodb_priv.h	1319	0644	edit dl rm
item.h	285458	0644	edit dl rm
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item_sum.h	72695	0644	edit dl rm
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item_vers.h	4413	0644	edit dl rm
item_windowfunc.h	35089	0644	edit dl rm
item_xmlfunc.h	4650	0644	edit dl rm
json_table.h	9667	0644	edit dl rm
key.h	2132	0644	edit dl rm
keycaches.h	1995	0644	edit dl rm
lex.h	29840	0644	edit dl rm
lex_charset.h	24313	0644	edit dl rm
lex_hash.h	143108	0644	edit dl rm
lex_ident.h	2122	0644	edit dl rm
lex_string.h	4077	0644	edit dl rm
lex_symbol.h	1323	0644	edit dl rm
lex_token.h	42641	0644	edit dl rm
lf.h	6462	0644	edit dl rm
lock.h	2220	0644	edit dl rm
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maria.h	5872	0644	edit dl rm
mariadb.h	1277	0644	edit dl rm
mdl.h	38893	0644	edit dl rm
mem_root_array.h	7106	0644	edit dl rm
message.h	1195	0644	edit dl rm
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myisam.h	17506	0644	edit dl rm
myisamchk.h	4734	0644	edit dl rm
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mysqld.h	41225	0644	edit dl rm
mysqld_default_groups.h	204	0644	edit dl rm
mysqld_suffix.h	1201	0644	edit dl rm
mysys_err.h	3022	0644	edit dl rm
my_alarm.h	2429	0644	edit dl rm
my_apc.h	4747	0644	edit dl rm
my_atomic.h	7281	0644	edit dl rm
my_atomic_wrapper.h	3051	0644	edit dl rm
my_base.h	27750	0644	edit dl rm
my_bit.h	6196	0644	edit dl rm
my_bitmap.h	5502	0644	edit dl rm
my_check_opt.h	2618	0644	edit dl rm
my_compare.h	11194	0644	edit dl rm
my_counter.h	1721	0644	edit dl rm
my_cpu.h	4885	0644	edit dl rm
my_crypt.h	904	0644	edit dl rm
my_decimal.h	14504	0644	edit dl rm
my_default.h	1880	0644	edit dl rm
my_handler_errors.h	4882	0644	edit dl rm
my_json_writer.h	18706	0644	edit dl rm
my_libwrap.h	1183	0644	edit dl rm
my_md5.h	1486	0644	edit dl rm
my_minidump.h	848	0644	edit dl rm
my_nosys.h	1438	0644	edit dl rm
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my_user.h	1126	0644	edit dl rm
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opt_range.h	65903	0644	edit dl rm
opt_subselect.h	14545	0644	edit dl rm
opt_trace.h	9505	0644	edit dl rm
opt_trace_context.h	3291	0644	edit dl rm
parse_file.h	4424	0644	edit dl rm
partition_element.h	5428	0644	edit dl rm
partition_info.h	19864	0644	edit dl rm
password.h	1621	0644	edit dl rm
pfs_file_provider.h	3153	0644	edit dl rm
pfs_idle_provider.h	1385	0644	edit dl rm
pfs_memory_provider.h	1626	0644	edit dl rm
pfs_metadata_provider.h	1898	0644	edit dl rm
pfs_socket_provider.h	2258	0644	edit dl rm
pfs_stage_provider.h	1556	0644	edit dl rm
pfs_statement_provider.h	4347	0644	edit dl rm
pfs_table_provider.h	2625	0644	edit dl rm
pfs_thread_provider.h	5560	0644	edit dl rm
pfs_transaction_provider.h	2846	0644	edit dl rm
privilege.h	28853	0644	edit dl rm
probes_mysql.h	973	0644	edit dl rm
probes_mysql_nodtrace.h	6087	0644	edit dl rm
procedure.h	6819	0644	edit dl rm
protocol.h	12537	0644	edit dl rm
proxy_protocol.h	548	0644	edit dl rm
queues.h	3477	0644	edit dl rm
records.h	3147	0644	edit dl rm
replication.h	16130	0644	edit dl rm
repl_failsafe.h	1585	0644	edit dl rm
rijndael.h	1711	0644	edit dl rm
rowid_filter.h	15474	0644	edit dl rm
rpl_constants.h	3357	0644	edit dl rm
rpl_filter.h	4774	0644	edit dl rm
rpl_gtid.h	29985	0644	edit dl rm
rpl_injector.h	9621	0644	edit dl rm
rpl_mi.h	16636	0644	edit dl rm
rpl_parallel.h	18228	0644	edit dl rm
rpl_record.h	1585	0644	edit dl rm
rpl_record_old.h	1407	0644	edit dl rm
rpl_reporting.h	3713	0644	edit dl rm
rpl_rli.h	36070	0644	edit dl rm
rpl_tblmap.h	3177	0644	edit dl rm
rpl_utility.h	9800	0644	edit dl rm
scheduler.h	3199	0644	edit dl rm
scope.h	4393	0644	edit dl rm
select_handler.h	2228	0644	edit dl rm
semisync.h	2287	0644	edit dl rm
semisync_master.h	25764	0644	edit dl rm
semisync_master_ack_receiver.h	8709	0644	edit dl rm
semisync_slave.h	3736	0644	edit dl rm
service_versions.h	2285	0644	edit dl rm
session_tracker.h	14275	0644	edit dl rm
set_var.h	17331	0644	edit dl rm
slave.h	12392	0644	edit dl rm
socketpair.h	842	0644	edit dl rm
source_revision.h	67	0644	edit dl rm
sp.h	22588	0644	edit dl rm
span.h	3931	0644	edit dl rm
spatial.h	22698	0644	edit dl rm
sp_cache.h	2037	0644	edit dl rm
sp_head.h	64917	0644	edit dl rm
sp_pcontext.h	25427	0644	edit dl rm
sp_rcontext.h	14441	0644	edit dl rm
sql_acl.h	14167	0644	edit dl rm
sql_admin.h	2915	0644	edit dl rm
sql_alloc.h	1732	0644	edit dl rm
sql_alter.h	15395	0644	edit dl rm
sql_analyse.h	11052	0644	edit dl rm
sql_analyze_stmt.h	12683	0644	edit dl rm
sql_array.h	6858	0644	edit dl rm
sql_audit.h	14162	0644	edit dl rm
sql_base.h	26050	0644	edit dl rm
sql_basic_types.h	9523	0644	edit dl rm
sql_binlog.h	895	0644	edit dl rm
sql_bitmap.h	7842	0644	edit dl rm
sql_bootstrap.h	1812	0644	edit dl rm
sql_cache.h	21676	0644	edit dl rm
sql_callback.h	1542	0644	edit dl rm
sql_class.h	271836	0644	edit dl rm
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sql_command.h	4800	0644	edit dl rm
sql_connect.h	4087	0644	edit dl rm
sql_const.h	11630	0644	edit dl rm
sql_crypt.h	1437	0644	edit dl rm
sql_cte.h	16494	0644	edit dl rm
sql_cursor.h	4428	0644	edit dl rm
sql_db.h	2438	0644	edit dl rm
sql_debug.h	5727	0644	edit dl rm
sql_delete.h	1343	0644	edit dl rm
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sql_digest.h	3819	0644	edit dl rm
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sql_do.h	954	0644	edit dl rm
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sql_expression_cache.h	4359	0644	edit dl rm
sql_get_diagnostics.h	7883	0644	edit dl rm
sql_handler.h	2910	0644	edit dl rm
sql_help.h	995	0644	edit dl rm
sql_hset.h	3401	0644	edit dl rm
sql_insert.h	5173	0644	edit dl rm
sql_i_s.h	8487	0644	edit dl rm
sql_join_cache.h	48659	0644	edit dl rm
sql_lex.h	174931	0644	edit dl rm
sql_lifo_buffer.h	9685	0644	edit dl rm
sql_limit.h	3187	0644	edit dl rm
sql_list.h	22391	0644	edit dl rm
sql_load.h	1276	0644	edit dl rm
sql_locale.h	2701	0644	edit dl rm
sql_manager.h	960	0644	edit dl rm
sql_mode.h	6735	0644	edit dl rm
sql_parse.h	8970	0644	edit dl rm
sql_partition.h	12674	0644	edit dl rm
sql_partition_admin.h	5940	0644	edit dl rm
sql_plist.h	7711	0644	edit dl rm
sql_plugin.h	7577	0644	edit dl rm
sql_plugin_compat.h	2237	0644	edit dl rm
sql_prepare.h	11409	0644	edit dl rm
sql_priv.h	18528	0644	edit dl rm
sql_profile.h	7816	0644	edit dl rm
sql_reload.h	1036	0644	edit dl rm
sql_rename.h	982	0644	edit dl rm
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sql_schema.h	3303	0644	edit dl rm
sql_select.h	89743	0644	edit dl rm
sql_sequence.h	5177	0644	edit dl rm
sql_servers.h	1777	0644	edit dl rm
sql_show.h	9819	0644	edit dl rm
sql_signal.h	3362	0644	edit dl rm
sql_sort.h	21964	0644	edit dl rm
sql_statistics.h	16504	0644	edit dl rm
sql_string.h	39009	0644	edit dl rm
sql_table.h	9812	0644	edit dl rm
sql_test.h	2665	0644	edit dl rm
sql_time.h	8505	0644	edit dl rm
sql_trigger.h	12332	0644	edit dl rm
sql_truncate.h	2079	0644	edit dl rm
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sql_type_geom.h	19039	0644	edit dl rm
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table.h	119009	0644	edit dl rm
table_cache.h	4232	0644	edit dl rm
threadpool.h	4810	0644	edit dl rm
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threadpool_winsockets.h	2290	0644	edit dl rm
thread_cache.h	5905	0644	edit dl rm
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transaction.h	1466	0644	edit dl rm
tzfile.h	5014	0644	edit dl rm
tztime.h	3397	0644	edit dl rm
t_ctype.h	5639	0644	edit dl rm
uniques.h	4217	0644	edit dl rm
unireg.h	7722	0644	edit dl rm
vers_string.h	2543	0644	edit dl rm
violite.h	9956	0644	edit dl rm
waiting_threads.h	4532	0644	edit dl rm
welcome_copyright_notice.h	1218	0644	edit dl rm
winservice.h	6019	0644	edit dl rm
win_tzname_data.h	6506	0644	edit dl rm
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wsrep_client_service.h	2602	0644	edit dl rm
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wsrep_status.h	1816	0644	edit dl rm
wsrep_storage_service.h	1809	0644	edit dl rm
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wsrep_types.h	1110	0644	edit dl rm
wsrep_utils.h	10251	0644	edit dl rm
wsrep_var.h	4612	0644	edit dl rm
wsrep_xid.h	1509	0644	edit dl rm
xa.h	1845	0644	edit dl rm

Edit: /usr/include/mysql/server/private/sql_sort.h (21964B)

#ifndef SQL_SORT_INCLUDED
#define SQL_SORT_INCLUDED

This program is free software; you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
   the Free Software Foundation; version 2 of the License.

This program is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   GNU General Public License for more details.

You should have received a copy of the GNU General Public License
   along with this program; if not, write to the Free Software
   Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1335  USA */

#include "my_base.h"                            /* ha_rows */
#include <my_cmp.h>
#include "queues.h"
#include "sql_string.h"
#include "sql_class.h"

class Field;
struct TABLE;

/* Defines used by filesort and uniques */

#define MERGEBUFF		7
#define MERGEBUFF2		15

/*
   The structure SORT_ADDON_FIELD describes a fixed layout
   for field values appended to sorted values in records to be sorted
   in the sort buffer.
   Only fixed layout is supported now.
   Null bit maps for the appended values is placed before the values 
   themselves. Offsets are from the last sorted field, that is from the
   record referefence, which is still last component of sorted records.
   It is preserved for backward compatiblility.
   The structure is used tp store values of the additional fields 
   in the sort buffer. It is used also when these values are read
   from a temporary file/buffer. As the reading procedures are beyond the
   scope of the 'filesort' code the values have to be retrieved via
   the callback function 'unpack_addon_fields'.
*/

typedef struct st_sort_addon_field
{
  /* Sort addon packed field */
  Field *field;          /* Original field */
  uint   offset;         /* Offset from the last sorted field */
  uint   null_offset;    /* Offset to null bit from the last sorted field */
  uint   length;         /* Length in the sort buffer */
  uint8  null_bit;       /* Null bit mask for the field */
} SORT_ADDON_FIELD;

struct BUFFPEK_COMPARE_CONTEXT
{
  qsort_cmp2 key_compare;
  void *key_compare_arg;
};

/**
  Descriptor for a merge chunk to be sort-merged.
  A merge chunk is a sequence of pre-sorted records, written to a
  temporary file. A Merge_chunk instance describes where this chunk is stored
  in the file, and where it is located when it is in memory.

It is a POD because
   - we read/write them from/to files.

We have accessors (getters/setters) for all struct members.
 */

struct Merge_chunk {
public:
  my_off_t file_position() const { return m_file_position; }
  void set_file_position(my_off_t val) { m_file_position= val; }
  void advance_file_position(my_off_t val) { m_file_position+= val; }

uchar *buffer_start() { return m_buffer_start; }
  const uchar *buffer_end() const { return m_buffer_end; }

void set_buffer(uchar *start, uchar *end)
  {
    m_buffer_start= start;
    m_buffer_end= end;
  }
  void set_buffer_start(uchar *start)
  {
    m_buffer_start= start;
  }
  void set_buffer_end(uchar *end)
  {
    DBUG_ASSERT(m_buffer_end == NULL || end <= m_buffer_end);
    m_buffer_end= end;
  }

void init_current_key() { m_current_key= m_buffer_start; }
  uchar *current_key() { return m_current_key; }
  void advance_current_key(uint val) { m_current_key+= val; }

void decrement_rowcount(ha_rows val) { m_rowcount-= val; }
  void set_rowcount(ha_rows val)       { m_rowcount= val; }
  ha_rows rowcount() const             { return m_rowcount; }

ha_rows mem_count() const { return m_mem_count; }
  void set_mem_count(ha_rows val) { m_mem_count= val; }
  ha_rows decrement_mem_count() { return --m_mem_count; }

ha_rows max_keys() const { return m_max_keys; }
  void set_max_keys(ha_rows val) { m_max_keys= val; }

size_t  buffer_size() const { return m_buffer_end - m_buffer_start; }

/**
    Tries to merge *this with *mc, returns true if successful.
    The assumption is that *this is no longer in use,
    and the space it has been allocated can be handed over to a
    buffer which is adjacent to it.
   */
  bool merge_freed_buff(Merge_chunk *mc) const
  {
    if (mc->m_buffer_end == m_buffer_start)
    {
      mc->m_buffer_end= m_buffer_end;
      mc->m_max_keys+= m_max_keys;
      return true;
    }
    else if (mc->m_buffer_start == m_buffer_end)
    {
      mc->m_buffer_start= m_buffer_start;
      mc->m_max_keys+= m_max_keys;
      return true;
    }
    return false;
  }

/// The current key for this chunk
  uchar *m_current_key= nullptr;
  /// Current position in the file to be sorted.
  my_off_t m_file_position= 0;
  /// Start of main-memory buffer for this chunk.
  uchar *m_buffer_start= nullptr;
  /// End of main-memory buffer for this chunk.
  uchar *m_buffer_end= nullptr;
  /// Number of unread rows in this chunk.
  ha_rows m_rowcount= 0;
  /// Number of rows in the main-memory buffer.
  ha_rows m_mem_count= 0;
  /// If we have fixed-size rows: max number of rows in buffer.
  ha_rows m_max_keys= 0;
};

typedef Bounds_checked_array<SORT_ADDON_FIELD> Addon_fields_array;
typedef Bounds_checked_array<SORT_FIELD> Sort_keys_array;

/**
  This class wraps information about usage of addon fields.
  An Addon_fields object is used both during packing of data in the filesort
  buffer, and later during unpacking in 'Filesort_info::unpack_addon_fields'.

@see documentation for the Sort_addon_field struct.
  @see documentation for get_addon_fields()
 */
class Addon_fields {
public:
  Addon_fields(Addon_fields_array arr)
    : m_field_descriptors(arr),
      m_addon_buf(),
      m_addon_buf_length(),
      m_using_packed_addons(false)
  {
    DBUG_ASSERT(!arr.is_null());
  }

SORT_ADDON_FIELD *begin() { return m_field_descriptors.begin(); }
  SORT_ADDON_FIELD *end()   { return m_field_descriptors.end(); }

/// rr_unpack_from_tempfile needs an extra buffer when unpacking.
  uchar *allocate_addon_buf(uint sz)
  {
    m_addon_buf= (uchar *)my_malloc(PSI_INSTRUMENT_ME, sz, MYF(MY_WME | MY_THREAD_SPECIFIC));
    if (m_addon_buf)
      m_addon_buf_length= sz;
    return m_addon_buf;
  }

void free_addon_buff()
  {
    my_free(m_addon_buf);
    m_addon_buf= NULL;
    m_addon_buf_length= 0;
  }

uchar *get_addon_buf() { return m_addon_buf; }
  uint   get_addon_buf_length() const { return m_addon_buf_length; }

void set_using_packed_addons(bool val)
  {
    m_using_packed_addons= val;
  }

bool using_packed_addons() const
  {
    return m_using_packed_addons;
  }

static bool can_pack_addon_fields(uint record_length)
  {
    return (record_length <= (0xFFFF));
  }

/**
    @returns Total number of bytes used for packed addon fields.
    the size of the length field + size of null bits + sum of field sizes.
   */
  static uint read_addon_length(uchar *p)
  {
    return size_of_length_field + uint2korr(p);
  }

/**
    Stores the number of bytes used for packed addon fields.
   */
  static void store_addon_length(uchar *p, uint sz)
  {
    // We actually store the length of everything *after* the length field.
    int2store(p, sz - size_of_length_field);
  }

static const uint size_of_length_field= 2;

private:
  Addon_fields_array m_field_descriptors;

uchar    *m_addon_buf;            ///< Buffer for unpacking addon fields.
  uint      m_addon_buf_length;     ///< Length of the buffer.
  bool      m_using_packed_addons;  ///< Are we packing the addon fields?
};

/**
  This class wraps information about usage of sort keys.
  A Sort_keys object is used both during packing of data in the filesort
  buffer, and later during unpacking in 'Filesort_info::unpack_addon_fields'.

@see SORT_FIELD struct.
*/

class Sort_keys :public Sql_alloc,
                 public Sort_keys_array
{
public:
  Sort_keys(SORT_FIELD* arr, size_t count):
    Sort_keys_array(arr, count),
    m_using_packed_sortkeys(false),
    size_of_packable_fields(0),
    sort_length_with_original_values(0),
    sort_length_with_memcmp_values(0),
    parameters_computed(false)
  {
    DBUG_ASSERT(!is_null());
  }

bool using_packed_sortkeys() const
  { return m_using_packed_sortkeys; }

void set_using_packed_sortkeys(bool val)
  {
    m_using_packed_sortkeys= val;
  }
  void set_size_of_packable_fields(uint len)
  {
    size_of_packable_fields= len;
  }

uint get_size_of_packable_fields()
  {
    return size_of_packable_fields;
  }

void set_sort_length_with_original_values(uint len)
  {
    sort_length_with_original_values= len;
  }

uint get_sort_length_with_original_values()
  {
    return sort_length_with_original_values;
  }

void set_sort_length_with_memcmp_values(uint len)
  {
    sort_length_with_memcmp_values= len;
  }

uint get_sort_length_with_memcmp_values()
  {
    return sort_length_with_memcmp_values;
  }

static void store_sortkey_length(uchar *p, uint sz)
  {
    int4store(p, sz - size_of_length_field);
  }

static uint read_sortkey_length(uchar *p)
  {
    return size_of_length_field + uint4korr(p);
  }

void increment_size_of_packable_fields(uint len)
  {
    size_of_packable_fields+= len;
  }

void increment_original_sort_length(uint len)
  {
    sort_length_with_original_values+= len;
  }

bool is_parameters_computed() { return parameters_computed; }
  void set_parameters_computed(bool val) { parameters_computed= val; }

static const uint size_of_length_field= 4;

private:
  bool m_using_packed_sortkeys;     // Are we packing sort keys
  uint size_of_packable_fields;     // Total length bytes for packable columns

/*
    The sort length for all the keyparts storing the original values
  */
  uint sort_length_with_original_values;

/*
    The sort length for all the keyparts storing the mem-comparable images
  */
  uint sort_length_with_memcmp_values;

/*
    TRUE       parameters(like sort_length_* , size_of_packable_field)
               are computed
    FALSE      otherwise.
  */
  bool parameters_computed;
};

/**
PACKED SORT KEYS

Description

In this optimization where we would like the pack the values of the sort key
inside the sort buffer for each record.

Contents:
1. Background
1.1 Implementation details
2. Solution : Packed Sort Keys
2.1 Packed key format
2.2 Which format to use
3. Special cases
3.1 Handling very long strings
3.2 Handling for long binary strings
3.3 Handling very long strings with Packed sort keys
4. Sort key columns in addon_fields

1. Background
Before this optimization of using packed sort keys, filesort() sorted the
data using mem-comparable keys.

That is, if we wanted to sort by

ORDER BY col1, col2, ... colN
then the filesort code would for each row generate one "Sort Key"
and then sort the rows by their Sort Keys.

The Sort Keys are mem-comparable (that is, are compared by memcmp()) and
they are of FIXED SIZE. The sort key has the same length regardless of
what value it represents. This causes INEFFICIENT MEMORY USAGE.

1.1 Implementation details

make_sortkey() is the function that produces a sort key
from a record.

The function treats Field and Item objects differently.

class Field has:

a) void make_sort_key(uchar *buff, uint length);
   make_sort_key is a non-virtual function which handles encoding of
   SQL null values.

b) virtual void sort_string(uchar *buff,uint length)=0;
    sort_string produces mem-comparable image of the field value
    for each datatype.

For Items, Type_handler has a virtual function:

virtual void make_sort_key(uchar *to, Item *item,
                             const SORT_FIELD_ATTR *sort_field,
                             Sort_param *param) const= 0;
  which various datatypes overload.

2. SOLUTION: PACKED SORT KEYS

Note that one can have mem-comparable keys are that are not fixed-size.
MyRocks uses such encoding for example.

However for this optimization it was decided to store the original
(non-mem-comparable) values instead and use a datatype-aware
key comparison function.

2.1 Packed key format
The keys are stored in a new variable-size data format called "packed".

The format is as follows:

<sort_key_length><packed_value_1><packed_value2> ....... <packed_valueN>

format for a n-part sort key

<sort_key_length> is the length of the whole key.
Each packed value is encoded as follows:

<null_byte=0>  // This is a an SQL NULL
  [<null_byte=1>] <packed_value>  // this a non-NULL value
null_byte is present if the field/item is NULLable.
SQL NULL is encoded as just one NULL-indicator byte. The value itself is omitted.

The format of the packed_value depends on the datatype.
For "non-packable" datatypes it is just their mem-comparable form, as before.

The "packable" datatypes are currently variable-length strings and the
packed format for them is (for binary blobs, see a note below):

<length> <string>
2.2 Which format to use

The advantage of Packed Key Format is potential space savings for
variable-length fields.

The disadvantages are:

a) It may actually take more space, because of sort_key_length and
   length fields.
b) The comparison function is more expensive.

Currently the logic is: use Packed Key Format if we would save 128 or more
bytes when constructing a sort key from values that have empty string
for each packable component.

3. SPECIAL CASES
3.1 HANDLING VERY LONG STRINGS
the size of sort key part was limited by @@max_sort_length variable.
It is defined as:

The number of bytes to use when sorting data values. The server uses only the
first max_sort_length bytes of each value and ignores the rest.

3.2 HANDLING VERY LONG BINARY STRINGS
Long binary strings receive special treatment. A sort key for the long
binary string is truncated at max_sort_length bytes like described above,
but then a "suffix" is appended which contains the total length of the
value before the truncation.

3.3 HANDLING VERY LONG STRINGS WITH PACKED SORT KEY
Truncating multi-byte string at N bytes is not safe because one can cut in the
middle of a character. One is tempted to solve this by discarding the partial
character but that's also not a good idea as in some collations multiple
characters may produce one weight (this is called "contraction").

This combination of circumstances:

The string value is very long, so truncation is necessary
The collation is "complex", so truncation is dangerous
is deemed to be relatively rare so it was decided to just use
the non-packed sort keys in this case.

4. SORT KEY COLUMNS IN ADDON FIELDS
Currently, each sort key column is actually stored twice
1. as part of the sort key
2. in the addon_fields
This made total sense when sort key stored the mem-comparable image
(from which one cannot restore the original value in general case).
But since we now store the original value, we could also remove it from the
addon_fields and further save space. This is still a limitation and needs
to be fixed later

@see Sort_keys

**/

/**
  The sort record format may use one of two formats for the non-sorted part of
  the record:

1. Use the rowid

|<sort_key>|   <rowid>  |
    /          / ref_length /

2. Use "addon fields"

|<sort_key>|<null bits>|<field a><field b>...|
    /          /         addon_length            /

The packed format for "addon fields"

<sort_key>  The key may use one of the two formats:
              A. fixed-size mem-comparable form. The record is always
                 sort_length bytes long.
              B. "PackedKeyFormat" - the records are variable-size.

<key>       Fields are fixed-size, specially encoded with
              Field::make_sort_key() so we can do byte-by-byte compare.

<length>    Contains the *actual* packed length (after packing) of
              everything after the sort keys.
              The size of the length field is 2 bytes,
              which should cover most use cases: addon data <= 65535 bytes.
              This is the same as max record size in MySQL.
  <null bits> One bit for each nullable field, indicating whether the field
              is null or not. May have size zero if no fields are nullable.
  <field xx>  Are stored with field->pack(), and retrieved with
              field->unpack(). Addon fields within a record are stored
              consecutively, with no "holes" or padding. They will have zero
              size for NULL values.

class Sort_param {
public:
  uint rec_length;            // Length of sorted records.
  uint sort_length;           // Length of sorted columns.
  uint ref_length;            // Length of record ref.
  uint addon_length;          // Length of addon_fields
  uint res_length;            // Length of records in final sorted file/buffer.
  uint max_keys_per_buffer;   // Max keys / buffer.
  uint min_dupl_count;
  ha_rows max_rows;           // Select limit, or HA_POS_ERROR if unlimited.
  ha_rows examined_rows;      // Number of examined rows.
  TABLE *sort_form;           // For quicker make_sortkey.
  /**
    ORDER BY list with some precalculated info for filesort.
    Array is created and owned by a Filesort instance.
   */
  Bounds_checked_array<SORT_FIELD> local_sortorder;
  Addon_fields *addon_fields;     // Descriptors for companion fields.
  Sort_keys *sort_keys;
  ha_rows *accepted_rows;         /* For ROWNUM */
  bool using_pq;
  bool set_all_read_bits;

uchar *unique_buff;
  bool not_killable;
  String tmp_buffer;
  // The fields below are used only by Unique class.
  qsort_cmp2 compare;
  BUFFPEK_COMPARE_CONTEXT cmp_context;

Sort_param()
  {
    memset(reinterpret_cast<void*>(this), 0, sizeof(*this));
    tmp_buffer.set_thread_specific();
    /*
      Fix memset() clearing the charset.
      TODO: The constructor should be eventually rewritten not to use memset().
    */
    tmp_buffer.set_charset(&my_charset_bin);
  }
  void init_for_filesort(uint sortlen, TABLE *table,
                         ha_rows maxrows, Filesort *filesort);

void  (*unpack)(TABLE *);
  /// Enables the packing of addons if possible.
  void try_to_pack_addons(ulong max_length_for_sort_data);

/// Are we packing the "addon fields"?
  bool using_packed_addons() const
  {
    DBUG_ASSERT(m_using_packed_addons ==
                (addon_fields != NULL &&
                 addon_fields->using_packed_addons()));
    return m_using_packed_addons;
  }

bool using_packed_sortkeys() const
  {
    DBUG_ASSERT(m_using_packed_sortkeys ==
                (sort_keys != NULL && sort_keys->using_packed_sortkeys()));
    return m_using_packed_sortkeys;
  }

/// Are we using "addon fields"?
  bool using_addon_fields() const
  {
    return addon_fields != NULL;
  }

uint32 get_result_length(uchar *plen)
  {
    if (!m_using_packed_addons)
      return res_length;
    return Addon_fields::read_addon_length(plen);
  }

uint32 get_addon_length(uchar *plen)
  {
    if (using_packed_addons())
      return Addon_fields::read_addon_length(plen);
    else
      return addon_length;
  }

uint32 get_sort_length(uchar *plen)
  {
    if (using_packed_sortkeys())
      return Sort_keys::read_sortkey_length(plen) +
              /*
                when addon fields are not present, then the sort_length also
                includes the res_length. For packed keys here we add
                the res_length
              */
             (using_addon_fields() ? 0: res_length);
    else
      return sort_length;
  }

uint get_record_length(uchar *plen)
  {
    if (m_packed_format)
    {
      uint sort_len= get_sort_length(plen);
      return sort_len + get_addon_length(plen + sort_len);
    }
    else
      return rec_length;
  }

/**
    Getter for record length and result length.
    @param record_start Pointer to record.
    @param [out] recl   Store record length here.
    @param [out] resl   Store result length here.
   */
  void get_rec_and_res_len(uchar *record_start, uint *recl, uint *resl)
  {
    if (m_packed_format)
    {
      uint sort_len= get_sort_length(record_start);
      uint addon_len= get_addon_length(record_start + sort_len);
      *recl= sort_len + addon_len;
      *resl= using_addon_fields() ? addon_len : res_length;
    }
    else
    {
      *recl= rec_length;
      *resl= res_length;
    }
  }

void try_to_pack_sortkeys();

qsort_cmp2 get_compare_function() const
  {
    return using_packed_sortkeys() ?
           get_packed_keys_compare_ptr() :
           get_ptr_compare(sort_length);
  }
  void* get_compare_argument(size_t *sort_len) const
  {
    return using_packed_sortkeys() ?
           (void*) this :
           (void*) sort_len;
  }

bool is_packed_format() const
  {
    return m_packed_format;
  }

private:
  uint m_packable_length;
  bool m_using_packed_addons; ///< caches the value of using_packed_addons()
  /* caches the value of using_packed_sortkeys() */
  bool m_using_packed_sortkeys;
  bool m_packed_format;
};

typedef Bounds_checked_array<uchar> Sort_buffer;

int merge_many_buff(Sort_param *param, Sort_buffer sort_buffer,
                    Merge_chunk *buffpek, uint *maxbuffer, IO_CACHE *t_file);
ulong read_to_buffer(IO_CACHE *fromfile, Merge_chunk *buffpek,
                     Sort_param *param, bool packing_format);
bool merge_buffers(Sort_param *param,IO_CACHE *from_file,
                   IO_CACHE *to_file, Sort_buffer sort_buffer,
                   Merge_chunk *lastbuff, Merge_chunk *Fb,
                   Merge_chunk *Tb, int flag);
int merge_index(Sort_param *param, Sort_buffer sort_buffer,
                Merge_chunk *buffpek, uint maxbuffer,
                IO_CACHE *tempfile, IO_CACHE *outfile);
void reuse_freed_buff(QUEUE *queue, Merge_chunk *reuse, uint key_length);

#endif /* SQL_SORT_INCLUDED */

cmd: