这是一个我的随笔记录，这些过程非常有用，也非常明显。
欢迎关注我的《深入理解MySQL主从原理 32讲 》，如下：
语句如下：
mysql> desc select id,count(*) from t110 group by id;
+----+-------------+-------+------------+------+---------------+------+---------+------+-------+----------+---------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+-------+----------+---------------------------------+
| 1 | SIMPLE | t110 | NULL | ALL | NULL | NULL | NULL | NULL | 99395 | 100.00 | Using temporary; Using filesort |
+----+-------------+-------+------------+------+---------------+------+---------+------+-------+----------+---------------------------------+
1 row in set, 1 warning (0.00 sec)
存入内部临时表的操作也在'sending data' 下面，下面的debug trace可以看出
操作是获取innodb层一条数据handler::ha_rnd_next，然后在内部临时表中查询
这行记录hp_search，如果存在则做相应的更改heap_update（如count累加，sum加值等）
使用内部临时表
1249 T@3: | | | | | | | | >handler::ha_rnd_next
1250 T@3: | | | | | | | | | >rnd_next
1251 T@3: | | | | | | | | | | >general_fetch
1252 T@3: | | | | | | | | | | | >row_search_mvcc
1253 T@3: | | | | | | | | | | | | >row_sel_store_mysql_rec
1254 T@3: | | | | | | | | | | | | | >row_sel_store_mysql_field_func
1255 T@3: | | | | | | | | | | | | | <row_sel_store_mysql_field_func 3267
1256 T@3: | | | | | | | | | | | | <row_sel_store_mysql_rec 3457
1257 T@3: | | | | | | | | | | | <row_search_mvcc 6453
1258 T@3: | | | | | | | | | | <general_fetch 9913
1259 T@3: | | | | | | | | | <rnd_next 10086
1260 T@3: | | | | | | | | <handler::ha_rnd_next 3159
1261 T@3: | | | | | | | | >evaluate_join_record
1262 T@3: | | | | | | | | | enter: join: 0x7ffe7c007778 join_tab index: 0 table: t112 cond: 0x0
1263 T@3: | | | | | | | | | counts: evaluate_join_record join->examined_rows++: 4
1264 T@3: | | | | | | | | | >sub_select_op
1265 T@3: | | | | | | | | | | >end_update
1266 T@3: | | | | | | | | | | | >handler::ha_index_read_map
1267 T@3: | | | | | | | | | | | | >heap_rkey
1268 T@3: | | | | | | | | | | | | | enter: info: 0x7ffe7caa74d0 inx: 0
1269 T@3: | | | | | | | | | | | | | >hp_search
1270 T@3: | | | | | | | | | | | | | | exit: hash: 0x105050504
1271 T@3: | | | | | | | | | | | | | | exit: found key at 0x7ffe7ceb4880
1272 T@3: | | | | | | | | | | | | | <hp_search 124
1273 T@3: | | | | | | | | | | | | | >hp_extract_record
1274 T@3: | | | | | | | | | | | | | <hp_extract_record 369
1275 T@3: | | | | | | | | | | | | <heap_rkey 81
1276 T@3: | | | | | | | | | | | <handler::ha_index_read_map 3261
1277 T@3: | | | | | | | | | | | >hanlder::ha_update_row
1278 T@3: | | | | | | | | | | | | >heap_update
1279 T@3: | | | | | | | | | | | | | >hp_copy_record_data_to_chunks
1280 T@3: | | | | | | | | | | | | | <hp_copy_record_data_to_chunks 308
1281 T@3: | | | | | | | | | | | | <heap_update 79
1282 T@3: | | | | | | | | | | | <hanlder::ha_update_row 8518
1283 T@3: | | | | | | | | | | <end_update 3482
1284 T@3: | | | | | | | | | <sub_select_op 1085
1285 T@3: | | | | | | | | <evaluate_join_record 1707下面是count(*)累加关于heap_update old值和new值的查看
Breakpoint 2, heap_update (info=0x7ffe7c9a9620, old_record=0x7ffe7c00f3b8 <incomplete sequence \375>, new_record=0x7ffe7c00f3a0 <incomplete sequence \375>)
at /root/mysqlall/percona-server-locks-detail-5.7.22/storage/heap/hp_update.c:24
24 my_bool auto_key_changed= 0;
(gdb) x/16bx 0x7ffe7c00f3b8
0x7ffe7c00f3b8: 0xfd 0x00 0x01 0x00 0x00 0x00 0x01 0x00
0x7ffe7c00f3c0: 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
(gdb) x/16bx 0x7ffe7c00f3a0
0x7ffe7c00f3a0: 0xfd 0x00 0x01 0x00 0x00 0x00 0x02 0x00
0x7ffe7c00f3a8: 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
(gdb) c
Continuing.
Breakpoint 2, heap_update (info=0x7ffe7c9a9620, old_record=0x7ffe7c00f3b8 <incomplete sequence \375>, new_record=0x7ffe7c00f3a0 <incomplete sequence \375>)
at /root/mysqlall/percona-server-locks-detail-5.7.22/storage/heap/hp_update.c:24
24 my_bool auto_key_changed= 0;
(gdb) x/16bx 0x7ffe7c00f3b8
0x7ffe7c00f3b8: 0xfd 0x00 0x01 0x00 0x00 0x00 0x02 0x00
0x7ffe7c00f3c0: 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
(gdb) x/16bx 0x7ffe7c00f3a0
0x7ffe7c00f3a0: 0xfd 0x00 0x01 0x00 0x00 0x00 0x03 0x00
0x7ffe7c00f3a8: 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
注意到key = 1 的这个值，第一次断点old值为0x01 new值为0x02，第二次断点old值为0x02 new值为0x03
然后遍历完所有的行过后，进入排序状态为Creating sort index
1526 T@3: | | | | | | | | | THD::enter_stage: 'Creating sort index' /root/mysqlall/percona-server-locks-detail-5.7.22/sql/sql_executor.cc:2604
1527 T@3: | | | | | | | | | >PROFILING::status_change
1528 T@3: | | | | | | | | | <PROFILING::status_change 384
1529 T@3: | | | | | | | | | >create_sort_index
1530 T@3: | | | | | | | | | | >my_raw_malloc
1531 T@3: | | | | | | | | | | | my: size: 376 my_flags: 48
1532 T@3: | | | | | | | | | | | exit: ptr: 0x7ffe7c9d1a90
1533 T@3: | | | | | | | | | | <my_raw_malloc 219
1534 T@3: | | | | | | | | | | >heap_info
1535 T@3: | | | | | | | | | | <heap_info 57
1536 T@3: | | | | | | | | | | >filesort
1537 T@3: | | | | | | | | | | | >make_sortorder
1538 T@3: | | | | | | | | | | | | >alloc_root
1539 T@3: | | | | | | | | | | | | | enter: root: 0x7ffe7c003c08
1540 T@3: | | | | | | | | | | | | | exit: ptr: 0x7ffe7caa4fc8
1541 T@3: | | | | | | | | | | | | <alloc_root 304
1542 T@3: | | | | | | | | | | | <make_sortorder 663
1543 T@3: | | | | | | | | | | | opt: (null): starting struct
1544 T@3: | | | | | | | | | | | opt: filesort_information: starting struct
1545 T@3: | | | | | | | | | | | opt: (null): starting struct
1546 T@3: | | | | | | | | | | | opt: direction: "asc"
1547 T@3: | | | | | | | | | | | opt: table: "intermediate_tmp_table"
1548 T@3: | | | | | | | | | | | opt: field: "id"
1549 T@3: | | | | | | | | | | | opt: (null): ending struct
1550 T@3: | | | | | | | | | | | opt: filesort_information: ending struct
1551 T@3: | | | | | | | | | | | info: sort_length: 5