MySQL新舊版本ORDER BY 處理方法

MySQL 的order by 涉及到三個參數:
A. sort_buffer_size 排序緩衝。
B. read_rnd_buffer_size 第二次排序緩衝。
C. max_length_for_sort_data 帶普通列的最大排序約束。


我來簡單說下MySQL的定序。
假設查詢語句select * from tb1 where 1 order by  a ; 欄位a沒有建立索引；以上三個參數都足夠大。
MySQL內部有兩種定序：
第一種，是普通的排序。這種排序的特點是節省記憶體，但是最終會對磁碟有一次隨機掃描。 大概主要過程如下：
1. 由於沒有WHERE條件，所以直接對磁碟進行全表掃描，把欄位a以及每行的物理ID（假設為TID）拿出來。然後把所有拿到的記錄全部放到sort_buffer_size中進行排序。
2. 根據排好序的TID，從磁碟隨機掃描所需要的所有記錄，排好序後再次把所有必須的記錄放到read_rnd_buffer_size中。
第二種，是冗餘排序。這種排序的特點是不需要二次對磁碟進行隨機掃描，但是缺點很明顯，太浪費記憶體空間。
跟第一種不同的是，在第一步裡拿到的不僅僅是欄位a以及TID，而是把所有請求的記錄全部拿到後，放到sort_buffer_size中進行排序。這樣可以直接從緩衝中返回記錄給用戶端，不用再次從磁碟上擷取一次。
從MySQL 5.7 後，對第二種排序進行了打包壓縮處理，避免太浪費記憶體。比如對於varchar(255)來說，實際儲存為varchar(3)。那麼相比之前的方式節約了好多記憶體，避免快取區域不夠時，建立磁碟暫存資料表。


以下為簡單的示範
mysql> use t_girl;
Database changed


三個參數的具體值：

mysql> select truncate(@@sort_buffer_size/1024/1024,2)||'MB' as 'sort_buffer_size',truncate(@@read_rnd_buffer_size/1024/1024,2)||'MB' as read_rnd_buffer_zie,@@max_length_for_sort_data as max_length_for_sort_data;+------------------+---------------------+--------------------------+| sort_buffer_size | read_rnd_buffer_zie | max_length_for_sort_data |+------------------+---------------------+--------------------------+| 2.00MB           | 2.00MB              |                     1024 |+------------------+---------------------+--------------------------+1 row in set (0.00 sec)

示範表的相關資料：

mysql> select table_name,table_rows,concat(truncate(data_length/1024/1024,2),'MB') as 'table_size' from information_schema.tables where table_name = 't1' and table_schema = 't_girl';+------------+------------+------------+| table_name | table_rows | table_size |+------------+------------+------------+| t1         |    2092640 | 74.60MB    |+------------+------------+------------+1 row in set (0.00 sec)

開啟最佳化器跟蹤：

mysql> SET OPTIMIZER_TRACE="enabled=on",END_MARKERS_IN_JSON=on;Query OK, 0 rows affected (0.00 sec)

從資料字典裡面拿到跟蹤結果：

mysql> select * from information_schema.optimizer_trace\G*************************** 1. row ***************************                            QUERY: select * from t1 where id < 10 order by id                            TRACE: {  "steps": [    {      "join_preparation": {        "select#": 1,        "steps": [          {            "expanded_query": "/* select#1 */ select `t1`.`id` AS `id`,`t1`.`log_time` AS `log_time` from `t1` where (`t1`.`id` < 10) order by `t1`.`id`"          }        ] /* steps */      } /* join_preparation */    },    {      "join_optimization": {        "select#": 1,        "steps": [          {            "condition_processing": {              "condition": "WHERE",              "original_condition": "(`t1`.`id` < 10)",              "steps": [                {                  "transformation": "equality_propagation",                  "resulting_condition": "(`t1`.`id` < 10)"                },                {                  "transformation": "constant_propagation",                  "resulting_condition": "(`t1`.`id` < 10)"                },                {                  "transformation": "trivial_condition_removal",                  "resulting_condition": "(`t1`.`id` < 10)"                }              ] /* steps */            } /* condition_processing */          },          {            "table_dependencies": [              {                "table": "`t1`",                "row_may_be_null": false,                "map_bit": 0,                "depends_on_map_bits": [                ] /* depends_on_map_bits */              }            ] /* table_dependencies */          },          {            "ref_optimizer_key_uses": [            ] /* ref_optimizer_key_uses */          },          {            "rows_estimation": [              {                "table": "`t1`",                "table_scan": {                  "rows": 2092640,                  "cost": 4775                } /* table_scan */              }            ] /* rows_estimation */          },          {            "considered_execution_plans": [              {                "plan_prefix": [                ] /* plan_prefix */,                "table": "`t1`",                "best_access_path": {                  "considered_access_paths": [                    {                      "access_type": "scan",                      "rows": 2.09e6,                      "cost": 423303,                      "chosen": true,                      "use_tmp_table": true                    }                  ] /* considered_access_paths */                } /* best_access_path */,                "cost_for_plan": 423303,                "rows_for_plan": 2.09e6,                "sort_cost": 2.09e6,                "new_cost_for_plan": 2.52e6,                "chosen": true              }            ] /* considered_execution_plans */          },          {            "attaching_conditions_to_tables": {              "original_condition": "(`t1`.`id` < 10)",              "attached_conditions_computation": [              ] /* attached_conditions_computation */,              "attached_conditions_summary": [                {                  "table": "`t1`",                  "attached": "(`t1`.`id` < 10)"                }              ] /* attached_conditions_summary */            } /* attaching_conditions_to_tables */          },          {            "clause_processing": {              "clause": "ORDER BY",              "original_clause": "`t1`.`id`",              "items": [                {                  "item": "`t1`.`id`"                }              ] /* items */,              "resulting_clause_is_simple": true,              "resulting_clause": "`t1`.`id`"            } /* clause_processing */          },          {            "refine_plan": [              {                "table": "`t1`",                "access_type": "table_scan"              }            ] /* refine_plan */          }        ] /* steps */      } /* join_optimization */    },    {      "join_execution": {        "select#": 1,        "steps": [          {            "filesort_information": [              {                "direction": "asc",                "table": "`t1`",                "field": "id"              }            ] /* filesort_information */,            "filesort_priority_queue_optimization": {              "usable": false,              "cause": "not applicable (no LIMIT)"            } /* filesort_priority_queue_optimization */,            "filesort_execution": [            ] /* filesort_execution */,            "filesort_summary": {              "rows": 62390,              "examined_rows": 2097152,              "number_of_tmp_files": 0,              "sort_buffer_size": 2097152,              "sort_mode": "<sort_key, additional_fields>"            } /* filesort_summary */          }        ] /* steps */      } /* join_execution */    }  ] /* steps */}MISSING_BYTES_BEYOND_MAX_MEM_SIZE: 0          INSUFFICIENT_PRIVILEGES: 01 row in set (0.00 sec)mysql> 

其中以上紅色部分<sort_key, additional_fields> 表示用了第二種定序。

其他的兩種<sort_key, rowid> 以及<sort_key, packed_additional_fields>分別代表第一種和後續版本MySQL的提升， 自己體驗去吧。