統計一個表的數據量是經常遇到的需求，但是不同的表設計及不同的寫法，統計性能差別會有較大的差異，下麵就簡單通過實驗進行測試(大家測試的時候註意緩存的情況，否則影響測試結果）。 1、準備工作為了後續測試工作的進行，先準備幾張用於測試的表及數據，為了使測試數據具有參考意義，建議測試表的數據量大一點，以 ...

統計一個表的數據量是經常遇到的需求，但是不同的表設計及不同的寫法，統計性能差別會有較大的差異，下麵就簡單通過實驗進行測試(大家測試的時候註意緩存的情況，否則影響測試結果）。

1、準備工作

為了後續測試工作的進行，先準備幾張用於測試的表及數據，為了使測試數據具有參考意義，建議測試表的數據量大一點，以免查詢時間太小，因此，可以繼續使用之前常用的連續數生成大法，如下：

/* 創建連續數表 */
CREATE TABLE nums(id INT primary key);

/* 生成連續數的存儲過程,優化過後的 */
DELIMITER $$
CREATE  PROCEDURE `sp_createNum`(cnt INT )
BEGIN
    DECLARE i INT  DEFAULT 1;
    TRUNCATE TABLE nums;
    INSERT INTO nums SELECT i;
    WHILE i < cnt DO
      BEGIN
        INSERT INTO nums SELECT id + i FROM nums WHERE id + i<=cnt;
        SET i = i*2;
      END;
    END WHILE;
END$$

DELIMITER ;

生成數據，本次準備生成1kw條記錄

/* 調用存儲過程 */
mysql> call sp_createNum(10000000);
Query OK, 1611392 rows affected (32.07 sec)

如果逐條迴圈，那時間相當長，大家可以自行測試，參考鏈接效率提升16800倍的連續整數生成方法

1.1 創建innodb表

生成3張表innodb表，如下：

nums_1表只有字元串主鍵欄位

/*  生成只有一個字元串類型欄位主鍵的表nums_1 */
mysql> create table  nums_1 (p1 varchar(32) primary key ) engine=innodb;
Query OK, 0 rows affected (0.01 sec)

/*  導入數據,將id通過md5函數轉換為字元串 */
mysql> insert into  nums_1 select md5(id) from nums;
Query OK, 10000000 rows affected (1 min 12.63 sec)
Records: 10000000  Duplicates: 0  Warnings: 0

nums_2表有5個欄位，其中主鍵為字元串類型欄位的p1，其他欄位為整型的id，非空的c1,可為空的c2,可為空的c3。

其中c1,c2欄位內容完全一致，差別是欄位約束不一樣（c1不可為空，c2可為空），c3與c1,c2的差別在於c1中aa開頭的值在c3中為null,其他內容一樣。

/* 創建表nums_2 */
mysql> create table nums_2(p1 varchar(32) primary key ,id int ,c1 varchar(10) not null, c2 varchar(10),c3 varchar(10)) engine=innodb;
Query OK, 0 rows affected (1.03 sec)

/*導入數據 */
mysql> insert into  nums_2(id,p1,c1,c2,c3) select id,md5(id),left(md5(id),10),left(md5(id),10),if(,left(md5(id),10) like 'aa%',null,,left(md5(id),10)) from nums;
Query OK, 10000000 rows affected (5 min 6.68 sec)
Records: 10000000  Duplicates: 0  Warnings: 0

nums_3表的內容與nums_2完全一樣，區別在於主鍵欄位不一樣，c3表為整型的id

/*  創建表nums_3 */
mysql> create table nums_3(p1 varchar(32) ,id int primary key  ,c1 varchar(10) not null, c2 varchar(10),c3 varchar(10)) engine=innodb;
Query OK, 0 rows affected (0.01 sec)

/* 因為內容完全一致，直接從nums_2 中導入 */
mysql> insert into nums_3 select  * from nums_2;
Query OK, 10000000 rows affected (3 min 18.81 sec)
Records: 10000000  Duplicates: 0  Warnings: 0

1.2 創建MyISAM引擎表

再創建一張MyISAM的表，表結構及內容均與nums_2也一致，只是引擎為MyISAM。

/* 創建MyISAM引擎的nums_4表*/
mysql> create table nums_4(p1 varchar(32) not null  primary key ,id int  ,c1 varchar(10) not null, c2 varchar(10),c3 varchar(10)) engine=MyISAM;
Query OK, 0 rows affected (0.00 sec)

/* 直接從nums_2表導入數據 */
mysql> insert into nums_4 select  * from nums_2;
Query OK, 10000000 rows affected (3 min 16.78 sec)
Records: 10000000  Duplicates: 0  Warnings: 0

2、查詢一張表數據量的方法

查詢一張表的數據量有如下幾種：

查詢大致數據量，可以查統計信息，2.1中會介紹具體方法

精確查找數據量，則可以通過count(主鍵欄位），count(*), count(1) [這裡的1可以替換為任意常量]

2.1 非精確查詢

如果只是查一張表大致有多少數據，尤其是很大的表只是查詢其表屬於什麼量級的（百萬、千萬還是上億條），可以直接查詢統計信息，查詢方式有如下幾種：

查詢索引信息，其中Cardinality 為大致數據量（查看主鍵PRIMARY行的值，如果為多列的複合主鍵，則查看最後一列的Cardinality 值）

mysql> show index from nums_2;
+--------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table  | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+--------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| nums_2 |          0 | PRIMARY  |            1 | p1          | A         |     9936693 |     NULL | NULL   |      | BTREE      |         |               |
+--------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
1 row in set (0.00 sec)

查看表狀態，其中Rows為大致數據量

mysql> show table status like  'nums_2';
+--------+--------+---------+------------+---------+----------------+-------------+-----------------+--------------+-----------+----------------+---------------------+-------------+------------+-----------------+----------+----------------+---------+
| Name   | Engine | Version | Row_format | Rows    | Avg_row_length | Data_length | Max_data_length | Index_length | Data_free | Auto_increment | Create_time         | Update_time | Check_time | Collation       | Checksum | Create_options | Comment |
+--------+--------+---------+------------+---------+----------------+-------------+-----------------+--------------+-----------+----------------+---------------------+-------------+------------+-----------------+----------+----------------+---------+
| nums_2 | InnoDB |      10 | Dynamic    | 9936693 |            111 |  1105182720 |               0 |   2250178560 |   4194304 |           NULL | 2020-04-04 19:31:34 | NULL        | NULL       | utf8_general_ci |     NULL |                |         |
+--------+--------+---------+------------+---------+----------------+-------------+-----------------+--------------+-----------+----------------+---------------------+-------------+------------+-----------------+----------+----------------+---------+
1 row in set (0.00 sec)

直接查看STATISTICS或TABLES表，內容與查看索引信息或表狀態類似，其中TABLE_ROWS的內容為大致的數據量

mysql> select   * from  information_schema.tables where table_schema='testdb' and table_name like  'nums_2';
+---------------+--------------+------------+------------+--------+---------+------------+------------+----------------+-------------+-----------------+--------------+-----------+----------------+---------------------+-------------+------------+-----------------+----------+----------------+---------------+
| TABLE_CATALOG | TABLE_SCHEMA | TABLE_NAME | TABLE_TYPE | ENGINE | VERSION | ROW_FORMAT | TABLE_ROWS | AVG_ROW_LENGTH | DATA_LENGTH | MAX_DATA_LENGTH | INDEX_LENGTH | DATA_FREE | AUTO_INCREMENT | CREATE_TIME         | UPDATE_TIME | CHECK_TIME | TABLE_COLLATION | CHECKSUM | CREATE_OPTIONS | TABLE_COMMENT |
+---------------+--------------+------------+------------+--------+---------+------------+------------+----------------+-------------+-----------------+--------------+-----------+----------------+---------------------+-------------+------------+-----------------+----------+----------------+---------------+
| def           | testdb       | nums_2     | BASE TABLE | InnoDB |      10 | Dynamic    |    9936693 |            111 |  1105182720 |               0 |   2250178560 |   4194304 |           NULL | 2020-04-04 19:31:34 | NULL        | NULL       | utf8_general_ci |     NULL |                |               |
+---------------+--------------+------------+------------+--------+---------+------------+------------+----------------+-------------+-----------------+--------------+-----------+----------------+---------------------+-------------+------------+-----------------+----------+----------------+---------------+
1 row in set (0.00 sec)

註意：

innodb引起的表通過以上3種方式均可查詢對應表的大致數據量，且結果相同，因為均是取自相同的統計信息
MyISAM表的結果是精確值（表數據量，不包含其他欄位）

mysql> select   * from  information_schema.tables where table_schema='testdb' and table_name like  'nums_4';
+---------------+--------------+------------+------------+--------+---------+------------+------------+----------------+-------------+-----------------+--------------+-----------+----------------+---------------------+---------------------+---------------------+-----------------+----------+----------------+---------------+
| TABLE_CATALOG | TABLE_SCHEMA | TABLE_NAME | TABLE_TYPE | ENGINE | VERSION | ROW_FORMAT | TABLE_ROWS | AVG_ROW_LENGTH | DATA_LENGTH | MAX_DATA_LENGTH | INDEX_LENGTH | DATA_FREE | AUTO_INCREMENT | CREATE_TIME         | UPDATE_TIME         | CHECK_TIME          | TABLE_COLLATION | CHECKSUM | CREATE_OPTIONS | TABLE_COMMENT |
+---------------+--------------+------------+------------+--------+---------+------------+------------+----------------+-------------+-----------------+--------------+-----------+----------------+---------------------+---------------------+---------------------+-----------------+----------+----------------+---------------+
| def           | testdb       | nums_4     | BASE TABLE | MyISAM |      10 | Dynamic    |   10000000 |             75 |   759686336 | 281474976710655 |    854995968 |         0 |           NULL | 2020-04-04 19:20:23 | 2020-04-04 19:21:45 | 2020-04-04 19:23:45 | utf8_general_ci |     NULL |                |               |
+---------------+--------------+------------+------------+--------+---------+------------+------------+----------------+-------------+-----------------+--------------+-----------+----------------+---------------------+---------------------+---------------------+-----------------+----------+----------------+---------------+
1 row in set (0.00 sec)

2.2 精確查找

因為2.1中innodb的表查詢的結果都是統計值，非準備值，實際工作中大多數情況下需要統計精確值，那麼查詢精確值的方法有如下幾種，且所有引擎的表都適用。

count(主鍵）

mysql> select count(p1) from nums_2;
+-----------+
| count(p1) |
+-----------+
|  10000000 |
+-----------+
1 row in set (1.60 sec)

count(1)

其中的1可以是任意常量，例如 count(2),count('a‘）等

mysql> select count(1) from nums_2;
+----------+
| count(1) |
+----------+
| 10000000 |
+----------+
1 row in set (1.45 sec)

count(*)

mysql> select count(*) from nums_2;
+----------+
| count(*) |
+----------+
| 10000000 |
+----------+
1 row in set (1.52 sec)

3、 count的性能對比

對比 count(主鍵） count(1) count(*) count（非空欄位） count（可為空欄位）性能對比

3.1 MyISAM引擎表

3.1.1 查詢整張表數據量

如果想精確查詢一張MyISAM表的數據量，使用 count(主鍵） count(1) count(*) 效率均一致，直接查出準確結果，耗時幾乎為0s

mysql> select count(p1) from nums_4;
+-----------+
| count(p1) |
+-----------+
|  10000000 |
+-----------+
1 row in set (0.00 sec)

mysql> select count(1) from nums_4;
+----------+
| count(1) |
+----------+
| 10000000 |
+----------+
1 row in set (0.00 sec)

mysql> select count(*) from nums_4;
+----------+
| count(*) |
+----------+
| 10000000 |
+----------+
1 row in set (0.00 sec)

執行計劃也均一致，可以看出沒有通過主鍵或其他索引掃描的方式統計

mysql> explain select count(*) from nums_4;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------------------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra                        |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------------------------+
|  1 | SIMPLE      | NULL  | NULL       | NULL | NULL          | NULL | NULL    | NULL | NULL |     NULL | Select tables optimized away |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------------------------+
1 row in set, 1 warning (0.00 sec)

mysql> explain select count(p1) from nums_4;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------------------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra                        |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------------------------+
|  1 | SIMPLE      | NULL  | NULL       | NULL | NULL          | NULL | NULL    | NULL | NULL |     NULL | Select tables optimized away |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------------------------+
1 row in set, 1 warning (0.00 sec)

mysql> explain select count(1) from nums_4;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------------------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra                        |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------------------------+
|  1 | SIMPLE      | NULL  | NULL       | NULL | NULL          | NULL | NULL    | NULL | NULL |     NULL | Select tables optimized away |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------------------------+
1 row in set, 1 warning (0.00 sec)

小結：

MyISAM的方法查整表數據量效率情況為 count(主鍵）= count(1) = count(*)

3.1.2 查詢部分數據

查詢部分數據的時候則無法直接從統計信息獲取，因此耗時情況大致如下：

mysql> select count(p1) from nums_4 where  p1 like 'aa%';
+-----------+
| count(p1) |
+-----------+
|     39208 |
+-----------+
1 row in set (0.14 sec)

mysql> select count(1) from nums_4 where  p1 like 'aa%';
+----------+
| count(1) |
+----------+
|    39208 |
+----------+
1 row in set (0.13 sec)

mysql> select count(*) from nums_4 where p1 like 'aa%';
+----------+
| count(*) |
+----------+
| 39208 |
+----------+
1 row in set (0.13 sec)

執行計劃其實均一樣：

mysql> explain select count(1) from nums_4 where  p1 like 'aa%';
+----+-------------+--------+------------+-------+---------------+---------+---------+------+-------+----------+--------------------------+
| id | select_type | table  | partitions | type  | possible_keys | key     | key_len | ref  | rows  | filtered | Extra                    |
+----+-------------+--------+------------+-------+---------------+---------+---------+------+-------+----------+--------------------------+
|  1 | SIMPLE      | nums_4 | NULL       | range | PRIMARY       | PRIMARY | 98      | NULL | 42603 |   100.00 | Using where; Using index |
+----+-------------+--------+------------+-------+---------------+---------+---------+------+-------+----------+--------------------------+
1 row in set, 1 warning (0.00 sec)

小結： MyISAM引擎表統計部分數據的時候直接得出數據量，也許掃描數據進行統計，幾種寫法效率相近。

3.2 innodb引擎表

innodb引擎因為要支持MVCC，因此不能整表數據量持久化保存，每次查詢均需遍歷統計，但是不同的寫法，查詢效率是有差別的，後面將進行不同維度進行對比。

3.2.1 不同寫法的性能對比

通過 count(主鍵),count(1) , count(*) 對比查詢效率

mysql> select count(p1) from nums_2  ;
+-----------+
| count(p1) |
+-----------+
|  10000000 |
+-----------+
1 row in set (1.68 sec)

mysql> select count(1) from nums_2  ;
+----------+
| count(1) |
+----------+
| 10000000 |
+----------+
1 row in set (1.37 sec)

mysql> select count(*) from nums_2  ;
+----------+
| count(*) |
+----------+
| 10000000 |
+----------+
1 row in set (1.38 sec)

簡單的對比發現，查詢性能結果為 count(主鍵) < count(1) ≈ count(*)

但是查看執行計劃都是如下情況

mysql> explain select count(p1) from nums_2;
+----+-------------+--------+------------+-------+---------------+---------+---------+------+---------+----------+-------------+
| id | select_type | table  | partitions | type  | possible_keys | key     | key_len | ref  | rows    | filtered | Extra       |
+----+-------------+--------+------------+-------+---------------+---------+---------+------+---------+----------+-------------+
|  1 | SIMPLE      | nums_2 | NULL

MySQL count知多少

1、 準備工作