Mysql Mysql 的不准确
Mysql的统计信息不准确
即使做了表分析，information_schema.tables 中的 DATA_LENGTH,TABLE_ROWS,AVG_ROW_LENGTH,INDEX_LENGTH 也都不是准确值
mysql> select table_schema,table_name,DATA_LENGTH,TABLE_ROWS,AVG_ROW_LENGTH,INDEX_LENGTH from information_schema.tables where table_name='isc_message1';
+--------------+--------------+-------------+------------+----------------+--------------+
| TABLE_SCHEMA | TABLE_NAME | DATA_LENGTH | TABLE_ROWS | AVG_ROW_LENGTH | INDEX_LENGTH |
+--------------+--------------+-------------+------------+----------------+--------------+
| test | isc_message1 | 2086649856 | 3844654 | 542 | 62488576 |
+--------------+--------------+-------------+------------+----------------+--------------+
1 row in set (0.00 sec)
insert into isc_message1(channel_code,create_time) values('2020030701','20200307');
insert into isc_message1(channel_code,create_time) values('2020030702','20200307');
mysql> select table_schema,table_name,DATA_LENGTH,TABLE_ROWS,AVG_ROW_LENGTH,INDEX_LENGTH from information_schema.tables where table_name='isc_message1';
+--------------+--------------+-------------+------------+----------------+--------------+
| TABLE_SCHEMA | TABLE_NAME | DATA_LENGTH | TABLE_ROWS | AVG_ROW_LENGTH | INDEX_LENGTH |
+--------------+--------------+-------------+------------+----------------+--------------+
| test | isc_message1 | 2086649856 | 3844654 | 542 | 62488576 |
+--------------+--------------+-------------+------------+----------------+--------------+
1 row in set (0.00 sec)
mysql> analyze table isc_message1;
+-------------------+---------+----------+----------+
| Table | Op | Msg_type | Msg_text |
+-------------------+---------+----------+----------+
| test.isc_message1 | analyze | status | OK |
+--------------------------+---------+----------+----------+
1 row in set (0.30 sec)
mysql> select table_schema,table_name,DATA_LENGTH,TABLE_ROWS,AVG_ROW_LENGTH,INDEX_LENGTH from information_schema.tables where table_name='isc_message1';
+--------------+--------------+-------------+------------+----------------+--------------+
| TABLE_SCHEMA | TABLE_NAME | DATA_LENGTH | TABLE_ROWS | AVG_ROW_LENGTH | INDEX_LENGTH |
+--------------+--------------+-------------+------------+----------------+--------------+
| test | isc_message1 | 2086649856 | 4021903 | 518 | 62488576 |
+--------------+--------------+-------------+------------+----------------+--------------+
1 row in set (0.00 sec)
mysql> select count(1) from isc_message1;
+----------+
| count(1) |
+----------+
| 3607910 |
+----------+
1 row in set (1.43 sec)
mysql> optimize table isc_message1;
+-------------------+----------+----------+-------------------------------------------------------------------+
| Table | Op | Msg_type | Msg_text |
+-------------------+----------+----------+-------------------------------------------------------------------+
| test.isc_message1 | optimize | note | Table does not support optimize, doing recreate + analyze instead |
| test.isc_message1 | optimize | status | OK |
+-------------------+----------+----------+-------------------------------------------------------------------+
2 rows in set (1 min 10.41 sec)
mysql> select table_schema,table_name,DATA_LENGTH,TABLE_ROWS,AVG_ROW_LENGTH,INDEX_LENGTH from information_schema.tables where table_name='isc_message1';
+--------------+--------------+-------------+------------+----------------+--------------+
| TABLE_SCHEMA | TABLE_NAME | DATA_LENGTH | TABLE_ROWS | AVG_ROW_LENGTH | INDEX_LENGTH |
+--------------+--------------+-------------+------------+----------------+--------------+
| test | isc_message1 | 2086649856 | 4606909 | 452 | 62488576 |
+--------------+--------------+-------------+------------+----------------+--------------+
1 row in set (0.00 sec)
mysql> select 3844654*542,4021903*518,4606909*452;
+-------------+-------------+-------------+
| 3844654*542 | 4021903*518 | 4606909*452 |
+-------------+-------------+-------------+
| 2083802468 | 2083345754 | 2082322868 |
+-------------+-------------+-------------+
1 row in set (0.00 sec)
注意：
1.DATA_LENGTH，INDEX_LENGTH一直没变化，即使收集了统计信息。说明Mysql统计信息收集也不是很准确，毕竟分析表完成的很快（300w行0.3s就完成了分析），也可能数据变化太小。
2.TABLE_ROWS，AVG_ROW_LENGTH 变化比较夸张，但肯定不符合实际情况，而他们的乘积却相对稳定些，上面显示变化在1M 以内
Mysql 执行计划的不靠谱
ID 是主键
create_time 可为空辅助索引
mysql> desc select count(1) from isc_message1;
+----+-------------+--------------+------------+-------+---------------+-------------+---------+------+---------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+--------------+------------+-------+---------------+-------------+---------+------+---------+----------+-------------+
| 1 | SIMPLE | isc_message1 | NULL | index | NULL | create_time | 6 | NULL | 3772852 | 100.00 | Using index |
+----+-------------+--------------+------------+-------+---------------+-------------+---------+------+---------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
mysql> desc select count(*) from isc_message1;
+----+-------------+--------------+------------+-------+---------------+-------------+---------+------+---------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+--------------+------------+-------+---------------+-------------+---------+------+---------+----------+-------------+
| 1 | SIMPLE | isc_message1 | NULL | index | NULL | create_time | 6 | NULL | 3772852 | 100.00 | Using index |
+----+-------------+--------------+------------+-------+---------------+-------------+---------+------+---------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
mysql> desc select count(id) from isc_message1;
+----+-------------+--------------+------------+-------+---------------+-------------+---------+------+---------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+--------------+------------+-------+---------------+-------------+---------+------+---------+----------+-------------+
| 1 | SIMPLE | isc_message1 | NULL | index | NULL | create_time | 6 | NULL | 3772852 | 100.00 | Using index |
+----+-------------+--------------+------------+-------+---------------+-------------+---------+------+---------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
mysql> desc select count(create_time) from isc_message1;
+----+-------------+--------------+------------+-------+---------------+-------------+---------+------+---------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+--------------+------------+-------+---------------+-------------+---------+------+---------+----------+-------------+
| 1 | SIMPLE | isc_message1 | NULL | index | NULL | create_time | 6 | NULL | 3772852 | 100.00 | Using index |
+----+-------------+--------------+------------+-------+---------------+-------------+---------+------+---------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
mysql> desc format=json select count(create_time) from isc_message1;
+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| EXPLAIN |
+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| {
"query_block": {
"select_id": 1,
"cost_info": {
"query_cost": "504644.20"
},
"table": {
"table_name": "isc_message1",
"access_type": "index",
"key": "create_time",
"used_key_parts": [
"create_time"
],
"key_length": "6",
"rows_examined_per_scan": 3772852,
"rows_produced_per_join": 3772852,
"filtered": "100.00",
"using_index": true,
"cost_info": {
"read_cost": "127359.00",
"eval_cost": "377285.20",
"prefix_cost": "504644.20",
"data_read_per_join": "101G"
},
"used_columns": [
"create_time"
]
}
}
} |
+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set, 1 warning (0.00 sec)
mysql> desc format=json select count(1) from isc_message1;
+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| EXPLAIN |
+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| {
"query_block": {
"select_id": 1,
"cost_info": {
"query_cost": "504644.20"
},
"table": {
"table_name": "isc_message1",
"access_type": "index",
"key": "create_time",
"used_key_parts": [
"create_time"
],
"key_length": "6",
"rows_examined_per_scan": 3772852,
"rows_produced_per_join": 3772852,
"filtered": "100.00",
"using_index": true,
"cost_info": {
"read_cost": "127359.00",
"eval_cost": "377285.20",
"prefix_cost": "504644.20",
"data_read_per_join": "101G"
}
}
}
} |
+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set, 1 warning (0.00 sec)
mysql> select count(create_time) from isc_message1;
+--------------------+
| count(create_time) |
+--------------------+
| 3157925 |
+--------------------+
1 row in set (0.47 sec)
mysql> select count(*) from isc_message1;
+----------+
| count(*) |
+----------+
| 3607910 |
+----------+
1 row in set (0.28 sec)
mysql> select count(id) from isc_message1;
+-----------+
| count(id) |
+-----------+
| 3607910 |
+-----------+
1 row in set (0.33 sec)
mysql> select count(1) from isc_message1;
+----------+
| count(1) |
+----------+
| 3607910 |
+----------+
1 row in set (0.28 sec)
mysql> select count(1) from isc_message1;
+----------+
| count(1) |
+----------+
| 3607910 |
+----------+
1 row in set (0.29 sec)
mysql> select count(create_time) from isc_message1;
+--------------------+
| count(create_time) |
+--------------------+
| 3157925 |
+--------------------+
1 row in set (0.45 sec)
mysql> select count(id) from isc_message1;
+-----------+
| count(id) |
+-----------+
| 3607910 |
+-----------+
1 row in set (0.34 sec)
注意：
通过以上可知，做count统计时
count(1)和count(*)的效率差不多，都比较高，而且准确
count(主键) 效率稍低，结果准确
count(可为空辅助索引) 效率低，结果不准确
而这几个count的执行计划，都不准确。