How does MySQL query the first few rows of specified data
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When there is a large amount of data in the data table, and only the values of some of these fields are needed for verification, you don't need all the data, you only need to check a few rows, use limit at this time
Such as:
1.select * from tags limit 4; just take four rows of data to see what the value of id or other required fields is.
The query result is: id=102 or., use id=102 below
It is effective for 2.explain select * from tags where id=102; to check whether the id or other fields in the table are indexed.
+-- +
| | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+-- +
| | 102 | SIMPLE | tags | ALL | NULL | NULL | NULL | NULL | 6226057 |
+-- +
You can see that the id index is not used here [the index built for id is usually PRIMARY]
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