Is there any point using MySQL “LIMIT 1” when querying on primary key/unique field?
It is not good practice to use LIMIT 1 when querying with filter criteria that is against either a primary key or unique constraint. A primary key, or unique constraint, means there is only one row/record in the table with that value, only one row/record will ever be returned. It’s contradictory to have LIMIT 1 on a primary key/unique field–someone maintaining the code later could mistake the importance & second guess your code.
But the ultimate indicator is the explain plan:
explain SELECT t.name FROM USERS t WHERE t.userid = 4
…returns:
id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra
-----------------------------------------------------------------------------------------------------
1 | SIMPLE | users | const | PRIMARY | PRIMARY | 4 | const | 1 |
…and:
explain SELECT t.name FROM USERS t WHERE t.userid = 4 LIMIT 1
…returns:
id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra
-----------------------------------------------------------------------------------------------------
1 | SIMPLE | users | const | PRIMARY | PRIMARY | 4 | const | 1 |
Conclusion
No difference, no need. It appears to be optimized out in this case (only searching against the primary key).
What about an indexed field?
An indexed field doesn’t guarantee uniqueness of the value being filtered, there could be more than one occurrence. So LIMIT 1 would make sense, assuming you want to return one row.