Is there any point using MySQL "LIMIT 1" when querying on indexed/unique field?

If the field has a unique index on it, I can't see this improving execution time at all - it's a stretch even by micro-optimization standards.

However, I can see it potentially masking other problems. Say you add LIMIT 1 to these queries, and then somehow the field upon which you're querying loses its unique index and the db gets multiple rows with the same value. This code will keep chugging along happily - I'd think you might want to fail fast instead, to become aware of (and fix) the bigger underlying problem.

In my own db interface code, I have a method queryOneRow() that runs some SQL and throws an exception if it gets more than one row back. It makes the most sense to me to handle this explicitly at the application layer rather than defensively in SQL.

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.