Why are no keys used in this EXPLAIN?

Question

I was expecting this query to use a key.

mysql> DESCRIBE TABLE Foo;
+-------+-------------+------+-----+---------+----------------+
| Field | Type        | Nu         


        

Answer 1

It could be because that the said table Foo very less volume of data. In such case optimizer will choose to do table scan rather than looking through index.

As MySQL Documentation clearly says

Indexes are less important for queries on small tables, or big tables where report queries process most or all of the rows. When a query needs to access most of the rows, reading sequentially is faster than working through an index. Sequential reads minimize disk seeks, even if not all the rows are needed for the query.