Does an index < or > MySQL queries?

Question

If I have a query like,

DELETE FROM table WHERE datetime_field < '2008-01-01 00:00:00'

does having the datetime_field column indexed help? i.e. is the index only useful when using equality (or inequality) testing, or is it useful when doing an ordered comparison as well?

(Suggestions for better executing this query, without recreating the table, would also be ok!)

Answer 1

Maybe. In general, if there is such an index, it will use a range scan on that index if there is no "better" index on the query. However, if the optimiser decides that the range would end up being too big (i.e. include more than, say 1/3 of the rows), it probably won't use the index at all, as a table scan would be likely to be faster.

Use EXPLAIN (on a SELECT; you can't EXPLAIN a delete) to determine its decision in a specific case. This is likely to depend upon

• How many rows there are in the table
• What the range is that you're specifying
• What else is specified in the WHERE clause. It won't use a range scan of one index if there is another index which "looks better".

Answer 2

From MySQL Reference manual:

A B-tree index can be used for column comparisons in expressions that use the =, >, >=, <, <=, or BETWEEN operators.

For a large number of rows, it can be much faster to lookup the rows through a tree index than through a table scan. But as the other answers point out, use EXPLAIN to find out MySQL's decision.