Index current records when new index is created?

[jasonla]

In MySQL, if I have a set of 5000 records and I create a new index for a column that is currently not indexed, will MySQL index all current records and all new records from that point on, or will it only index new records?

Also, it doesn't seem like I can index a column of type DATETIME in MySQL. Is this correct?

[bastien]

1. should index all records

2. you should be able to. note: mysql may not use the index when searching the db based on the column if it returns more than 30% of the rows