MySQL Index Updates

Question

Say I have 3 columns, "Col1, Col2 and Col3" and I have an index on Col1. If I make an update to the data in Col3 (EG: change an int), does the index have to get re-made or updated again, even though I've not touched the indexed columns (Col1's) data at all?

Answer 1

Short answer: No

Long answer: The index provides a mapping from the value of a column to the location of the column. Neither value nor location changed, so the index entry hasn't changed, so it isn't updated.