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?
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http://stackoverflow.com/questions/3567981/how-do-mysql-indexes-work – John Woo Aug 13 '12 at 08:21
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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.
Eugen Rieck
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