<p>I have a MySQL table where an indexed <code>INT</code> column is going to be 0 for 90% of the rows. If I change those rows to use <code>NULL</code> instead of 0, will they be left out of the index, making the index about 90% smaller?</p>

It looks like it does index the
```
NULL
```
s too.
Be careful when you run this because MySQL will LOCK the table for WRITES during the index creation. Building the index can take a while on large tables even if the column is empty (all nulls).
Reference
.