MySQL Index for searching for NULL values in text fields I need to do the simple query SELECT id FROM table WHERE blob_field IS NULL. 

blob_field is a longblob. If I create an index on blob_field with prefix length 1, will this be sufficient to use for this query?
Yes, a prefix length of one will allow you to have a working index for looking up NULLs.

Here's the SQLFiddle: http://sqlfiddle.com/#!2/12bf3/1/0

The EXPLAIN says that it is using the desired index.

Of course you might want to try a larger test to make sure it performs as well in reality as you hope.