New answers tagged blob
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.
Why don't you do like this : SELECT OCTET_LENGTH(blob_field ) FROM table WHERE predicate ;
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