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I realize that the exact question has been asked before on stack overflow with only a vaguely conclusive answer: link
So here it is again.
Table A: Has an integer column which is nullable, along with an index.
Query:
SELECT *
FROM table
WHERE column IS NULL
LIMIT 10;
Will an index be used, or will mysql do a full table scan until it has found 10 entries?
i think this should be helpful:
http://dev.mysql.com/doc/refman/5.0/en/mysql-indexes.html