Is there any performance gain in indexing a boolean field?

Question

I\'m just about to write a query that includes a WHERE isok=1. As the name implies, isok is a boolean field (actually a TINYINT(1) UNSIGNED

Answer 1

It depends on the actual queries and the selectivity of the index/query combination.

Case A: condition WHERE isok = 1 and nothing else there:

SELECT *
FROM tableX
WHERE isok = 1

• If the index is selective enough (say you have 1M rows and only 1k have isok = 1), then the SQL engine will probably use the index and be faster than without it.

• If the index is not selective enough (say you have 1M rows and more than 100k have isok = 1), then the SQL engine will probably not use the index and do a table scan.

Case B: condition WHERE isok = 1 and more stuff:

SELECT *
FROM tableX
WHERE isok = 1
  AND another_column = 17

Then, it depends on what other indexes you have. An index on another_column would probably be more selective than the index on isok which has only two possible values. An index on (another_column, isok) or (isok, another_column) would be even better.