How can a bit-field be so slow?

[jel]

In a (rather complex) T-SQL statement I used the expression:
WHERE .... AND fieldx = 1 ....
fieldx has datatype 'bit'.
The query performed very poor, and removing this specific constraint speeded things up more then 10 times.
Because I needed the constraint anyways, I made a wild experiment: I changed the datatype to int, and put an index on it. Guess what: performance still 10 times better then before.
Not being an SQL-guru at all: what is happening here? I'd think a bit-field would me much faster then an indexed int-field?

 

[stravis]

The bit data type can not have an index, and is not a good data type for data that you wish to filter on. The advantage of the bit datatype is that it takes up a small amount of space and up to 8 bit values from a single record can take up just one byte of data. As your tests showed, choose another data type and your conditional queries will run much faster.

 

[jel]

Thanks stravis, glad to know my uneducated guess was correct.