BIT字段是否比SQL Server中的int字段快？

Question

I have table with some fields that the value will be 1 0. This tables will be extremely large overtime. Is it good to use bit datatype or its better to use different type for performance? Of course all fields should be indexed.

Answer 1

I can't give you any stats on performance, however, you should always use the type that is best representative of your data. If all you want is 1-0 then absolutely you should use the bit field.

The more information you can give your database the more likely it is to get it's "guesses" right.

Answer 2

Officially bit will be fastest, especially if you don't allow nulls. In practice it may not matter, even at large usages. But if the value will only be 0 or 1, why not use a bit? Sounds like the the best way to ensure that the value won't get filled with invalid stuff, like 2 or -1.

Answer 3

It depends.

If you would like to maximize speed of selects, use int (tinyint to save space), because bit in where clause is slower then int (not drastically, but every millisecond counts). Also make the column not null which also speeds things up. Below is link to actual performance test, which I would recommend to run at your own database and also extend it by using not nulls, indexes and using multiple columns at once. At home I even tried to compare using multiple bit columns vs multiple tinyint columns and tinyint columns were faster ( select count(*) where A=0 and B=0 and C=0 ). I thought that SQL Server (2014) would optimize by doing only one comparison using bitmask, so it should by three times faster but that wasn't the case. If you use indexes, you would need more than 5000000 rows (as used in the test) to notice any difference (which I didn't have the patience to do since filling table with multiple millions of rows would take ages on my machine).

https://www.mssqltips.com/sqlservertip/4137/sql-server-performance-test-for-bit-data-type-in-a-where-clause/

If you would like to save space, use bit, since 8 of them can ocuppy one byte whereas 8 tinyints will ocupy 8 bytes. Which is around 7 Megabytes saved on each million of rows.

The differences between those two cases are basically negligable and since using bit has the upside of signalling that the column represents merely a flag, I would recommend using bit.

Answer 4

As I understand it, you still need a byte to store a bit column (but you can store 8 bit columns in a single byte). So having a large number (how many?) of these bit columns could save you a bit on storage. As Yishai said it probably won't make much of a difference in performance (though a bit will translate to a boolean in application code more nicely).

If you can state with 100% confidence that the two options for this column will NEVER change then by all means use the bit. But if you can see a third value popping up in the future it could make life a little easier when that day comes to use a tinyint.

Just a thought, but I'm not sure how much good an index will do you on this column either, unless you see the vast majority of rows going to one side or the other. In a roughly 50/50 distribution you might actually take more of a hit keeping the index up to date than it gains you'd see in querying the table.