MySQL Query Cost

Can someone tell me the theoretical difference in the e开发者_如何学JAVAxpensive of the following queries?

• SELECT * FROM posts WHERE category IN(1)
• SELECT * FROM posts WHERE category IN(1,2)
• SELECT * FROM posts WHERE category IN(1,2,3)

... etc ...

Thanks.

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Theoretically, if you had an index on category AND the table was sizable enough AND the index is selective, then

Cost for 1st as baseline = N
Cost of 2nd = 2 x N
Cost of 3rd = 3 x N

If there is no index or the index is not selective enough (e.g. category=1 covers 10% of the table) then it will perform a table scan. If the table is small, it will not even bother to use the index. In which case

Cost of 1st as baseline = N + X
Cost of 2nd = N + 2X
Cost of 3rd = N + 3X

The reason is that a full table scan is cost N. During the scan, it will use X time pick out the entire row (select *) so there is a little bit more involved in processing more categories, but not so significant as to affect the baseline time required to scan the table.