Question

Suppose I have simple table like this:

ID (PRIMARY)
time (INT)
stage (TINYINT)
other fields...

I have to do range search on time while normally selecting stage. With SQL query example:

SELECT * FROM table WHERE time>10000 AND (stage=1 OR stage=3 OR stage=4)

VERY IMPORTANT: There are a lot of rows with stage = 2, let’s say 99% of the table. There are only 5 distinct stage values.

What would be the propper indexing of this table?

Answer 1

It depends on the distribution of the values in the different columns.
If you have very few possible stage values you will probably have the best performance with either a separate index on time and one on stage or with a combined index time, stage.

But if you have lots of distinct stage values it might be faster to order the index the other way around: stage, time.

But using OR makes the stage search more fragmented compared to an AND search. Therefore I would try to have time first in the index.

The only way to know for sure on your specific set of data is to try and measure, but above three mentioned candidates are my top candidates for indexes.

Edit
You might want to create a clustered index on time, possibly time, stage if most of your queries search by time range. This way you minimize lookup in the table once you found the correct rows in the index.
Beware that this can create a fragmented dataspace if time is strictly increasing when you insert new records.