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I have two fields a and b, where b has substantially higher selectivity than a.
Now, if I am only querying on both a and b (never on either field by itself), which of the following two indexes is better and why:
{a: 1, b : 1}{b: 1, a : 1}Explain seems to return almost identical results, but I read somewhere that you should put higher selectivity fields first. I don’t know why that would make sense though.
After doing some further analysis the two indexes are in fact pretty much identical from a performance point of view.
Really if you are in a similar situation, the real consideration should be whether in the future you might be more likely to query on a alone or b alone, and put that field first in the index.