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I have a SQL table with these columns (id, A, B, C, D, E).
I usually query on (A and C) and (B and C) so I decide to have index on these 3. There are some options for this:
Which option should I choose? Database size may be an issue but let assume it’s not a big deal right now.
Build an index on the two pairs. With most indexes (although this may vary between types and databases), an index on A,B is uses as well if you query only A, but A,B is not used if you query only B. So make sure the first columns in your indexes are the columns you query for. If you query A,B and A,C, but not B or C alone, you only need the indexes on those two pairs.