Do i use Unique Index with additional Column or Index with less Column?,

Question

For a Table that i'll just SELECT rows from it,

It has 3 Columns [A, B, C] .

I can create one of those two INDEXES

ALTER TABLE `table` ADD INDEX `A_B` (`A', `B`),ADD INDEX `B_C` (`B`, `C`);

OR

ALTER TABLE `table` ADD UNIQUE INDEX `A_B_C` (`A`, `B`, `C`),ADD INDEX `B_C` (`B`, `C`);

Both works at the same speed, and has the same result in EXPLAIN .

How do i decide what to chose?

Normal one is 4 Columns, and Unique one is 5 Columns to prevent Duplication,

But i'll just SELECT on the table , So is it better to use The Normal ? Or using the Unique won't make any different?

Answer 1

There is a slight overhead to having more columns in an index.

However, I prefer the unique index because it also ensures uniqueness -- that is, it enforces a property about the table that you want.

Note that if you have defined the trio of columns as unique , there is not need to have a separate index on them.

I should add that if all the conditions in your queries are equality conditions, then one unique index might suffice.