Does a composite index have a direction in MySQL?

Question

When the following is required:

create index i_t_a_b on t(a,b);

create index i_t_b_a on t(b,a);

Answer 1

If you want maximum search speed and have both columns connected or where the conditions are, BUT sometimes columns a have higher selectivity, and sometimes column b has higher selectivity, and you want to use this fact from a single index.

In addition, I think that your machine data size / performance ratio should be quite high, and at the same time you will have to (be willing) to call any improvement necessary (at least by a few percent).

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Answer 2

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, , a b , :

Select Max(b) From t Where a=1  --# Would use i_t_a_b

Select Max(a) From t Where b=1  --# Would use i_t_b_a

---

, :

a  b  c  d  e
-  -  -  -  -
0  8  x  x  x
0  9  x  x  x
1  8  x  x  x
1  9  x  x  x

i_t_a_b :

0
  8
  9
1
  8
  9

i_t_b_a :

8
  0
  1
9
  0
  1

---

Select Max(b) From t Where a=1

8 9 of i_t_b_a, a=1. , ( x), , i_t_a_b.