Re: Will a composed index help me?

[Rick James]

It depends. Without the schema and the whole SELECT, I can't tell.

If you have PRIMARY KEY(F11) (and it is InnoDB), then the PK and the data occupy the same BTree, so you can get to F12 for 'free'. That is, you won't need another index.

If you have UNIQUE(F11), well, did it turn into the PK?

If your JOIN needs to fetch other fields from T1, then the index won't help.

Also, the JOIN might start with T2. That opens up another can of worms.

Otherwise, INDEX(F11, F12) would help because then it would be "using index". See the EXPLAIN to see this.

To assist in analyzing slow SELECTs, please provide
* SHOW CREATE TABLE tbl\G
* SHOW TABLE STATUS LIKE 'tbl'\G
* EXPLAIN SELECT ...\G
and surround them with [ code ] and [ / code ]