I think i was the one who asked you the question in the SFO training.

Thanks

I had the same problem with the various indexes on tables. The query would not use the same execution plan for each execution with Index Hints, but when we analyze the table the execution plan is as per the Index hints. I am not sure why is this possible, due to data fragmentation on the datafile and the estimates are not the most recent.

Lets consider

CREATE TABLE T (
A int NOT NULL,
B int NOT NULL,
C int NOT NULL,
S varchar(80) NOT NULL,
KEY ix_a_b (A, B),
KEY ix_a_c (A, C)
) ENGINE=InnoDB

Our typical queries are

select S from T where A = ? and B = ?;
select S from T where A = ? and C = ?;

Well, here sampling comes to the play. We will get cardinalities
ix_a_b (1) x1
ix_a_b (2) x2
ix_a_c (3) y1
ix_a_c (4) y2

Because of sampling x1 is never equal to y1. So, for our queries it will always prefer one index over another. It means that one of the queries is always executed using wrong index!!!

I talked to Percona people. In their opinion the behavior is inherent, as sampling of indexes happen independently. I guess, expectations of optimizer are:
1) x1 = y1
2) x1 <= x2 <= rowcount && y1 <= y2 <= rowcount