Choose the better unique index

[elsa0520]

Enhancement

If both of unique index need to double scan, we should better choose the unique index which can match all of where predicates.
The reason is that if the unique index can match the all of where predicates, it will skip the double scan when the qualify row is zero.

For example

1. create table

CREATE TABLE t ( a int NOT NULL ,  b int NOT NULL,
			c varchar(64) NOT NULL ,  d varchar(64) NOT NULL  ,
			UNIQUE KEY ua (a),
			UNIQUE KEY uab (a, b));

2. query

select * from t where a=1 and b=1;

3. choose the uab index

uab index is better than ua. When there is no row can match the where predicates "a=1 and b=1", the uab index can skip the double scan while the ua index can not.

"Point_Get 1.00 root table:t, index:uab(a, b) "