Hello list!
I've noticed in MySQL 5.0 partial keys on character fields aren't always
used. In 4.1 they were.
They seem not to be used when using inequality comparison.
I'm not sure whether this is a bug or intended (in the latter case I have to
work around it to get the speed I got with 4.1 back into 5.0)
You can try for yourselves:
Setup:
CREATE TABLE t (T varchar(16) NOT NULL,KEY (T(1)));
INSERT INTO t SET T="test1";
INSERT INTO t SET T="test2";
INSERT INTO t SET T="test2";
Test:
EXPLAIN SELECT * FROM t WHERE T="x";
Result: key T used
EXPLAIN SELECT * FROM t WHERE T!="x";
Result: key T _unused_
Adding a key on entire field T works, but that's wasting a lot of space
because I only want to test whether a certain field is empty or not.
Important note: MySQL4.1 *did* use key T in the second query!
Can anybody concur whether this happens for them too?
And if so, is this a bug?
grtz,
Thomas
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