Hi Thomas, all!
Thomas van Gulick wrote:
[[...]]
Test:
EXPLAIN SELECT * FROM t WHERE T="x";
Result: key T used
EXPLAIN SELECT * FROM t WHERE T!="x";
Result: key T _unused_
To be expected:
An "unequal" condition will evaluate to "true" for a very large
proportion of the index entries, so the selectivity will be too low.
The mere existence of an index does not imply that using it to access
the data is a good decision, doing this only pays off if the extra
accesses (to the index) are less work than is saved by not accessing
those data rows which are not returned by the index.
You wouldn't use a catalogue in manual search if a sequential scan would
return 1 out of 4 (or more) items, right ? The same holds here.
Joerg
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Joerg Bruehe, Senior Production Engineer
MySQL AB, www.mysql.com
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