Characteristics of Bad Keys
Bad keys have the following features:
• Wide
– Use wide keys with caution.
– You get fewer rows per page.
– Evaluating the hash function takes more time with wide keys.
– A wide key deepens the index level of B-tree and ISAM logarithmically, with respect to key width. B-tree is the least affected table structure.
– Consider using a surrogate key as an alternative.
• Non-static
Updating the index can slow performance.
• Non-uniform duplication
A mix of high and low duplication can cause inconsistent query performance.
• Sequential
– Sequential keys must be used with care.
– ISAM tables can be lopsided and the overflow chains can cause concurrency problems.
– Control sequential key problems with a frequent modify schedule.