Re: Slow Performance / Index not used

[Rick James]

Were your timings run twice? The first run is likely to be I/O bound; the second is likely to be at least partially cached.

PK can, and should, be used in many situations. Some simple examples:
* SELECT * FROM tbl WHERE id = 123 -- fetch one row very efficiently.
* SELECT * FROM tbl WHERE id BETWEEN 123 AND 234 -- fetch consecutive rows very efficiently
* SELECT ... FROM a, b WHERE a... AND b.id=a.id -- after deciding on which rows from 'a' to fetch, then drill directly into 'b' to find the rest of the stuff.

An example of where having a clustered PK is less efficient (as in InnoDB, vs MyISAM):
SELECT id FROM tbl WHERE id BETWEEN 123 AND 234
Now it is doing as you fear -- stepping over whole records to get just the id. On the other hand, if "id" were a secondary key, this would be more efficient and the EXPLAIN would say "Using index".