On 07/17/2012 12:41 PM, David Johnston wrote:

So the question that needs to be asked is what kind of performance increase
can be had during DML (insert/update) statements and whether those gains are
worth pursuing.  Since these other engines appear to allow both cases you
should be able to get at least a partial idea of the performance gains
between "index (a,b,c,d)" and "index (a,b) covering (c,d)".

Tom's recent answer to me on this point (as I understood it) was that he would expect performance to degrade, not improve, since the btree code is known not to perform well when there are many non-unique values.



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