Re: Covering Indexes

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.

cheers

andrew