MVCC and index-only read - Mailing list pgsql-general

Hi,

if I got it right the reason some aggregates (such as COUNT) using only index columns are "slow" on postgresql is that
ituses MVCC, so it has to read the data as well as the index. It makes sense to me, but I don't understand is how other
databases(such as Oracle) do it. 
Can someone explain?

Thank you.



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