On 20.2.2015 21:01, Peter Eisentraut wrote:
> Is there a case where the combining function is different from the
> transition function, other than for count?
It's different in all the cases when the aggregate state is not
identical to a single value - for example the usual avg(), sum() and
stddev() aggregates keep state which is equal to
{count(X), sum(X), sum(X*X)}
The 'combine' function gets two such 'state' values, while transition
gets 'state' + next value.
I'm inclined to say that 'combinefn == transfn' is a minority case.
--
Tomas Vondra http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
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