Parallel Aggregate costs don't consider combine/serial/deserial funcs

Hi,

I realised a few days ago that the parallel aggregate code does not
cost for the combine, serialisation and deserialisation functions at
all.

I've attached a patch which fixes this.

One small point which I was a little unsure of in the attached is,
should the "if (aggref->aggdirectargs)" part of
count_agg_clauses_walker() be within the "if
(!context->combineStates)". I simply couldn't decide. We currently
have no aggregates which this affects anyway, per; select * from
pg_aggregate where aggcombinefn <> 0 and aggkind <> 'n'; so for now
I've left it outwith.

Another thing I thought of is that it's not too nice that I have to
pass 3 bools to count_agg_clauses() in order to tell it what to do. I
was tempted to invent some bitmask flags for this, then modify
create_agg_path() to use the same flags, but I thought I'd better not
cause too much churn with this patch.

--
David Rowley http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services