Improving avg performance for numeric

Revisiting:
http://www.postgresql.org/message-id/45661BE7.4050205@paradise.net.nz

I think the reasons which the numeric average was slow were:
(1) Using Numeric for count, which is slower than int8 to increment,
(2) Constructing/deconstructing arrays at each transition step.

This is also discussed at:
http://www.citusdata.com/blog/53-postgres-performance-to-avg-or-to-sum-divided-by-count

So, I think we can improve the speed of numeric average by keeping the
transition state as an struct in the aggregate context, and just passing
the pointer to that struct from/to the aggregate transition function.

The attached patch uses this method.

I tested it using the data generated using:
CREATE TABLE avg_test AS SELECT (random() * 999)::decimal(5,2) as d FROM
generate_series(1, 10000000) s;

After applying this patch, run time of "SELECT avg(d) FROM avg_test;"
improves from 10.701 seconds to 5.204 seconds, which seems to be a huge
improvement.

I think we may also be able to use a similar method to improve performance
of some other numeric aggregates (like stddev). But I want to know your
feedback first.

Is this worth working on?

Thanks,
-- Hadi