Improving avg performance for numeric

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