Re: Using 128-bit integers for sum, avg and statistics aggregates

On Mon, Mar 9, 2015 at 5:37 PM, Andreas Karlsson <andreas@proxel.se> wrote:
> int128-agg-v7.patch

I see a spelling error:

"+ * On platforms which support 128-bit integers some aggergates instead use a"

Other than that, the patch looks pretty good to me. You're
generalizing from the example of existing routines for
int128_to_numericvar(), and other such code in a fairly mechanical
way. The performance improvements are pretty real and tangible.

You say:

/** Integer data types use Numeric accumulators to share code and avoid* risk of overflow.  For int2 and int4 inputs,
Numericaccumulation* is overkill for the N and sum(X) values, but definitely not overkill* for the sum(X*X) value.
Hence,we use int2_accum and int4_accum only* for stddev/variance --- there are faster special-purpose accumulator*
routinesfor SUM and AVG of these datatypes.** Similarily we can, where available, use 128-bit integer accumulators* for
sum(X)for int8 and sum(X*X) for int2 and int4, but not sum(X*X)* for int8.*/
 

I think you should talk about the new thing first (just after the
extant, first sentence "Integer data types use Numeric..."). Refer to
where 128-bit integers are used and how, and only then the other stuff
(exceptions). After that, put the  PolyNumAggState struct definition
and basic functions. Then int2_accum() and so on.
-- 
Peter Geoghegan