Compute decimal average by storing overflow

[karteekmurthys]

This technique is similar to JAVA approach here:https://github.com/prestodb/presto/blob/master/presto-main/src/main/java/com/facebook/presto/operator/aggregation/DecimalAverageAggregation.java#L222.

The accumulator is of the form:

struct DecimalAccumulator {
   int128_t sum;
   int64_t count;
   int64_t overflow;
};

We track overflow and use the overflow to compute the final average:

Average = overflow ( overflow_multiplier /  count    +      sum / (count * overflow))

Here we try to minimize integer multiplication that is likely to cause int128 bit overflows.

The division operations used above return integer quotient and an integer remainder. The remainder part is further divided by count to get a double value. So, the accuracy of the final result depends the (double)remainder/count value.

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[karteekmurthys]

@Yuhta Would you please review the PR. Now we are covering partial and intermediate aggregations.

[facebook-github-bot]

@Yuhta has imported this pull request. If you are a Meta employee, you can view this diff on Phabricator.

[karteekmurthys]

@Yuhta I see that build tests and linter workflows have failed. Is there anything I can help with?

[Yuhta]

@karteekmurthys The errors I saw are trivial ones so far, no worry for now