Speed up SUM calculation in numeric aggregates.

This introduces a numeric sum accumulator, which performs better than
repeatedly calling add_var(). The performance comes from using wider digits
and delaying carry propagation, tallying positive and negative values
separately, and avoiding a round of palloc/pfree on every value. This
speeds up SUM(), as well as other standard aggregates like AVG() and
STDDEV() that also calculate a sum internally.

Reviewed-by: Andrey Borodin
Discussion: <c0545351-a467-5b76-6d46-4840d1ea8aa4@iki.fi>

src/test/regress/expected/numeric.out

@@ -1909,3 +1909,19 @@ select scale(-13.000000000000000);
     15
 (1 row)
 
+--
+-- Tests for SUM()
+--
+-- cases that need carry propagation
+SELECT SUM(9999::numeric) FROM generate_series(1, 100000);
+    sum    
+-----------
+ 999900000
+(1 row)
+
+SELECT SUM((-9999)::numeric) FROM generate_series(1, 100000);
+    sum     
+------------
+ -999900000
+(1 row)
+

src/test/regress/sql/numeric.sql

@@ -997,3 +997,11 @@ select scale(1.12345);
 select scale(110123.12475871856128);
 select scale(-1123.12471856128);
 select scale(-13.000000000000000);
+
+--
+-- Tests for SUM()
+--
+
+-- cases that need carry propagation
+SELECT SUM(9999::numeric) FROM generate_series(1, 100000);
+SELECT SUM((-9999)::numeric) FROM generate_series(1, 100000);