review: Reduce palloc's in numeric operations

Hello all

I tested this patch and I can confirm, so this patch can increase
speed about 16-22% (depends on IO waits, load type).

I tested real query (anonymyzed)

SELECT SUM(COALESCE((a * b * c * (d + ((1 -(d)) * (1 -(e))))),0)) m1 FROM tab;

-- patched
4493 26.5591 postgres slot_deform_tuple
1327 7.8442 postgres AllocSetAlloc
1313 7.7614 postgres ExecMakeFunctionResultNoSets
1105 6.5319 postgres set_var_from_num_nocopy
924 5.4620 postgres make_result
637 3.7654 postgres mul_var
635 3.7536 postgres numeric_mul
560 3.3103 postgres MemoryContextAlloc
405 2.3940 postgres AllocSetFree
389 2.2995 postgres ExecEvalScalarVarFast
332 1.9625 postgres slot_getsomeattrs
322 1.9034 postgres numeric_add
313 1.8502 postgres add_abs
304 1.7970 postgres pfree
238 1.4069 postgres slot_getattr
216 1.2768 postgres numeric_sub
200 1.1822 postgres heap_tuple_untoast_attr
183 1.0818 postgres strip_var
180 1.0640 postgres ExecEvalConst
173 1.0226 postgres alloc_var
172 1.0167 postgres check_stack_depth

-- origin
4419 22.8325 postgres slot_deform_tuple
2041 10.5456 postgres AllocSetAlloc
1248 6.4483 postgres set_var_from_num
1186 6.1279 postgres ExecMakeFunctionResultNoSets
886 4.5779 postgres MemoryContextAlloc
856 4.4229 postgres make_result
757 3.9113 postgres numeric_mul
731 3.7770 postgres AllocSetFree
625 3.2293 postgres mul_var
601 3.1053 postgres alloc_var
545 2.8160 postgres pfree
503 2.5989 postgres free_var
458 2.3664 postgres slot_getsomeattrs
378 1.9531 postgres numeric_add
360 1.8601 postgres add_abs
336 1.7361 postgres ExecEvalScalarVarFast
266 1.3744 postgres slot_getattr
221 1.1419 postgres numeric_sub

Review:

1) this patch was clearly applied and compilation was without warning

2) regress tests - All 133 tests passed.

4) don't see any memory leaks there (verified by following code)

CREATE OR REPLACE FUNCTION public.fx(_m integer)
RETURNS void
LANGUAGE plpgsql
AS $function$
declare m numeric = 10;
n numeric = 20022.222;
r numeric;
begin
for i in 1.._m
loop
r := m * n;
end loop;
end;
$function$

postgres=# select fx(10000000);
fx
----

(1 row)

Time: 5312.623 ms --- original ( 4798.103 ms -- patched 10% speedup)

5) it respect PostgreSQL's coding standards

6) we would to accept this patch - it can carry 10% speedup
calculations with numerics.

Regards

Pavel Stehule