Reduce palloc's in numeric operations.

Hello, I will propose reduce palloc's in numeric operations.

The numeric operations are slow by nature, but usually it is not
a problem for on-disk operations. Altough the slowdown is
enhanced on on-memory operations.

I inspcted them and found some very short term pallocs. These
palloc's are used for temporary storage for digits of unpaked
numerics.

The formats of numeric digits in packed and unpaked forms are
same. So we can kicked out a part of palloc's using digits in
packed numeric in-place to make unpakced one.

In this patch, I added new function set_var_from_num_nocopy() to
do this. And make use of it for operands which won't modified.

The performance gain seems quite moderate....

'SELECT SUM(numeric_column) FROM on_memory_table' for ten million
rows and about 8 digits numeric runs for 3480 ms aganst original
3930 ms. It's 11% gain.  'SELECT SUM(int_column) FROM
on_memory_table' needed 1570 ms.

Similary 8% gain for about 30 - 50 digits numeric. Performance of
avg(numeric) made no gain in contrast.

Do you think this worth doing?

regards,

-- 
Kyotaro Horiguchi
NTT Open Source Software Center