Re: review: Reduce palloc's in numeric operations

On 19.11.2012 15:17, Pavel Stehule wrote:
> I tested this patch and I can confirm, so this patch can increase
> speed about 16-22% (depends on IO waits, load type).

Thanks for the review.

I spent some more time on this, continuing with the thought that perhaps 
it would be better if get_str_from_var() didn't scribble on its input. I 
ended up with the attached patch, which contains a bunch of small tweaks:

* Add init_var_from_num() function. This is the same as 
set_var_from_num_nocopy in the original patch, but it doesn't require 
the caller to have called init_var() first. IMHO this makes the calling 
code slightly more readable. Also, it's now more evident what these vars 
are: the digits array points to original array in the original Datum, 
but 'buf' is NULL. This is the same arrangement that's used in the 
constant NumericVars like const_zero.

* get_str_from_var() no longer scribbles on its input. I noticed that 
it's always called with a dscale that comes from the input var itself. 
In other words, the rounding was unnecessary to begin with. I simply 
removed the dscale argument and the round_var() call from 
get_str_from_var(). If a someone wants to display a string with 
different dscale in the future, he can simply call round_var() before 
get_str_from_var().

* numericvar_to_int8() no long scribbles on its input either. It creates 
a temporary copy to avoid that. To compensate, the callers no longer 
need to create a temporary copy, so the net # of pallocs is the same, 
but this is nicer. (there's room for a micro-optimization to avoid 
making the temporary copy numericvar_to_int8() when the argument is 
already suitably rounded - I left that our for now, dunno if it would 
make any difference in practice)

* use a constant for the number 10 in get_str_from_var_sci(), when 
calculating 10^exponent. Saves a palloc() and some cycles to convert 
integer 10 to numeric.

Comments? Assuming no-one sees some fatal flaw in this, I'll commit this 
tomorrow.

- Heikki