Re: free space % calculation in pgstathashindex

On Mon, Aug 7, 2017 at 6:07 PM, Ashutosh Sharma <ashu(dot)coek88(at)gmail(dot)com> wrote:
> Hi,
>
..
> In step #1, assuming '*' as an arithmetic operator, the left operand
> i.e. 'stats.unused_pages' is of type uint32 whereas the right operand
> i.e. 'stats.space_per_page' is of type int32 and arithmetic
> conversions of the ANSI C standard states: 'if either operand has type
> unsigned int, the other operand is converted to unsigned int' which
> means stats.space_per_page would be converted to uint32 and then the
> multiplication would be performed. But, if the result of
> multiplication is greater than the max value accepted by 32 bits
> number, we would get an incorrect result. Hence, it is important to do
> proper typecasting of operands before performing any arithmetic
> operation. In this case, i would typecast both stats.unused_pages and
> stats.space_per_page to uint64 and the
> perform multiplication on them. Similar handling needs to be done in
> step #2 as well.
>

I think even if you have typecasted the first variable, it would have
served the purpose.

/* Count unused pages as free space. */

- stats.free_space += stats.unused_pages * stats.space_per_page;
+ stats.free_space += ((uint64) stats.unused_pages *
+ (uint64) stats.space_per_page);

Why an extra parenthesis in above case whereas not in below case? I
think the code will look consistent if you follow the same coding
practice. I suggest don't use it unless you need it.

/*
* Total space available for tuples excludes the metapage and the bitmap
* pages.
*/
- total_space = (nblocks - (stats.bitmap_pages + 1)) * stats.space_per_page;
+ total_space = (uint64) (nblocks - (stats.bitmap_pages + 1)) *
+ (uint64) stats.space_per_page;

--
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com