Re: Optimize partial TOAST decompression

Hi!

> Andrey Borodin <x4mmm(at)yandex-team(dot)ru> 于2019年6月29日周六 下午9:48写道：

> Hi!
> Please, do not use top-posting, i.e. reply style where you quote whole
> message under your response. It makes reading of archives terse.
>
> > 24 июня 2019 г., в 7:53, Binguo Bao <djydewang(at)gmail(dot)com> написал(а):
> >
> >> This is not correct: L bytes of compressed data do not always can be
> decoded into at least L bytes of data. At worst we have one control byte
> per 8 bytes of literal bytes. This means at most we need (L*9 + 8) / 8
> bytes with current pglz format.
> >
> > Good catch! I've corrected the related code in the patch.
> > ...
> > <0001-Optimize-partial-TOAST-decompression-2.patch>
>
> I've took a look into the code.
> I think we should extract function for computation of max_compressed_size
> and put it somewhere along with pglz code. Just in case something will
> change something about pglz so that they would not forget about compression
> algorithm assumption.
>
> Also I suggest just using 64 bit computation to avoid overflows. And I
> think it worth to check if max_compressed_size is whole data and use min of
> (max_compressed_size, uncompressed_data_size).
>
> Also you declared needsize and max_compressed_size too far from use. But
> this will be solved by function extraction anyway.
>
> Thanks!
>
> Best regards, Andrey Borodin.

Thanks for the suggestion.
I've extracted function for computation for max_compressed_size and put the
function into pg_lzcompress.c.

Best regards, Binguo Bao.