Re: Optimize partial TOAST decompression

> 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.

> Also, I'm not sure you use SET_VARSIZE_COMPRESSED correctly...
I followed the code in toast_fetch_datum function[1], and I didn't see any
wrong with it.

Best regards, Binguo Bao

[1]
https://github.com/postgres/postgres/blob/master/src/backend/access/heap/tuptoaster.c#L1898

Andrey Borodin <x4mmm(at)yandex-team(dot)ru> 于2019年6月23日周日 下午5:23写道：

> Hi, Binguo!
>
> > 2 июня 2019 г., в 19:48, Binguo Bao <djydewang(at)gmail(dot)com> написал(а):
> >
> > Hi, hackers!
> ....
> > This seems to have a 10x improvement. If the number of toast data chunks
> is more, I believe that patch can play a greater role, there are about 200
> related TOAST data chunks for each entry in the case.
>
> That's really cool that you could produce meaningful patch long before end
> of GSoC!
>
> I'll describe what is going on a little:
> 1. We have compressed value, which resides in TOAST table.
> 2. We want only some fraction of this value. We want some prefix with
> length L.
> 3. Previously Paul Ramsey submitted patch that omits decompression of
> value beyond desired L bytes.
> 4. Binguo's patch tries to do not fetch compressed data which will not bee
> needed to decompressor. In fact it fetches L bytes from TOAST table.
>
> 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.
>
> Also, I'm not sure you use SET_VARSIZE_COMPRESSED correctly...
>
> Best regards, Andrey Borodin.