Re: [proposal] de-TOAST'ing using a iterator - Mailing list pgsql-hackers
From | Binguo Bao |
---|---|
Subject | Re: [proposal] de-TOAST'ing using a iterator |
Date | |
Msg-id | CAL-OGktjn9wHLHhEem58qJXMBUAVWa8-VrDoCmceAxXGTUM_BA@mail.gmail.com Whole thread Raw |
In response to | Re: [proposal] de-TOAST'ing using a iterator (Alvaro Herrera <alvherre@2ndquadrant.com>) |
Responses |
Re: [proposal] de-TOAST'ing using a iterator
Re: [proposal] de-TOAST'ing using a iterator Re: [proposal] de-TOAST'ing using a iterator |
List | pgsql-hackers |
Alvaro Herrera <alvherre@2ndquadrant.com> 于2019年9月17日周二 上午5:51写道:
On 2019-Sep-10, Binguo Bao wrote:
> +/*
> + * Support for de-TOASTing toasted value iteratively. "need" is a pointer
> + * between the beginning and end of iterator's ToastBuffer. The marco
> + * de-TOAST all bytes before "need" into iterator's ToastBuffer.
> + */
> +#define PG_DETOAST_ITERATE(iter, need) \
> + do { \
> + Assert((need) >= (iter)->buf->buf && (need) <= (iter)->buf->capacity); \
> + while (!(iter)->done && (need) >= (iter)->buf->limit) { \
> + detoast_iterate(iter); \
> + } \
> + } while (0)
> /* WARNING -- unaligned pointer */
> #define PG_DETOAST_DATUM_PACKED(datum) \
> pg_detoast_datum_packed((struct varlena *) DatumGetPointer(datum))
In broad terms this patch looks pretty good to me. I only have a small
quibble with this API definition in fmgr.h -- namely that it forces us
to export the definition of all the structs (that could otherwise be
private to toast_internals.h) in order to satisfy callers of this macro.
I am wondering if it would be possible to change detoast_iterate and
PG_DETOAST_ITERATE in a way that those details remain hidden -- I am
thinking something like "if this returns NULL, then iteration has
finished"; and relieve the macro from doing the "->buf->buf" and
"->buf->limit" checks. I think changing that would require a change in
how the rest of the code is structured around this (the textpos internal
function), but seems like it would be better overall.
(AFAICS that would enable us to expose much less about the
iterator-related structs to detoast.h -- you should be able to move the
struct defs to toast_internals.h)
Then again, it might be just wishful thinking, but it seems worth
considering at least.
--
Álvaro Herrera https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
I've tied to hide the details of the struct in patch v11 with checking "need" pointer
inside detoast_iterate function. I also compared the performance of the two versions.
patch v10 patch v11
comp. beg. 1413ms 1489ms
comp. end 24327ms 28011ms
uncomp. beg. 1439ms 1432ms
uncomp. end 25019ms 29007ms
comp. beg. 1413ms 1489ms
comp. end 24327ms 28011ms
uncomp. beg. 1439ms 1432ms
uncomp. end 25019ms 29007ms
We can see that v11 is about 15% slower than v10 on suffix queries since this involves
the complete de-TOASTing and detoast_iterate() function is called frequently in v11.
Personally, I prefer patch v10. Its performance is superior, although it exposes some struct details.
Best regards,
Binguo Bao
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