Re: [proposal] de-TOAST'ing using a iterator

From: Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>
To: Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>
Cc: Binguo Bao <djydewang(at)gmail(dot)com>, John Naylor <john(dot)naylor(at)2ndquadrant(dot)com>, Thomas Munro <thomas(dot)munro(at)gmail(dot)com>, Andrey Borodin <x4mmm(at)yandex-team(dot)ru>, Atri Sharma <atri(dot)jiit(at)gmail(dot)com>, Stephen Frost <sfrost(at)snowman(dot)net>, Владимир Лесков <vladimirlesk(at)yandex-team(dot)ru>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: [proposal] de-TOAST'ing using a iterator
Date: 2019-09-17 13:34:20
Message-ID: 20190917133420.7q65fdbszj6zesw7@development
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On Mon, Sep 16, 2019 at 06:22:51PM -0300, Alvaro Herrera wrote:
>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.

I do agree hiding the exact struct definition would be nice. IMHO if the
only reason for exposing it is the PG_DETOAST_ITERATE() macro (or rather
the references to buf fields in it) then we can simply provide functions
to return those fields.

Granted, that may have impact on performance, but I'm not sure it'll be
even measurable. Also, the other detoast macros right before this new
one are also ultimately just a function calls.


Tomas Vondra
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

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