From: | Peter Geoghegan <pg(at)heroku(dot)com> |
---|---|
To: | Andreas Karlsson <andreas(at)proxel(dot)se> |
Cc: | Petr Jelinek <petr(at)2ndquadrant(dot)com>, David Rowley <dgrowleyml(at)gmail(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com>, Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>, Andres Freund <andres(at)2ndquadrant(dot)com> |
Subject: | Re: Using 128-bit integers for sum, avg and statistics aggregates |
Date: | 2015-03-13 21:22:18 |
Message-ID: | CAM3SWZTkAUZQo21_d4_xu7a8drGaVTN5+Ts9bo4KbKUgoqDP2Q@mail.gmail.com |
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On Thu, Mar 12, 2015 at 6:23 PM, Andreas Karlsson <andreas(at)proxel(dot)se> wrote:
> Fixed.
Did you intend to attach a patch here?
>> I think you should talk about the new thing first (just after the
>> extant, first sentence "Integer data types use Numeric..."). Refer to
>> where 128-bit integers are used and how, and only then the other stuff
>> (exceptions). After that, put the PolyNumAggState struct definition
>> and basic functions. Then int2_accum() and so on.
>
>
> Good idea! Do you think the rewritten comment is clear enough, or do I need
> to go into more detail?
>
> /*
> * Integer data types use Numeric accumulators to share code and avoid risk
> * of overflow. To speed up aggregation 128-bit integer accumulators are
> * used instead where sum(X) or sum(X*X) fit into 128-bits, and there is
> * platform support.
> *
> * For int2 and int4 inputs sum(X) will fit into a 64-bit accumulator, hence
> * we use faster special-purpose accumulator routines for SUM and AVG of
> * these datatypes.
> */
>
> #ifdef HAVE_INT128
> typedef struct Int128AggState
Not quite. Refer to the 128-bit integer accumulators as
"special-purpose accumulator routines" instead. Then, in the case of
the extant 64-bit accumulators, refer to them by the shorthand
"integer accumulators". Otherwise it's the wrong way around.
--
Peter Geoghegan
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