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Re: Speed dblink using alternate libpq tuple storage

From: Marko Kreen <markokr(at)gmail(dot)com>
To: Kyotaro HORIGUCHI <horiguchi(dot)kyotaro(at)oss(dot)ntt(dot)co(dot)jp>
Cc: pgsql-hackers(at)postgresql(dot)org, greg(at)2ndQuadrant(dot)com
Subject: Re: Speed dblink using alternate libpq tuple storage
Date: 2012-01-20 14:49:45
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
On Tue, Jan 17, 2012 at 05:53:33PM +0900, Kyotaro HORIGUCHI wrote:
> Hello,  This is revised and rebased version of the patch.
> a. Old term `Add Tuple Function' is changed to 'Store
>    Handler'. The reason why not `storage' is simply length of the
>    symbols.
> b. I couldn't find the place to settle PGgetAsCString() in. It is
>    removed and storeHandler()@dblink.c touches PGresAttValue
>    directly in this new patch. Definition of PGresAttValue stays
>    in lipq-fe.h and provided with comment.
> c. Refine error handling of dblink.c. I think it preserves the
>    previous behavior for column number mismatch and type
>    conversion exception.
> d. Document is revised.

First, my priority is one-the-fly result processing,
not the allocation optimizing.  And this patch seems to make
it possible, I can process results row-by-row, without the
need to buffer all of them in PQresult.  Which is great!

But the current API seems clumsy, I guess its because the
patch grew from trying to replace the low-level allocator.

I would like to propose better one-shot API with:

    void *(*RowStoreHandler)(PGresult *res, PGresAttValue *columns);

where the PGresAttValue * is allocated once, inside PQresult.
And the pointers inside point directly to network buffer.
Ofcourse this requires replacing the current per-column malloc+copy
pattern with per-row parse+handle pattern, but I think resulting
API will be better:

1) Pass-through processing do not need to care about unnecessary
   per-row allocations.

2) Handlers that want to copy of the row (like regular libpq),
   can optimize allocations by having "global" view of the row.
   (Eg. One allocation for row header + data).

This also optimizes call patterns - first libpq parses packet,
then row handler processes row, no unnecessary back-and-forth.

Summary - current API has various assumptions how the row is
processed, let's remove those.


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