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Re: double and numeric conversion

From: Theo Schlossnagle <jesus(at)omniti(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Theo Schlossnagle <jesus(at)omniti(dot)com>, PostgreSQL-development Development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: double and numeric conversion
Date: 2010-03-02 14:37:12
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Lists: pgsql-hackers
On Mar 1, 2010, at 4:35 PM, Tom Lane wrote:

> Theo Schlossnagle <jesus(at)omniti(dot)com> writes:
>> I'm writing some extension and I have a hot code path that has a lot of double (C type) data and needs to output NUMERIC tuple data.  The current methods I can find in the code to convert sprintf the double to a buffer and then invoke the numeric_in function on them.  I've profile my stuff and I'm spending (wasting) all my time in that conversion.  Is there a more efficient method of converting a double into a postgres numeric value?
> If you're worried about micro-optimization, why are you using NUMERIC at
> all?  It's no speed demon.
> Although you might be able to shave some cycles with a dedicated code
> path for this conversion, binary to decimal is fundamentally not cheap.

I feared that was the case.  I spent an hour or so coding that last night and the speedups for me were worth it, I see a 2 fold speedup in conversion operations (or a 50% reduction in CPU cycles per conversion).  The integer ones were trivial, the double one has the imperfect issue of reasonably guessing the dscale, but seems to work in my tests.

I didn't look deeply at the postgres internals to see if there was a way to do double -> numeric and integer-types -> numeric without intermediary string format.  If that sort of thing is easy to leverage, I'd be happy to share the code.

Theo Schlossnagle

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