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From: Steinar H. Gunderson <sgunderson@bigfoot.com>
To: pgsql-performance@postgresql.org
Subject: Re: [GENERAL] Creation of tsearch2 index is very slow
Date: Fri, 20 Jan 2006 22:44:02 +0100
Message-ID: <20060120214402.GA22493@uio.no> (raw)
In-Reply-To: <20060120213754.GG31908@svana.org>
References: <43D0ED57.1020608@gmx.de>
	<17454.1137771321@sss.pgh.pa.us>
	<20060120170452.GC31908@svana.org>
	<26153.1137784469@sss.pgh.pa.us>
	<20060120201036.GE31908@svana.org>
	<26583.1137788505@sss.pgh.pa.us>
	<20060120205132.GF31908@svana.org>
	<26958.1137791955@sss.pgh.pa.us>
	<20060120213754.GG31908@svana.org>

On Fri, Jan 20, 2006 at 10:37:54PM +0100, Martijn van Oosterhout wrote:
> Given that all it's doing is counting bits, a simple fix would be to
> loop over bytes, use XOR and count ones. For extreme speedup create a
> lookup table with 256 entries to give you the answer straight away...

For extra obfscation:

  unsigned v = (unsigned)c;
  int num_bits = (v * 0x1001001001001ULL & 0x84210842108421ULL) % 0x1f;

(More more-or-less intelligent options at
http://graphics.stanford.edu/~seander/bithacks.html#CountBitsSetNaive :-) )

/* Steinar */
-- 
Homepage: http://www.sesse.net/



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