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Re: Proposal: String key space for advisory locks

From: Kenneth Marshall <ktm(at)rice(dot)edu>
To: Christophe Pettus <xof(at)thebuild(dot)com>
Cc: Itagaki Takahiro <itagaki(dot)takahiro(at)oss(dot)ntt(dot)co(dot)jp>,Josh Berkus <josh(at)agliodbs(dot)com>,Pgsql Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Proposal: String key space for advisory locks
Date: 2009-10-27 12:52:58
Message-ID: 20091027125258.GH16724@it.is.rice.edu (view raw or flat)
Thread:
Lists: pgsql-hackers
On Mon, Oct 26, 2009 at 06:35:13PM -0700, Christophe Pettus wrote:
>
> On Oct 26, 2009, at 5:24 PM, Itagaki Takahiro wrote:
>
>> Hmmm, hashtext() returns int32. ,
>> Can you reduce the collision issue if we had hashtext64()?
>
> That would certainly reduce the chance of a collison considerably, assuming 
> the right algorithm.
>
> --
> -- Christophe Pettus
>    xof(at)thebuild(dot)com
>
The current hash function can already support generating a 64-bit
hash value by using both the b and c values. The function is called
hashlittle2 and has this comment in the original Bob Jenkins 2006
code:

/*
 * hashlittle2: return 2 32-bit hash values
 *
 * This is identical to hashlittle(), except it returns two 32-bit hash
 * values instead of just one.  This is good enough for hash table
 * lookup with 2^^64 buckets, or if you want a second hash if you're not
 * happy with the first, or if you want a probably-unique 64-bit ID for
 * the key.  *pc is better mixed than *pb, so use *pc first.  If you want
 * a 64-bit value do something like "*pc + (((uint64_t)*pb)<<32)".
 */

This should be a simple change. It would be worth running it by
the developer community to figure out how to add this functionality
in a way that will make 64-bit hashes available easily to other
code in the DB, perhaps even using them in very large hash indexes.

Regards,
Ken

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