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Re: Hash twice

From: Robert Haas <robertmhaas(at)gmail(dot)com>
To: Simon Riggs <simon(at)2ndquadrant(dot)com>
Cc: PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Hash twice
Date: 2013-01-14 19:12:24
Message-ID: CA+TgmoYB0L8oDLopPC81s63NGOvqzyaY4o_kRH09_wgEpJnWUw@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-hackers
On Sat, Jan 12, 2013 at 11:39 AM, Simon Riggs <simon(at)2ndquadrant(dot)com> wrote:
> Lock code says it calculates "hash value once and then pass around as needed".
>
> But it actually calculates it twice for new locks.
>
> Trivial patch attached to make it follow the comments in
> LockTagHashCode and save a few cycles.

Hmm.  This is a nice idea, but it doesn't look right to me, because
you're searching LockMethodLocalHash with a hash code intended to be
used in LockMethodLockHash, and the two hashing schemes are not
compatible, because the former is hashing a LOCALLOCKTAG, and the
latter is hashing a LOCKTAG, and both are just using tag_hash.

On the flip side if I'm wrong and the hashing schemes are compatible,
there are other places in the file where the same trick could be
employed.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company


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  • Hash twice at 2013-01-12 16:39:15 from Simon Riggs

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Subject: Re: Hash twice
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