From: Craig James <craig_james@emolecules.com>
To: Alvaro Herrera <alvherre@commandprompt.com>
Cc: pgsql-performance@postgresql.org
Subject: Re: LIKE search and performance
Date: Thu, 24 May 2007 15:08:16 -0700
Message-ID: <46560CD0.404@emolecules.com> (raw)
In-Reply-To: <20070524214654.GR4320@alvh.no-ip.org>
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<4655E64B.5050503@hagander.net>
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<1180040560.31471.262.camel@archimedes>
<46560264.2020406@emolecules.com>
<20070524214654.GR4320@alvh.no-ip.org>
Alvaro Herrera wrote:
>> Just out of curiosity: Does Postgress store a duplicate of the data in the
>> index, even for long strings? I thought indexes only had to store the
>> string up to the point where there was no ambiguity, for example, if I have
>> "missing", "mississippi" and "misty", the index only needs "missin",
>> "missis" and "mist" in the actual index.
>
> What would happen when you inserted a new tuple with just "miss"? You
> would need to expand all the other tuples in the index.
That's right. This technique used by some index implementations is a tradeoff between size and update speed. Most words in most natural languages can be distinguished by the first few characters. The chances of having to modify more than a few surrounding nodes when you insert "miss" is small, so some implementations choose this method. Other implementations choose to store the full string. I was just curious which method Postgres uses.
Craig
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To: pgsql-performance@postgresql.org
Cc: craig_james@emolecules.com, alvherre@commandprompt.com
Subject: Re: LIKE search and performance
In-Reply-To: <46560CD0.404@emolecules.com>
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