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From: Heikki Linnakangas <heikki.linnakangas@enterprisedb.com>
To: Alexander Korotkov <aekorotkov@gmail.com>
Cc: Robert Haas <robertmhaas@gmail.com>
Cc: pgsql-hackers <pgsql-hackers@postgresql.org>
Subject: Re: WIP: Fast GiST index build
Date: Thu, 01 Sep 2011 12:37:42 +0300
Message-ID: <4E5F5266.4010602@enterprisedb.com> (raw)
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On 01.09.2011 12:23, Alexander Korotkov wrote:
> On Thu, Sep 1, 2011 at 12:59 PM, Heikki Linnakangas<
> heikki.linnakangas@enterprisedb.com>  wrote:
>
>> So I changed the test script to generate the table as:
>>
>> CREATE TABLE points AS SELECT random() as x, random() as y FROM
>> generate_series(1, $NROWS);
>>
>> The unordered results are in:
>>
>>           testname           |   nrows   |    duration     | accesses
>> -----------------------------+**-----------+-----------------+**----------
>>   points unordered buffered   | 250000000 | 05:56:58.575789 |  2241050
>>   points unordered auto       | 250000000 | 05:34:12.187479 |  2246420
>>   points unordered unbuffered | 250000000 | 04:38:48.663952 |  2244228
>>
>> Although the buffered build doesn't lose as badly as it did with more
>> overlap, it still doesn't look good :-(. Any ideas?
>
>
> But it's still a lot of overlap. It's about 220 accesses per small area
> request. It's about 10 - 20 times greater than should be without overlaps.

Hmm, those "accesses" numbers are actually quite bogus for this test. I 
changed the creation of the table as you suggested, so that all x and y 
values are in the range 0.0 - 1.0, but I didn't change the loop to 
calculate those accesses, so it still queried for boxes in the range 0 - 
100000. That makes me wonder, why does it need 220 accesses on average 
to satisfy queries most of which lie completely outside the range of 
actual values in the index? I would expect such queries to just look at 
the root node, conclude that there can't be any matching tuples, and 
return immediately.

-- 
   Heikki Linnakangas
   EnterpriseDB   http://www.enterprisedb.com



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