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
