Re: why is gist index taking so much space on the disc

On Mon, 21 Nov 2005, Martijn van Oosterhout wrote:

> On Mon, Nov 21, 2005 at 08:14:44PM +0100, Grzegorz Jaskiewicz wrote:
>>> You mean you sometimes put the same elements in the two halves? You
>>> shouldn't do that. The whole point is that the search will descend any
>>> node that matches consistant, but any single key should only appear
>>> once in each index.
>>>
>>> picksplit should *split* the set, not return two sets about the same
>>> size as you started...
>>
>> Nope, I mean that 'masks' created to match either 'half' sometimes
>> match elements in the other one.
>> This shouldn't be a big deal, just one level to go down on query to
>> much more specific result set.
>> I have fixed that with, somewhat hack.
>
> It's not a hack, that's how it's supposed to work. An entry should only
> appear once in the index, but it could appear in multiple places. Like
> you say, some entries can go into either half.
>
> B-Trees are the rather special case that you can always split a set of
> values into two non-overlapping sets. With geometric types (like your
> bitmasks) you can't avoid overlap sometimes so you have to follow
> multiple branches to check if an element is there or not.
>
> Your pseudo code is good and will work fine. However, ideally you want
> to divide the overlap in such a way that later splits work better.
> Maybe by trying to decide which mask is "closer".
>
> The better the splitting the more efficient your tree will become.
> Ofcourse, perfect splitting may be expensive but then it depends on how
> many inserts vs how many selects. If you do a lot of searches it may be
> worth the time.

Martijn is perfectly right here. You, probably, need to read a bit
some classical papers, for example,
"R-TREES: A dynamic index structure for spatial searching" by Antonin 
Guttman.



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Oleg Bartunov, sci.researcher, hostmaster of AstroNet,
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