Re: scoring differences between bitmasks - Mailing list pgsql-general

From Todd A. Cook
Subject Re: scoring differences between bitmasks
Date
Msg-id 43402A95.5090701@blackducksoftware.com
Whole thread Raw
In response to scoring differences between bitmasks  (Ben <bench@silentmedia.com>)
List pgsql-general
Hi,

It may be that I don't understand your problem. :)

Are you searching the table for the closest vector?  If so, is
"closeness" defined only as the number of bits that are different?
Or, do you need to know which bits as well?

-- todd


Ben wrote:
> Hrm, I don't understand. Can you give me an example with some
> reasonably sized vectors?
>
> On Oct 2, 2005, at 10:59 AM, Todd A. Cook wrote:
>
>> Hi,
>>
>> Try breaking the vector into 4 bigint columns and building a multi-
>> column
>> index, with index columns going from the most evenly distributed to  the
>> least.  Depending on the distribution of your data, you may only  need 2
>> or 3 columns in the index.  If you can cluster the table in that  order,
>> it should be really fast.  (This structure is a tabular form of a  linked
>> trie.)
>>
>> -- todd
>>
>>
>> Ben wrote:
>>
>>> Yes, that's the straightforward way to do it. But given that my
>>> vectors are 256 bits in length, and that I'm going to eventually
>>> have  about 4 million of them to search through, I was hoping
>>> greater minds  than mine had figured out how to do it faster, or  how
>>> compute some  kind of indexing....... somehow.


pgsql-general by date:

Previous
From: Ben
Date:
Subject: Re: scoring differences between bitmasks
Next
From: Martijn van Oosterhout
Date:
Subject: Re: 8.1 'make check' fails