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Cube extension improvement, GSoC

From: Stas Kelvich <stanconn(at)gmail(dot)com>
To: pgsql-hackers(at)postgresql(dot)org
Subject: Cube extension improvement, GSoC
Date: 2013-03-22 23:10:38
Message-ID: 6A7E75B1-64DD-4F5F-A991-435E3E5A24FB@gmail.com (view raw or flat)
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Lists: pgsql-hackers
Hello,

some time ago I started working on the data search system (about 100-200M of records) with queries consisted of several diapason and equality conditions, e.g.:

  WHERE dim1 BETWEEN 128 AND 137 AND
  WHERE dim2 BETWEEN 4815 AND 162342 AND
  WHERE dim3 = 42
  ORDER BY dim1 ASC

There are 6 or 7 search criteria in my task. In order to avoid full table scan I started using R-Tree over cube data type:

  CREATE INDEX ON my_table USING GiST(cube(array[dim1, dim2, dim3]))

For fast sorting I used Alexander Korotkov's patch for knngist (http://www.mail-archive.com/pgsql-hackers(at)postgresql(dot)org/msg153676.html), which changes metric for nearest neighbors search and allows to obtain cubes ordered by a specific coordinate. Having changed some data types and function/operator definitions I ported this to 9.2 (https://github.com/kelvich/postgres/commit/bb372). Then, after this I could select ordered records right from the index:

  SELECT * FROM my_table
  WHERE cube(array[dim1, dim2, dim3]) <@ cube '(128,4815,42),(137,162342,42)'
  ORDER BY cube(array[dim1, dim2, dim3]) <*> 5;

The main issue of such approach is the index size. In my case it was about 100GB for 100M of records. Therefore index building becomes very expensive disk-related operation. For the same reason reading a large number of records from the index is slow too.

I came to Oleg Bartunov, Theodor Sigaev and after a while to Alexander Korotkov for any help. (I'm very thankful to them and glad that they agreed to meet, listen to me and give useful advices). Having discussed it we decided that there was several possible improvements for the cube extension:

  * Adding point data type support to the cube extension in order to avoid storing of coincident upper left and lower right vertices, which may reduce the volume that leaf nodes occupy almost twice.
  * Checking the split algorithm with big datasets and, if possible, improving it.
  * Learning cube extension to store dimensions with different data types. Such index would be good alternative to compound key B-Tree multi-index (suitable for diapason queries and data ordering).
  * Providing support for KNN with metrics induced by the different norms: euclidean, taxicab norm, p-norm. This can be useful in fields where we can extract signature: similar images search, similar audio search.

I'd like to participate in GSoC with this improvements, and I'm very interested in any comments or suggestions about this feature list.



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