Re: [GENERAL] Understanding and implementing a GiST Index

postgres=# SELECT smlar(
postgres(# '{1,0,0,0, 1,1,1,1, 1,1,1,0, 0,1,0,0}'::int[],
postgres(# '{1,0,0,1, 0,1,0,0, 0,1,0,0, 0,1,1,0}'
postgres(# );
 smlar
-------
     1
(1 row)

postgres=# SELECT smlar(
postgres(# '{1,0,0,0, 1,1,1,1, 1,1,1,0, 0,1,0,0}'::int[],
postgres(# '{1,0,0,1, 0,1,0,0, 0,1,0,0, 0,1,1,0}'
postgres(# ); smlar
-------- 0.4375
(1 row)

postgres=# SELECT smlar(
'{0,1,1,0}'::int[],
'{1,0,1,0}'
); smlar
-------   0.5
(1 row)

Just quick recommendation.
You need to ask -hackers mailing list for that. Also, do you want GiST or SP-GiST ?

We already use hamming distance in rd-tree, which implemented in GiST, see our presentation http://www.sai.msu.su/~megera/postgres/talks/pgcon-2012.pdf, for example.

Oleg

On Thu, Oct 9, 2014 at 11:09 AM, Connor Wolf <wolf@imaginaryindustries.com> wrote:

Hi there!

I'm trying to implement a custom indexing system using the GiST index framework, and I have a few questions.
Basically, I'm trying to implement a system that allows me to search across a set of 64 bit integers by hamming distance. This is intended to be used in searching for similar images, where the 64 bit integer is actually a phash of an image, and similarity between two images is reflected in the hamming distance between two integers.

Anyways, The appropriate approach here is to use something called a BK tree, for which I've written some test code and I think I have a decent grip of (my test code seems to work, in any event).

That said:

• Is there a simple piece of example-code for implementing a GiST index for a single index? I've been looking through the /contrib/ directory, particularly the /contrib/btree_gist/ files, but it looks like 90% of the complexity there is related to supporting all the column types Postgres has, rather then actually tied to producing a functional index.
  
• Once I have something compiling, how can I check to be sure that I'm actually using the indexing module I created? I can enable my compiled extension using `CREATE EXTENSION`, but is there an option for `CREATE INDEX test_index ON testing USING gist (val);` that lets me specify *which* GiST index is actually employed? Is this even a valid question?
  This is probably something that's available in one of the system tables somewhere, but I haven't had much luck with google finding out where.
• Testing: What's the appropriate way to examine the generated tree structure of the index? I certainly went through a few bugs with my test tree system (and that was in python!). Is there any way to examine the index structure for debugging purposes?
• Also, it looks like `ereport()` is the proper way to emit debugging information. Is this correct?
• In that vein, is there any way to have information that is on the operations of an entire query? Looking at the number of tree nodes touched for a scan would be nice (and I would not be surprised if there is already a facility for it).

Project code is here if anyone is interested, any help would be great. I have very little idea what I'm doing.

Thanks,
Connor