Re: knngist questions

> 1. Is KNNGIST intended to work if there's more than one pathkey? If
> so, how? Example:
>
> SELECT * FROM tab ORDER BY this_point<-> '(0,0)', this_point<-> '(1,1)'

Why not, if distances from two points to '(0,0)' are equal, it's need to compare
distances to '(1,1)'. Nothing new here, KNN-GiST supports it, that a reason why
StackElem struct has variable-size array for distances.

In practice, it could be used for finding closest restaurant with some fish in
its name (pg_trgm has a support of disttance):
SELECT * FROM restaurants ORDER BY MY_COORDS <-> r_coords, 'fish' <-> r_name;

> 2. I'm concerned by the fact that the new consistent() methods only
> return 0 or -1.0 for non-leaf pages. If we have a query like:
M-m, it's returns 0 or -1.0 only for operation from WHERE clause, for ORDER BY
operation it should return >= 0 value. That's why consistentFn shoulf be able to
distinguish source of operation (actually, it's need because now we allow
boolean distances, if distance could not be a boolean then operation could not
come from WHERE clause)

> SELECT * FROM tab WHERE this_point<-> '(0,0)'

For point, it's incorrect query: non-boolean operations could not present in
WHERE clause.

> you'd need to have the consistent function for each page return a
> minimum and maximum distance to '(0,0)'. If you have one page that
Only minimum distance which guarantees that in it's sub-tree there is no more
close point. For R-tree it's rather obvious because keys on inner pages are
actually a bounding box of its subtree.

> has a minimum distance of 0 and a maximum distance of 100 and another
> page which has a minimum distance of 101 and a maximum distance of
> 200, you don't even need to look at the second page until all the keys
> from the first one have been processed. If there's a third page with
Right, it works so, until I made a big mistake in code. But performance test
allows me to believe that I didn't :)

It's read a page (for the start - root page) and puts all keys in binary tree
ordered by distance and then takes left-most key from binary tree. If that key
is pointer to heap, GiST returns it to postgres, if it's a pointer to index's
page then repeat loop with new page.

> tree up front. If you are already doing something like this somewhere
> in the code, please point me in the right direction...

getNextNearest() function: it gets next pointer from tree with a help of
getNextDataPointer(), and then, depending of pointer type (to index or heap
page) returns a pointer or call processPage() which puts new pointers into tree.

May be, not obvious thing that all pointers on the same distance are stored in
single binary tree node (struct StackElem). Each pointer is represented by
DataPointer which contains information needed for support concurrency. List of
that structs is keeped "semi-ordered": pointer to heap are always in the
beginning of list to increase response time and memory requirement.

>
> 3. I've been scratching my head over the following bit of code and it
> doesn't make any sense to me. As far as I can tell, this is
> effectively comparing the number of columns in the ORDER BY clause to
> the number of restriction clauses applicable to the relation being
> scanned. Those two quantities don't seem to have much to do with each
> other, so either I'm confused or the code is. It doesn't seem like it
Oops, it seems to me that's artefact of developing, sorry. I'm not very familiar
with planner/optimizer so that required a lot of my brain and I just missed that
after another attempt to get it work.

> should matter anyway, since I don't think we're planning on any AMs
> being both amoptionalkey and amcanorderbyop.

Agree.
--
Teodor Sigaev E-mail: teodor(at)sigaev(dot)ru
WWW: http://www.sigaev.ru/