Re: Very specialised query

On Mon, 30 Mar 2009, Віталій Тимчишин wrote:
> What is the bad plan? Is it like the first plan from your first message?

It's the plan a few messages back. The UNION ALL query I showed 
effectively got the database to do it both ways round.

It's the case that a "between" index scan will return much fewer rows than 
an open-ended index scan.

> BTW: About aggregates: they can return arrays, but I can't imagine what you can group by on... May be windowing functions from 8.4
> could help.

A normal function seems the best way to go about this - they can return 
multiple rows.

So, I have written a plpgsql function to calculate overlaps. It works 
reasonably quickly where there aren't that many overlaps. However, it 
seems to go very slowly when there are a large number of rows to return. I 
am considering recoding it as a C function instead.

1. The docs say that returning multiple rows from plpgsql waits until the
     whole lot are done before returning any. Does this happen with the C
     functions too?
2. What sort of speedup would I be likely to see?
3. How do I RAISE NOTICE in a C function?

> Also, if your maximum length (select max(end-start) from location) is low enough, you can try adding some more constraints to make
> optimizer happy (have it more precise row count to select correct plan).

Alas:

select min(start), max(start), min(end), max(end), max(end - start) from location;

  min |   max    | min |   max    |   max
-----+----------+-----+----------+----------
    1 | 61544858 |   1 | 61545105 | 21512431
(1 row)

Matthew

-- 
 I suppose some of you have done a Continuous Maths course. Yes? Continuous
 Maths?  Whoah, it was like that, was it!
                                        -- Computer Science Lecturer