FW: [Qgis-developer] Reverse Changeset 13499

[Chris Crook]

Hi Jürgen

Interesting point about top posting .. .  It's just the way MS Outlook sets up the reply, and I just go along with it. I'd never really thought about it before .. And here I go doing it again.

The answer to the "where" clause is, yes, sometimes.  And sometimes not!  Basically we have large datasets (national cadastral dataset, 3.5 million polygons for parcels, 19 million vectors for boundary dimensions etc, approximate counts!).  We use Qgis for investigative visualisation as well as in production processes using plugins etc.  In both instances we may be using where clauses to access subsets of the data (for example to exclude road/railway parcels from the view).  

We are generally using scale based rendering, so that we never view these layers over the whole country at once. In effect the main "where" clause is always the bounding box of the current view, generally a very small part of the full dataset.  We aren't really interested in how many features there are over the whole country.  And we don't want to wait for postgres to count them before we get to see the bit we are actually interested in.

Postgres/Qgis manages this scenario brilliantly provided that the postgres provider isn't trying to calculate statistics or other information over the whole dataset (or the whole dataset restricted by a non-spatial where clause).  

So that is the premise on which the estimated metadata option is based.  If you call it a "no non-spatial query" option may that would make more sense?

I hope that makes it clearer what why we are very keen on reversing or modifying the change!

Cheers
Chris

-----Original Message-----
From: Jürgen E. Fischer [mailto:jef at norbit.de]
Sent: Thursday, 20 May 2010 9:14 a.m.
To: Chris Crook
Subject: Re: [Qgis-developer] Reverse Changeset 13499

Hi Chris,

On Thu, 20. May 2010 at 07:02:33 +1200, Chris Crook wrote:
> Maybe the ideal solution is to retain the speed, but make it obvious 
> that the feature count is wrong by displaying it as zero (or not 
> displaying it at all) rather than the potentially very inaccurate estimated value.

Ok, so are you trying to prove all my points against top posting or are you intentionally avoiding to tell me whether or not you are using a where clause?
;)


Jürgen

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