[postgis-users] Rv: Postgis for Postgres 64 bits

[Darrel Maddy]

Thanks again for the rapid response.  I have been using gdal_translate as you suggest to reconstitute images, although, as yet, I have only successfully done this when the tiled target image is in a table of its own. I am probably being a bit slow but have not yet figured out how to extract one target image from a table which contains multiple tiled images using only gdal_translate - ideally I do not want to have to create a separate table for the target image but rather simply extract the image of interest from a PG table containing all of the images (hope that makes sense).

Darrel



-----Original Message-----
From: postgis-users-bounces at postgis.refractions.net [mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of Paragon Corporation
Sent: 04 February 2012 21:44
To: 'PostGIS Users Discussion'
Subject: Re: [postgis-users] Rv: Postgis for Postgres 64 bits

 

> "First thing 1000x1000 blocks is too big.  If you are going to be 
> doing a lot of ST_Value calls, you probably want to chunk your tiles 
> to 100x100 or below.
> 
> ST_Value currently requires doing a memcopy for each call so the 
> bigger your tiles the more memcopying it will have to do.
> 
> On top of that the bigger your tiles, the less useful ST_Intersects 
> is.  Its kind of the same issue with big geometries -- like if you 
> stuffed all of africa in one record, and then were searching for a 
> city in Africa, the ST_Intersects will not be that useful."
> 
> Thanks for this. I did do a few tests. This is what I got:
> 
> Tile Size	Time
> 2000		350873ms
> 1000		235415
> 500		288298
> 300		234895	
> 200		231374
> 
> I guess I was concerned that at 200x200 the images were split across 
> 3344 rows as opposed to the 176 of the 1000x1000.  I am learning here, 
> so I will give 100x100 a go (tomorrow) - I just worry about when I 
> need to put the images back together (images will be uploaded as tiles 
> and then the originals - model outputs - will be deleted). The 
> geometry is composed of
> 100 points (this is for testing only - final configuration has yet to 
> be determined) -  I figured it would be faster if they were all on the 
> same tile, but may be not?
> 
> Thanks
> 
> Darrel
> 

Bborie probably has more info on this since he's working it and can hopefully chime in.  Right now the ST_Union you'd use in the db is not very efficient since its using plpgsql. Doing it with the gdal_translate is a lot faster and you can do basic where conditions with that like where file_name='...'.

 In 2.0 ST_Union will be done mostly in C so should be faster.

As far as reconstituting, yes and no.  If your tiles are smaller and you don't need the full tile, doing clip operations and union subset that covers a geometric region will be faster again because of the whole memcopy issue.

However if you need to export the tile -- reconsitute the original, then you are right a bigger tile would be better, though you may want to try gdal_translate as that I think is much more efficient for export.
http://www.postgis.org/documentation/manual-svn/RT_FAQ.html#id2997931

A lot fo these performance issues we'll be dealing with in the 2.1 release, but for 2.0, we are just focussing on functionality and stability.

Hope that helps,
Regina


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