[postgis-users] MORE THOUGHTS ABOUT PERFORMANCE IN POSTGIS

[christopher.faulkner at ntlworld.com]

Hello Alvaro

I had a similar problem a while back. I was getting very slow response times. In my case, the selection itself was not slow, the astext() function was taking the time. Remove the astext() and see how quickly it executes. 

If it then executes quickly, you can see that spatial indexing is not going to help. 

This happened to me because I was selecting polgons or lines with many points, within a rectangular geographic area. IN most cases only a small part of the feature was required but astext() had to work on the whole geometry !

My solution was complex and inelegant... It would be nice if astext could clip to your selection area ...

Chris


> 
> From: "Alvaro Zabala" <alvaro_zabala at hotmail.com>
> Date: 2004/03/03 Wed AM 09:37:42 GMT
> To: postgis-users at postgis.refractions.net
> Subject: [postgis-users] MORE THOUGHTS ABOUT PERFORMANCE IN POSTGIS
> 
> We are making some aditional probes, and we would like to ask/argue some 
> questions:
> 
> Ive noticed that PostGIS spatial indexes are very fast when you ask concrete 
> regions. For example: when you work with a complete layer (500.000 rows) , 
> and you ask 1/15 of total region, we are recovering many thousand of rows in 
> 1000 ms (very good times).
> 
> The problem is when we ask regions near to complete region. (3/4). Im 
> speaking about the time postgis resolve the query 
> (statement.executeQuery(query) with Java JDBC). It looks that if for these 
> regions, the use of spatial index doesnt help.
> 
> What is the reason? The number of points of the table? The type of query? 
> etc.
> 
> Regards!
> 
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