[postgis-users] mechanix of a query

[Pavel Iacovlev]

1)
I would start investigating from MapServer. What are your layer values for
TOLERANCEUNITS pixels
TOLERANCE 10 (your value here)

2)
Do you debug your requests with firebug ?

Your speed should not be constant, you should get faster responses for
smaller bboxes.

On Sun, Sep 20, 2009 at 4:28 PM, P Kishor <punk.kish at gmail.com> wrote:
> On Sun, Sep 20, 2009 at 2:47 AM, Pavel Iacovlev
> <iacovlev.pavel at gmail.com> wrote:
>> You can enable debug of Mapserver
>> http://mapserver.org/development/rfc/ms-rfc-28
>>
>> You can configure PostgreSQL to log all your queries. Here is a tool
>> to analyze them and how to make PostgreSQL log them
>> http://pgfouine.projects.postgresql.org/tutorial.html
>>
>> Mapserver computes the current bbox view and sends the query to
>> PostGIS and then you get all the geometries that are inside that bbox.
>> Thats why you should create a GisT index to speed things up.
>>
>
> Thanks for the logging info. I will do so. Actually, I do have a Gist
> index on my layer. It is a simple, lat/lon grid over my study area. I
> have a WMSGetFeatureInfo control in OpenLayers that returns the xmin,
> ymin, xmax, ymax for the grid square containing the user's mouse
> click.
>
> When I am zoomed out (example queries below), the bbox is
>
> xmax: -88.026581        ymax: 45.970243
> xmin: -89.435577        ymin: 45.325117
>
> The result comes back immediately.
>
> When I am zoomed in, the result never comes back. The bbox sent in the query is
>
> xmax: -88.643017        ymax: 45.688914
> xmin: -88.819141        ymin: 45.608274
>
>
> As you can see, the bbox in the second query is very small, and my
> guess is, no geometry is found within that bbox. That itself is
> erroneous, however, the funny thing is, I don't even get a "0" result.
> There is simply no response from the server, and my user interface
> just hangs, waiting for a response to WMSGetFeatureInfo.
>
> From the user's perspective, this is confusing. Since I am requesting
> the info for the grid square in which my click is contained, its speed
> should be constant, no matter how much I am zoomed in or out. After
> all, the mouse can have only one click x,y, and that click can only be
> contained within one grid square, no matter what my zoom level. Of
> course, when I am zoomed out, then the precision of which grid square
> my click falls within is very coarse, and I may not get the correct
> grid square back, however, when I am zoomed in, I should be able to
> more accurately click within the grid square I desire. So, actually,
> zooming in is, from the user's perspective, a desired state.
>
> I am not sure which technology here is at fault. I have OpenLayers,
> MapServer and PostGIS in play, and something is failing to work as
> intuited.
>
> Any suggestions?
>
>
>
>
>> On Sun, Sep 20, 2009 at 7:15 AM, P Kishor <punk.kish at gmail.com> wrote:
>>> I am trying to understand the mechanics of a getfeatureinfo query. My
>>> setup is data in Pg/PostGIS, served by MapServer via WMS, with
>>> OpenLayers in the front. When I click on a map and send a
>>> GetFeatureInfo query, MapServer grabs the x,y sent in the URL, and,
>>> for the layer (also sent in the URL), finds the row in the table which
>>> contains the x,y of my click.
>>>
>>> How does MapServer do that?
>>>
>>> Does it run a Pg/PostGIS query for this?
>>> How can I find out the syntax of this query?
>>>
>>> If a GetFeatureInfo takes too long, what can I do to speed it up?
>>>
>>>
>>> --
>>> Puneet Kishor
>>> _______________________________________________
>>> postgis-users mailing list
>>> postgis-users at postgis.refractions.net
>>> http://postgis.refractions.net/mailman/listinfo/postgis-users
>>>
>>
>>
>>
>> --
>> http://iap.md, The future is open
>>
>
>
>
> --
> Puneet Kishor http://www.punkish.org
> Carbon Model http://carbonmodel.org
> Charter Member, Open Source Geospatial Foundation http://www.osgeo.org
> Science Commons Fellow, http://sciencecommons.org/about/whoweare/kishor
> Nelson Institute, UW-Madison http://www.nelson.wisc.edu
> -----------------------------------------------------------------------
> Assertions are politics; backing up assertions with evidence is science
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