[postgis-users] Huge query, out of memory

[Nicklas Avén]

Great I have several different versions but I think the one I sent is the one best working. I have tried it on a big table and as far as I recall this version does it right. I forgot to mention:I see that I have used both CTE and window functionSo you will need PostgreSQL 8.4 to get it working /Nicklas 
2010-01-15 Nicolas Gillet - MARKET-IP wrote:

>
Hello,
 
 
 
Thank you for this input.
 
 
 
At a first glance I did not understand it fully but Ill take time to go deeper.
 
 
 
Indeed a tutorial might be useful for several users as I think merging road elements is a redundant issue for GIS users.
 
 
 
Nicolas.
 
 
 
>
>

De : postgis-users-bounces at postgis.refractions.net [mailto:postgis-users-bounces at postgis.refractions.net] De la part de Nicklas Avén
>Envoyé : jeudi 14 janvier 2010 14:38
>À : PostGIS Users Discussion
>Objet : Re: [postgis-users] Huge query, out of memory
 
 
 
Hallo 
 
 
 
 
 
I have been struggling with the same task. I don't know if I have solved it because I ran out of time a few months ago. But I attach the sql as a file with some descriptive texts. I intend to check it more properly and write some tutorial, but that will take some time before I can do.
 
 
 
So, no guarantees :-)
 
 
 
/Nicklas
>
> 2010-01-14 Nicolas Gillet - MARKET-IP wrote:
>
> >
 
Hello,
 
I have a big shape file to process (about 300 Mo,  1.5 million records)
 
I have to aggregate the touching linestrings having the same attributes therefore the best way I could find (with help of this mailing list) was to aggregate all the linestrings with the same attribute and then splitting the ones that dont touch.
 
Here is the query :
 
SELECT subQ.id, subQ.feattyp, subQ.frc, subQ.shieldnum, subQ.name, null, (st_dump(merged_geom)).geom AS the_geom
 
FROM (
 
                SELECT MAX(id) AS id, feattyp, frc, MAX(shieldnum) AS shieldnum, nw.name, ST_LineMerge(ST_Collect(the_geom)) AS merged_geom
 
                FROM nw
 
                WHERE nw.frc >= 0
 
                GROUP BY nw.name, nw.feattyp, nw.frc
 
) AS subQ;
 
This has worked fine on smaller shapes  but on this one I get an out of memory error.
 
I used pgTune to change database default settings and the result is as follow :
 
#custom_variable_classes = ''                   # list of custom variable class names
 
default_statistics_target = 50 # pgtune wizard 2010-01-13
 
maintenance_work_mem = 120MB # pgtune wizard 2010-01-13
 
constraint_exclusion = on # pgtune wizard 2010-01-13
 
effective_cache_size = 1408MB # pgtune wizard 2010-01-13
 
work_mem = 12MB # pgtune wizard 2010-01-13
 
wal_buffers = 8MB # pgtune wizard 2010-01-13
 
checkpoint_segments = 16 # pgtune wizard 2010-01-13
 
shared_buffers = 480MB # pgtune wizard 2010-01-13
 
max_connections = 80 # pgtune wizard 2010-01-13
 
All the other parameters have their default values.
 
What can I do to run my script through this heavy shape files ?
 
Thank you
 
Nicolas.
 
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