[postgis-users] Hints or tips on Large Intersect

[Ben Jubb]

If you are comfortable with building your own version of PostGIS, you 
could get the latest development version from the SVN repository (you'll 
need to get the latest dev version of GEOS too),  and try using the new 
'prepared' predicate ST_IntersectsPrepared.  This is a replacement for 
the intersects() function in your code, that (depending on the 
geometries involved) could speed up that test by 100s of times.  It 
optimized for the case when the first argument is changing slowly, and 
the second argument is changing every invocation.

What do you think is bottleneck in your code though?  Is it the 
intersection() operation or the intersects() test?  It depends on the 
distribution of your data, and the odds of any pair of geometries 
intersecting.

b

Stephen Woodbridge wrote:
> Yeah, this sounds like it will run for a very very long time. A couple 
> of thoughts that have more to do with managing the process:
>
> 1) make sure you run an is_valid() check on both tables and remove/fix 
> any geometries that are not valid. It is a pain when you hit one of 
> these and it nukes your transaction or crashes the server and you have 
> to start over.
>
> 2) you might want to break this into multiple queries based on some 
> subset of the record in the smaller table. Like do 1-10000, 
> 10001-20000, etc. This would allow you to get the results of each 
> commited so a restart would be less painful, also this would allow you 
> to get some timing statistics to better predict how long the remainder 
> of the rows will take.
>
> -Steve
>
> niels hoffmann wrote:
>> Hi,
>>  
>> I am fairly new to Postgis so I like some feedback whether I am going 
>> through the right moves.
>> I am trying to create a new table with the intersected results from 
>> two input polygon tables.
>> Both tables are in NZMG (2193) the first table has 100000+ records, 
>> the second table has 400000+ records.
>> The query I am using is:
>>  
>> Create table ablc_pol with OIDS as
>> SELECT intersection(a.geom, l.geom) as intersect_geom, a.*, 
>> l."CLASS",l."NAME", l."REPLID"
>> from first_table a, second_table l
>> Where a.geom && l.geom
>> AND intersects(a.geom, l.geom);
>>  
>> Currently this query is taking >200 hours before I cancelled it 
>> because I wasn't sure it would ever end. However, running it on a 
>> small subset showed satisfactory results...
>> I am using version 1.2 on Windows.
>> Does it matter which table I put first in the query or would the 
>> optimizer take care of that?
>>  
>>  
>> Cheers,
>> Niels
>>  
>>
>>
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