[postgis-users] replicating features on self-join

Martin Davis mtnclimb at telus.net
Tue Sep 25 21:19:46 PDT 2012

This is one of those cases that SQL seems to make unnecessarily 
difficult.  Writing this query in a procedural way would be trivial, so 
why is it so hard to express in a declarative relational way?

DISTINCT ON (a.id) is the classic non-standard Postgres way of handling 

The other hack to get uniqueness of the driving table is to GROUP BY 
a.id.  You then need to use appropriate aggregation functions to 
maintain the dependent columns from a.  You might be able to use MAX on 
newclass.  MAX might work for the other invariate a columns too, but it 
might be faster to use FIRST. This is not built into Postgres, but is 
available as an extension:


Or, you could just SELECT a.id, MAX(newclass) and then join back to a to 
recover the dependent columns.

No idea whether any of these ideas will be performant.  Maybe someone 
else has a more clever way of doing this.

On 9/25/2012 10:48 AM, William Kyngesburye wrote:
> I'm trying to do a series of joins to classify some lines based on other lines they are touching in the same table and whether they are in polygons in another table, and lines are replicating.
> in a simplified form, it's something like (I imported with GDAL, so all my geometry fields are wkb_geometry):
> select a.wkb_geometry, a.id, a.name, a.class,
>    case
>      when a.class = 'a' and b.class is not NULL and (d1.newclass = 'b' or d2.newclass = 'b') then 'b'
>      ...
>      else a.class
>    as newclass
> from source as a left outer join polys as b st_coveredby(a.wkb_geometry, b.wkb_geometry)
> left outer join source as c1 on st_touches(st_startpoint(a.wkb_geometry), c1.wkb_geometry)
> left outer join source as c2 on st_touches(st_endpoint(a.wkb_geometry), c2.wkb_geometry)
> All lines are replicated 4-9 times (maybe more that I didn't notice), each duplicate with different newclass values though some newclass values duplicated also (I suppose dependent on which join it came from).
> How can I stop the replication?  Probably some SQL thing, the different join types still confuse me some.
> Adding a DISTINCT on the id to the select (and a sort to make that work) adds a LOT to the processing time processing time.
> -----
> William Kyngesburye <kyngchaos*at*kyngchaos*dot*com>
> http://www.kyngchaos.com/
> "Oh, look, I seem to have fallen down a deep, dark hole.  Now what does that remind me of?  Ah, yes - life."
> - Marvin
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