[postgis-users] ST_Union (8.4) and TopologyException

[Simon Greener]

Paul and Regina,

> The "high speed" union can only kick in when run against collections
> consisting entirely of polygons. 

So I have to use ST_Accum as in:

create table simon.si_result 
as 
select i.si_grouping_f_id, ST_Difference(s.geom,i.geom) as geom
  from simon.si_grouping_f s
       inner join 
       (select si_grouping_f_id, ST_Union(ST_Accum(a.geom)) as geom
          from simon.si_grouping_f_store_f a
         where geom is not null 
         group by si_grouping_f_id
        ) as i
        on (s.id = i.si_grouping_f_id);

Result: Query returned successfully with no result in 3056063 ms.

That is just over 50 minutes which is about twice as fast as without. Not bad.

Recasting it the way Regina wanted:

> So I would write your below as
>
> create table simon.si_result
> as
> WITH i AS
> (select si_grouping_f_id, ST_Union(a.geom) as geom
>           from simon.si_grouping_f_store_f a
>          where geom is not null
>          group by si_grouping_f_id
>         )
> select i.si_grouping_f_id, ST_Difference(s.geom,i.geom) as geom
>   from simon.si_grouping_f s
>        inner join i
>         on (s.id = i.si_grouping_f_id);

Ran slowly until I put ST_Accum() in the ST_Union. 

> (select si_grouping_f_id, ST_Union(ST_Accum(a.geom)) as geom

The result was it processed in 3538468ms or 59mins.

S
-- 
SpatialDB Advice and Design, Solutions Architecture and Programming,
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