[postgis-devel] Split By function

[Obe, Regina]

I recall we were discussing a need for a split by function and that ST_GeometryN etc. is slow for large geometries. Unfortunately I don't have any humongous geometries to test with.
 
I just looked at ST_Dump (not sure why I never used it before, I guess because generate_series is more versatile.).  Anyrate it seems to me it comes very close to a split by and with range ability.  
 
I presume ST_Dump is much more efficient than ST_GeometryN since it would make only one call out.  Kevin can you test out that old generate_series one you had with dump?
 
E.g. I can do this
 

SELECT a.somefield1, a.somefield2, a.geom
FROM
(SELECT somefield1, somefield2, (ST_Dump(the_geom)).*
FROM sometablewithmulties) as a
WHERE
a.path[1] between 2 and 5;
 
To grab all geometries in all records in a MULTI between 2 and 5 slot. Granted I can't think of a use for such a thing.
 
Which allows me to wrap this in an sql function wrapper to generate a 
pgis_GeometryRangeN(start int, end int) 
 
 
or 
pgis_GeometryAll() returns geometry[]
 
similarly the below denatures a MULTIPOLYGON/LINESTRING to single POLYGON/LINESTRINGS
SELECT a.somefield, (ST_Dump(the_geom)).geom
FROM sometable as a;
 
Which allows me to wrap this in a simple SQL function that does all sorts of split by stuff.  E.g. for geometry collections I suppose it could get interesting with those multidimensional arrays.  Unfortunately I guess ST_Dump is not going to deal with single geometries and breaking into points etc.  But seems if its efficient for the other case - we can create a similar one to deal with the other cases by cutting and pasting some of the logic.
 
Thanks,
Regina
 

 
 
 


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