What is ST_Dump, Dump
ST_Dump is a function that takes a geometry and returns a set of Postgis geometry_dump structure. Geometry_dump is composed of 2 properties. geom and path. The geom is a geometry and path is a multi-dimensional integer array that has the exact location of the geom in the MULTI, Geometry Collection. For MULTI geometries, the path is one dimensional looking like {1}. For single geometries, the path is an empty array. For GeometryCollection it has multiple array elements depending on the complexity of the Geometry Collection.
ST_Dump comes in very handy for expanding a MULTI geometry into single geometries. Below is an example. That expands a set of MULTIPOLYGONs into single POLYGONs
SELECT somefield1, somefield2,
(ST_Dump(the_geom)).geom As the_geom
FROM sometable
It comes in very handy in conjunction with ST_Collect. ST_Collect will return a MULTI geom if you give it single geoms, but will return a Geometry collection if you try to feed it MULTI geometries. In general ST_Collect is faster than ST_Union. To take advantage of the speed and simplicity of ST_Collect without generation of GeometryCollections, you can expand your MULTIPOLYGONS, MULTILINESTRINGS, MULTIPOINTS into single geoms and then recollect them according to the grouping you desire. If you want to regroup a set of MULTI.. Into a new grouping, you can do something like the below.
SELECT stusps,
ST_Multi(ST_Collect(f.the_geom)) as singlegeom FROM (SELECT stusps,
(ST_Dump(the_geom)).geom As the_geom FROM somestatetable ) As f
GROUP BY stusps
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