Hello Neil,
One thing you can do to create some sort of tileindex that can be part
of the where clause is create a view in postGIS that contains the
boundingboxes of a table based upon an attribute:
CREATE VIEW MYBBOX AS
SELECT
table1.uniqueID,
extent(table1.the_geom) as box,
box2d(scale(extent(table1.the_geom),1.01,1.01)) as bbox
FROM
table1
GROUP BY
table1.UniqueID;
Where BOX is the exact extent and bbox is the extent + a little margin,
this I used to zoom with an area around it because my original MYBBOX
view was based on a points table.
This view could be used as a helper to build the right queries.
Wouldn't it be more performance effective
Neil Best schreef:
Last fall I got some help for the list on how to use Postgres as a
source for a tile index that gives locations of raster files on disk
and that has been working beautifully.
My new question is this: Is it possible for a tile index location to
indicate something other than a path to a file, such as a PostGIS table?
The reason that I ask is that I have implemented table partitioning
for large county-by-county (or smaller) data sets such as SSURGO and
USDA CLU. At the outset I convinced myself mistakenly that Postgres
could leverage spatial check constraints for query planning using the
constraint_exclusion = on mechanism, but it can't. It only works with
fields where the check constraint is based on a straight
equal/not-equal comparison to a constant, AFAICT. Neither &&
::geometry or ~ ::text checks work for this reason.
Getting Mapserver to leverage this advanced query planning means
adding logic to my application to inject appropriate values into a
WHERE clause in the layer's DATA query string. To do this I need some
kind of generalized layer that indicates the spatial organization of
the attributes used for the partitioning. This is essentially a tile
index, so if I could just create a layer of bounding boxes for each
table and let Mapserver decide which tables it needs to scan rather
than adding complexity to my application in order to have the query
planner do the same thing, that would be ideal.
Anybody have any thoughts or experiences to share? Thanks.
Neil
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