Thanks, that's true. I think the performance benefit with be good
because I have approx 10.000 building_type's (although, that's a lot
of indices to choose between). A typical distance query for a specific
building_type will result in 10-100 matches. Not using partial
indexing will thus result in appox 500.000 rows for each query (all
building types), which I would then have to filter on building_type,
reducing the resultset with a factor 10.000
Giving it some more thought, I think splitting the building table into
individual building_type tables (and attaching spatial indices to
those) might be the best solution.
From a technical point, I think the postgresql CREATE INDEX should
support multiple USING to allow for mixed patial / non-spatial
indices, something like this:
CREATE INDEX blabla ON buildings (USING HASH(building_type), USING
GIST(the_geom))
internally that could be rewritten as
CREATE INDEX blabla_hash_value1 ON buildings USING GIST(the_geom)
WHERE hash(building_type)=value1
CREATE INDEX blabla_hash_value2 ON buildings USING GIST(the_geom)
WHERE hash(building_type)=value2
..
On Mar 18, 2009, at 12:43 PM, Kemal Bayram wrote:
Well according to EXPLAIN a partial index is taken into account, how
much of
a performance benefit you gain ultimately depends on how many
records lie
within your average distance.
If your data set is relatively static you may also want to consider
clustering your table too.
-----Original Message-----
From: [email protected]
[mailto:[email protected]] On
Behalf Of M.A. (Thijs) van den Berg
Sent: 18 March 2009 13:00
To: PostGIS Users Discussion
Subject: Re: [postgis-users] Can I build an index on combined
non-spatial&spatial columns?
Thanks :)
Does PostGIS support partial indices?
http://www.postgresql.org/docs/8.3/interactive/indexes-partial.html
If so.. I could do something like this:
CREATE INDEX building_type_spindx_school ON buildings USING
GIST(the_geom) WHERE building_type='School';
CREATE INDEX building_type_spindx_bar
ON buildings
USING GIST(the_geom)
WHERE building_type='Bar';
...
Yet another option would be to partition the buildings table
into child tables per building_type (ie have child table
containing all the schools, another child table having all
the bars), and have individua spatial indices on the child
tables. Is that an option?
On Mar 18, 2009, at 11:36 AM, Kemal Bayram wrote:
I don't know about combining indexes but you want to
include a spatial
operator such as && in addition to Distance so that you can take
advantage of an index on "the_geom".
-----Original Message-----
From: [email protected]
[mailto:[email protected]] On
Behalf Of
M.A. (Thijs) van den Berg
Sent: 18 March 2009 11:57
To: [email protected]
Subject: [postgis-users] Can I build an index on combined
non-spatial
&spatial columns?
Suppose I want to speedup the following type of queries with an
index:
SELECT the_geom, building_name
FROM buildings
WHERE Distance(the_geom, 'POINT(100312 102312)') < 5000 AND
building_type = 'School'
Is that possible, and if not, why not? My current option
is to write
my own index storage in C++, using a hashmap to filter
building_type,
and build separate spatial indices for each building type.
Can I do
something similar in postgresql?
Would my only option be to split the building_types into separate
tables?
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