JDBCDataStore.getCount should get a full Query object
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Key: GEOT-2321
URL: http://jira.codehaus.org/browse/GEOT-2321
Project: GeoTools
Issue Type: Improvement
Components: data jdbc-ng
Affects Versions: 2.5.3
Reporter: Andrea Aime
Assignee: Justin Deoliveira
Fix For: 2.5.4
Currently the JDBCDataStoure count method looks like:
{code}
int getCount(SimpleFeatureType featureType, Filter filter, Connection cx)
{code}
This means the sql encoded does not have a chance to include offset and limit.
In case of tables with millions of rows a count can take minutes, but if you
add limit and offset natively, the time needed can go down very significantly.
And sometimes, not :)
I've been told Oracle can optimize this quite aggressively, but tried with
postgis, and the time did not improve, at least not with the naive approach.
I have a OSM table with 20 million lines, a {{select count(*)}} over it takes
1m20s, doing a {{select count(*) from planet_osm_line offset 10000000 limit
100}} (offset at 10M, halfway thru) takes exactly the same time and returns a
wrong result, but a {{select count(*) from (select * from planet_osm_line
offset 10000000 limit 100) as foo}} takes just 40s and gives the right result.
Ah, I get it. Limit and offset work on the result, which in this case is a
single line. So in the case of count + limit/offset we actually need to wrap
the query like I did...
Hum... Justin, opinions?
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