Hi,
I made my first large PostGIS table with nearly ten million rows. Untill this I
had thought that OpenJUMP works very well with PostGIS but now I am not sure
anymore. What I say is about the native PostGIS driver, I have not tried the
others with this large table.
I believe that the first problem I notised actually makes all big PostGIS tables
unusable slow and I am not sure if it really needs to be so.
OpenJUMP seems to start PostGIS query by sending:
SELECT AsBinary(find_extent( 'table', 'geom' ))
I could not find function find_extent from PostGIS manual but I suppose it is
doing the same as
SELECT extent(geom)from table;
This will mean full scan to geometry table every time than PostGIS layer is
accessed. Index does not help with this function and it takes several minutes to
run the query with my 10 million row table. The final query returning the
geometries takes just seconds. I wonder why these exact extents are queried? It
looks like coming from here (taken from CVN.txt):
2006-05-08 16:20 jaquino
* com/vividsolutions/jump/datastore/postgis/PostgisDSMetadata.java:
Use find_extent for PostGIS extents
Alternative query
SELECT estimated_extent('table','geom');
is returning the result immediately, but it is not accurate. I do not know for
what OpenJUMP is using and needing the exact extents of the database layer, or
if estimated extents could be used instead.
Another problem is that with a big database table it is too simple to send
queries which will leed to memory running out and a total jam. All that is
needed is to zoom out too far. Perhaps situation could be done more safe for the
user by adding an option to limit the number or returned features? This option
should be selectable in the Add datastore layer and Run datastore query dialogs,
I believe it would need just adding LIMIT max_number to PostGIS queries. I
understand this would be dangerous as well if the user does not realise that the
query did not give a full set of data. So I am not if this is a good idea of
all. Perhaps there could be a warning message in case of
max_features=returned_features?
Regards,
-Jukka Rahkonen-
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