[Qgis-developer] Table join branch
marco.hugentobler at sourcepole.ch
Mon Jul 19 03:10:33 EDT 2010
Hi QGIS devs
There is now the table_join_branch (svn co
https://svn.osgeo.org/qgis/branches/table_join_branch) which contains an
initial table join prototype. Here is a short description how to use it:
- Load a vector layer
- Load a table layer. This can be done e.g. using 'Layer->Add Vector Layer...'
on a .dbf or .csv file
- Go to vector layer properties->join and click '+' to add a new join
- Select join field of table and join-to field of vector layer
- Now you should see the new fields in the attribute tab of the vector layer
properties, in the attribute table and in the info-tool window
Internally, QgsVectorLayer stores the join info (fields and join table provider
key) and remaps the field ids (first the provider ids, then the joined field ids
and last the added field ids). When querying the joined field values, a subset
string is set to the table provider to query a feature with the value of the
As you might expect, there are still a number of issues:
- Joined fields cannot be edited (should they?). So attribute table and info-
tool should disable those rows. QgsVectorLayer probably needs a method to tell
attribute table and info tool which rows are not editable.
- Performance is an issue. The current implementation tries to improve
performance by automatically creating attribute indices in the ogr provider
(could be implemented in other providers too). Still there is a siginificant
performance penalty when doing classifications based on joined attributes or
attributes searches (info tool and opening attribute table are usually fast,
because not many features are queried at once).
One possibility to enhance this could be to load tables into memory providers
and implementing attribute index capability there. Are there other
Looking forward to you feedback and suggestions. I'm sure there are a lot of
issues I didn't consider.
@Martin: in your gsoc blog, you write that you are refactoring the editing
buffer to a utility class. Do you think the join info and the id remaping can
be ported to the new design easily?
Dr. Marco Hugentobler
Sourcepole - Linux & Open Source Solutions
Webereistr. 66, CH-8134 Adliswil, Switzerland
marco.hugentobler at sourcepole.ch http://www.sourcepole.ch
Technical Advisor QGIS Project Steering Committee
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