This chapter describes the various operations the Nominatim database administrator may use to clean and maintain the database. None of these operations is mandatory but they may help improve the performance and accuracy of results.
Updating postcodes
Command: nominatim refresh --postcodes
Postcode centroids (aka 'calculated postcodes') are generated by looking at all postcodes of a country, grouping them and calculating the geometric centroid. There is currently no logic to deal with extreme outliers (typos or other mistakes in OSM data). There is also no check if a postcodes adheres to a country's format, e.g. if Swiss postcodes are 4 digits.
When running regular updates, postcodes results can be improved by running this command on a regular basis. Note that only the postcode table and the postcode search terms are updated. The postcode that is assigned to each place is only updated when the place is updated.
The command takes around 70min to run on the planet and needs ca. 40GB of temporary disk space.
Updating word counts
Command: nominatim refresh --word-counts
Nominatim keeps frequency statistics about all search terms it indexes. These
statistics are currently used to optimise queries to the database. Thus better
statistics mean better performance. Word counts are created once after import
and are usually sufficient even when running regular updates. You might want
to rerun the statistics computation when adding larger amounts of new data,
for example, when adding an additional country via nominatim add-data.
Forcing recomputation of places and areas
Command: nominatim refresh --data-object [NWR]<id> --data-area [NWR]<id>
When running replication updates, Nominatim tries to recompute the search and address information for all places that are affected by a change. But it needs to restrict the total number of changes to make sure it can keep up with the minutely updates. Therefore it will refrain from propagating changes that affect a lot of objects.
The administrator may force an update of places in the database.
nominatim refresh --data-object invalidates a single OSM object.
nominatim refresh --data-area invalidates an OSM object and all dependent
objects. That are usually the places that inside its area or around the
center of the object. Both commands expect the OSM object as an argument
of the form OSM type + OSM id. The type must be N (node), W (way) or
R (relation).
After invalidating the object, indexing must be run again. If continuous
update are running in the background, the objects will be recomputed together
with the next round of updates. Otherwise you need to run nominatim index
to finish the recomputation.
Removing large deleted objects
Nominatim refuses to delete very large areas because often these deletions are
accidental and are reverted within hours. Instead the deletions are logged in
the import_polygon_delete table and left to the administrator to clean up.
There is currently no command to do that. You can use the following SQL query to force a deletion on all objects that have been deleted more than a certain timespan ago (here: 1 month):
SELECT place_force_delete(p.place_id) FROM import_polygon_delete d, placex p
WHERE p.osm_type = d.osm_type and p.osm_id = d.osm_id
and age(p.indexed_date) > '1 month'::interval