docs: add new maintenance section

currently used for postcode updates, word count updates and
deleted relations.

docs/admin/Maintenance.md

@@ -0,0 +1,51 @@
+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`.
+
+
+## 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):
+
+```sql
+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
+```

docs/develop/Postcodes.md

@@ -1,45 +0,0 @@
-# Postcodes in Nominatim
-
-The blog post
-[Nominatim and Postcodes](https://www.openstreetmap.org/user/lonvia/diary/43143)
-describes the handling implemented since Nominatim 3.1.
-
-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.
-
-
-## Regular updating calculated postcodes
-
-The script to rerun the calculation is
-`nominatim refresh --postcodes`
-and runs once per night on nominatim.openstreetmap.org.
-
-
-## Finding places that share a specific postcode
-
-In the Nominatim database run
-
-```sql
-SELECT address->'postcode' as pc,
-       osm_type, osm_id, class, type,
-       st_x(centroid) as lon, st_y(centroid) as lat
-FROM placex
-WHERE country_code='fr'
-  AND upper(trim (both ' ' from address->'postcode')) = '33210';
-```
-
-Alternatively on [Overpass](https://overpass-turbo.eu/) run the following query
-
-```
-[out:json][timeout:250];
-area["name"="France"]->.boundaryarea;
-(
-nwr(area.boundaryarea)["addr:postcode"="33210"];
-);
-out body;
->;
-out skel qt;
-```

docs/mkdocs.yml

@@ -21,6 +21,7 @@ pages:
         - 'Deploy' : 'admin/Deployment.md'
         - 'Nominatim UI'  : 'admin/Setup-Nominatim-UI.md'
         - 'Advanced Installations' : 'admin/Advanced-Installations.md'
+        - 'Maintenance' : 'admin/Maintenance.md'
         - 'Migration from older Versions' : 'admin/Migration.md'
         - 'Troubleshooting' : 'admin/Faq.md'
     - 'Customization Guide':
@@ -37,7 +38,6 @@ pages:
         - 'Architecture Overview' : 'develop/overview.md'
         - 'OSM Data Import' : 'develop/Import.md'
         - 'Tokenizers' : 'develop/Tokenizers.md'
-        - 'Postcodes' : 'develop/Postcodes.md'
         - 'Testing' : 'develop/Testing.md'
         - 'External Data Sources': 'develop/data-sources.md'
     - 'Appendix':