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
+```