add experimental option for enabling forward dependencies

docs/customize/Settings.md

@@ -148,6 +148,29 @@ Setting this option to 'yes' means that Nominatim skips reindexing of contained
 objects when the area becomes too large.
 
 
+#### NOMINATIM_UPDATE_FORWARD_DEPENDENCIES
+
+| Summary            |                                                     |
+| --------------     | --------------------------------------------------- |
+| **Description:**   | Forward geometry changes to dependet objects |
+| **Format:**        | bool |
+| **Default:**       | no |
+| **Comment:**       | EXPERT ONLY. Must not be enabled after import. |
+
+The geometry of OSM ways and relations may change when a node that is part
+of the object is moved around. These changes are not propagated per default.
+The geometry of ways/relations is only updated the next time that the object
+itself is touched. When enabling this option, then dependent objects will
+be marked for update when one of its member objects changes.
+
+Enabling this option may slow down updates significantly.
+
+!!! warning
+    If you want to enable this option, it must be set already on import.
+    Do not enable this option on an existing database that was imported with
+    NOMINATIM_UPDATE_FORWARD_DEPENDENCIES=no.
+    Updates will become unusably slow.
+
 #### NOMINATIM_LANGUAGES
 
 | Summary            |                                                     |

nominatim/clicmd/args.py

@@ -187,6 +187,7 @@ class NominatimArgs:
                     osm2pgsql_style_path=self.config.config_dir,
                     threads=self.threads or default_threads,
                     dsn=self.config.get_libpq_dsn(),
+                    forward_dependencies=self.config.get_bool('UPDATE_FORWARD_DEPENDENCIES'),
                     flatnode_file=str(self.config.get_path('FLATNODE_FILE') or ''),
                     tablespaces=dict(slim_data=self.config.TABLESPACE_OSM_DATA,
                                      slim_index=self.config.TABLESPACE_OSM_INDEX,

nominatim/tools/exec_utils.py

@@ -117,7 +117,6 @@ def run_osm2pgsql(options: Mapping[str, Any]) -> None:
     env = get_pg_env(options['dsn'])
     cmd = [str(options['osm2pgsql']),
            '--hstore', '--latlon', '--slim',
-           '--with-forward-dependencies', 'false',
            '--log-progress', 'true',
            '--number-processes', str(options['threads']),
            '--cache', str(options['osm2pgsql_cache']),
@@ -131,14 +130,14 @@ def run_osm2pgsql(options: Mapping[str, Any]) -> None:
     else:
         cmd.extend(('--output', 'gazetteer'))
 
-    if options['append']:
+    cmd.append('--append' if options['append'] else '--create')
-        cmd.append('--append')
-    else:
-        cmd.append('--create')
 
     if options['flatnode_file']:
         cmd.extend(('--flat-nodes', options['flatnode_file']))
 
+    if not options.get('forward_dependencies', False):
+        cmd.extend(('--with-forward-dependencies', 'false'))
+
     for key, param in (('slim_data', '--tablespace-slim-data'),
                        ('slim_index', '--tablespace-slim-index'),
                        ('main_data', '--tablespace-main-data'),

settings/env.defaults

@@ -33,6 +33,15 @@ NOMINATIM_MAX_WORD_FREQUENCY=50000
 # If true, admin level changes on places with many contained children are blocked.
 NOMINATIM_LIMIT_REINDEXING=yes
 
+# When set to 'yes' changes to OSM objects will be propagated to dependent
+# objects. This means that geometries of way/relations are updated when a
+# node on a way or a way in a relation changes.
+# EXPERT ONLY: Use with care. Enabling this option might lead to significantly
+#              more load when updates are applied.
+#              Do not enable this option on an existing database.
+# The default is to not propagate these changes.
+NOMINATIM_UPDATE_FORWARD_DEPENDENCIES=no
+
 # Restrict search languages.
 # Normally Nominatim will include all language variants of name:XX
 # in the search index. Set this to a comma separated list of language