docs: add overview over indexing

docs/develop/Indexing.md

@@ -0,0 +1,152 @@
+# Indexing Places
+
+In Nominatim, the word __indexing__ refers to the process that takes the raw
+OpenStreetMap data from the place table, enriches it with address information
+and creates the search indexes. This section explains the basic data flow.
+
+
+## Initial import
+
+After osm2pgsql has loaded the raw OSM data into the place table,
+the data is copied to the final search tables placex and location_property_osmline.
+While they are copied, some basic properties are added:
+
+ * country_code, geometry_sector and partition
+ * initial search and address rank
+
+In addition the column `indexed_status` is set to `1` marking the place as one
+that needs to be indexed.
+
+All this happens in the triggers `placex_insert` and `osmline_insert`.
+
+## Indexing
+
+The main work horse of the data import is the indexing step, where Nominatim
+takes every place from the placex and location_property_osmline tables where
+the indexed_status != 0 and computes the search terms and the address parts
+of the place.
+
+The indexing happens in three major steps:
+
+1. **Data preparation** - The indexer gets the data for the place to be indexed
+   from the database.
+
+2. **Search name processing** - The prepared data is given to the
+   tokenizer which computes the search terms from the names
+   and potentially other information.
+
+3. **Address processing** - The indexer then hands the prepared data and the
+   tokenizer information back to the database via an `INSERT` statement which
+   also sets the indexed_status to `0`. This triggers the update triggers
+   `placex_update`/`osmline_update` which do the work of computing address
+   parts and filling all the search tables.
+
+When computing the address terms of a place, Nominatim relies on the processed
+search names of all the address parts. That is why places are processed in rank
+order, from smallest rank to largest. To ensure correct handling of linked
+place nodes, administrative boundaries are processed before all other places.
+
+Apart from these restrictions, each place can be indexed independently
+from the others. This allows a large degree of parallelization during the indexing.
+It also means that the indexing process can be interrupted at any time and
+will simply pick up where it left of when restarted.
+
+### Data preparation
+
+The data preparation step computes and retrieves all data for a place that
+might be needed for the next step of processing the search name. That includes
+
+* location information (country code)
+* place classification (class, type, ranks)
+* names (including names of linked places)
+* address information (`addr:*` tags)
+
+Data preparation is implemented in pl/PgSQL mostly in the functions
+`placex_indexing_prepare()` and `get_interpolation_address()`.
+
+#### `addr:*` tag inheritance
+
+Nominatim has limited support for inheriting address tags from a building
+to POIs inside the building. This only works when the address tags are on the
+building outline. Any rank 30 object inside such a building or on its outline
+inherits all address tags when it does not have any address tags of its own.
+
+The inheritance is computed in the data preparation step.
+
+### Search name processing
+
+The prepared place information is handed to the tokenizer next. This is a
+Python module responsible for processing the names  from both name and address
+terms and building up the word index from them. The process is explained in
+more detail in the [Tokenizer chapter](Tokenizer.md).
+
+### Address processing
+
+Finally, the preprocessed place information and the results of the search name
+processing are written back to the database. At this point the update trigger
+of the placex/location_property_osmline tables take over and fill all the
+dependent tables. This makes up the most work-intensive part of the indexing.
+
+Nominatim distinguishes between dependent and independent places.
+**Dependent places** are all places on rank 30: house numbers, POIs etc. These
+places don't have a full address of their own. Instead they are attached to
+a parent street or place and use the information of the parent for searching
+and displaying information. Everything else are **independent places**: streets,
+parks, water bodies, suburbs, cities, states etc.  They receive a full address
+on their own.
+
+The address processing for both types of places is very different.
+
+#### Independent places
+
+To compute the address of an independent place Nominatim searches for all
+places that cover the place to compute the address for at least partially.
+For places with an area, that area is used to check for coverage. For place
+nodes an artificial square area is computed according to the rank of
+the place. The lower the rank the lager the area. The `location_area_large_X`
+tables are there to facilitate the lookup. All places that can function as
+the address of another place are saved in those tables.
+
+`addr:*` and `isin:*` tags are taken into account to compute the address, too.
+Nominatim will give preference to places with the same name as in these tags
+when looking for places in the vicinity. If there are no matching place names
+at all, then the tags are at least added to the search index. That means that
+the names will not be shown in the result as the 'address' of the place, but
+searching by them still works.
+
+Independent places are always added to the global search index `search_name`.
+
+#### Dependent places
+
+Dependent places skip the full address computation for performance reasons.
+Instead they just find a parent place to attach themselves to.
+
+![parenting of dependent places](parenting-flow.svg)
+
+By default a POI
+or house number will be attached to the closest street. That can be any major
+or minor street indexed by Nominatim. In the default configuration that means
+that it can attach itself to a footway but only when it has a name.
+
+When the dependent place has an `addr:street` tag, then Nominatim will first
+try to find a street with the same name before falling back to the closest
+street.
+
+There are also addresses in OSM, where the housenumber does not belong
+to a street at all. These have an `addr:place` tag. For these places, Nominatim
+tries to find a place with the given name in the indexed places with an
+address rank between 16 and 25. If none is found, then the dependent place
+is attached to the closest place in that category and the addr:place name is
+added as *unlisted* place, which indicates to Nominatim that it needs to add
+it to the address output, no matter what. This special case is necessary to
+cover addresses that don't really refer to an existing object.
+
+When an address has both the `addr:street` and `addr:place` tag, then Nominatim
+assumes that the `addr:place` tag in fact should be the city part of the address
+and give the POI the usual street number address.
+
+Dependent places are only added to the global search index `search_name` when
+they have either a name themselves or when they have address tags that are not
+covered by the places that make up their address. The latter ensures that
+addresses are always searchable by those address tags.
+

docs/develop/parenting-flow.plantuml

@@ -0,0 +1,31 @@
+@startuml
+skinparam monochrome true
+
+start
+
+if (has 'addr:street'?) then (yes)
+  if (street with that name\n nearby?) then (yes)
+    :**Use closest street**
+     **with same name**;
+     kill
+  else (no)
+    :** Use closest**\n**street**;
+     kill
+  endif
+elseif (has 'addr:place'?) then (yes)
+  if (place with that name\n nearby?) then (yes)
+    :**Use closest place**
+     **with same name**;
+     kill
+  else (no)
+    :add addr:place to adress;
+    :**Use closest place**\n**rank 16 to 25**;
+     kill
+  endif
+else (otherwise)
+ :**Use closest**\n**street**;
+ kill
+endif
+
+
+@enduml

docs/mkdocs.yml

@@ -36,6 +36,7 @@ pages:
     - 'Developers Guide':
         - 'Architecture Overview' : 'develop/overview.md'
         - 'Database Layout' : 'develop/Database-Layout.md'
+        - 'Indexing' : 'develop/Indexing.md'
         - 'Tokenizers' : 'develop/Tokenizers.md'
         - 'Setup for Development' : 'develop/Development-Environment.md'
         - 'Testing' : 'develop/Testing.md'