Merge pull request #3367 from lonvia/address-word-counts

[nominatim.git] / docs / library / Low-Level-DB-Access.md

diff --git a/docs/library/Low-Level-DB-Access.md b/docs/library/Low-Level-DB-Access.md
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 # Low-level connections
 
+The `NominatimAPIAsync` class allows to directly access the underlying
+database connection to explore the raw data. Nominatim uses
+[SQLAlchemy](https://docs.sqlalchemy.org/) for building queries. Please
+refer to the documentation of the library to understand how to write SQL.
+
+To get access to a search connection, use the `begin()` function of your
+API object. This returns a `SearchConnection` object described below
+wrapped in a context manager. Its
+`t` property has definitions for all Nominatim search tables. For an
+overview of available tables, refer to the
+[Development Layout](../develop/Database-Layout.md) in in the development
+chapter. Note that only tables that are needed for search are accessible
+as SQLAlchemy tables.
+
+!!! warning
+    The database layout is not part of the API definition and may change
+    without notice. If you play with the low-level access functions, you
+    need to be prepared for such changes.
+
+Here is a simple example, which prints how many places are available in
+the placex table:
+
+```
+import asyncio
+from pathlib import Path
+import sqlalchemy as sa
+from nominatim.api import NominatimAPIAsync
+
+async def print_table_size():
+    api = NominatimAPIAsync(Path('.'))
+
+    async with api.begin() as conn:
+        cnt = await conn.scalar(sa.select(sa.func.count()).select_from(conn.t.placex))
+        print(f'placex table has {cnt} rows.')
+
+asyncio.run(print_table_size())
+```
+
+!!! warning
+    Low-level connections may only be used to read data from the database.
+    Do not use it to add or modify data or you might break Nominatim's
+    normal functions.
+
+## SearchConnection class
+
 ::: nominatim.api.SearchConnection
     options:
+        members:
+            - scalar
+            - execute
+            - get_class_table
+            - get_db_property
+            - get_property
         heading_level: 6