use constant expressions to select partial indexes in reverse

When expressions are generated with SQLAlchemy, any constants are
replaced with bind parameters. The bind parameters become parameters of
prepared statements. The result is that the query planner tends to
oversee that the partial indexes can be used.

nominatim/db/sqlalchemy_functions.py

@@ -0,0 +1,34 @@
+# SPDX-License-Identifier: GPL-3.0-or-later
+#
+# This file is part of Nominatim. (https://nominatim.org)
+#
+# Copyright (C) 2023 by the Nominatim developer community.
+# For a full list of authors see the git log.
+"""
+Custom functions and expressions for SQLAlchemy.
+"""
+
+import sqlalchemy as sa
+
+def select_index_placex_geometry_reverse_lookuppolygon(table: str) -> 'sa.TextClause':
+    """ Create an expression with the necessary conditions over a placex
+        table that the index 'idx_placex_geometry_reverse_lookupPolygon'
+        can be used.
+    """
+    return sa.text(f"ST_GeometryType({table}.geometry) in ('ST_Polygon', 'ST_MultiPolygon')"
+                   f" AND {table}.rank_address between 4 and 25"
+                   f" AND {table}.type != 'postcode'"
+                   f" AND {table}.name is not null"
+                   f" AND {table}.indexed_status = 0"
+                   f" AND {table}.linked_place_id is null")
+
+def select_index_placex_geometry_reverse_lookupplacenode(table: str) -> 'sa.TextClause':
+    """ Create an expression with the necessary conditions over a placex
+        table that the index 'idx_placex_geometry_reverse_lookupPlaceNode'
+        can be used.
+    """
+    return sa.text(f"{table}.rank_address between 4 and 25"
+                   f" AND {table}.type != 'postcode'"
+                   f" AND {table}.name is not null"
+                   f" AND {table}.linked_place_id is null"
+                   f" AND {table}.osm_type = 'N'")