Improve city name lookups

In the course of finding some way to “optimize” support for the planet on our server, here is an index which speeds things up greatly for city name lookups:

create index admin_city_names on planet_osm_line (boundary,admin_level,name) where (boundary='administrative' and admin_level='8');

This creates a partial index which greatly optimizes queries such as:

select 1 from planet_osm_line where boundary='administrative' and admin_level='8' and name='Tours';

Figures speak for themselves… Prior to the index, such a query would take around 3 to 4 minutes. With that index, “explain analyze” tells me it takes 40.554ms on our current server.

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