Fix: Delete to be expired tiles at the same time as reading them

The TRUNCATE didn't work, because there can be tiles from more than one
zoom level in the same table.

Using DELETE ... RETURNING we are ensured to always delete exactly what
we are getting back.

Author: joto

src/gen/osm2pgsql-gen.cpp

@@ -175,7 +175,7 @@ void get_tiles_from_table(pg_conn_t const &connection, std::string const &table,
                           std::vector<std::pair<uint32_t, uint32_t>> *tiles)
 {
     auto const result = connection.exec(
-        R"(SELECT x, y FROM "{}" WHERE zoom = {})", table, zoom);
+        R"(DELETE FROM "{}" WHERE zoom = {} RETURNING x, y)", table, zoom);
 
     tiles->reserve(result.num_tuples());
 
@@ -476,8 +476,6 @@ class genproc_t
             log_debug("Running generalizer for expire list from table '{}'...",
                       table);
             get_tiles_from_table(db_connection, table, zoom, &tile_list);
-            log_debug("Truncating table '{}'...", table);
-            db_connection.exec("TRUNCATE {}", table);
         } else {
             auto const extent =
                 get_extent_from_db(db_connection, m_dbschema, params, zoom);