Move DISTINCT from queries generating list to when we read the data

We had some problems with the query planner getting confused with those
queries. Lets make them as simple as possible to not confuse it
unnecessarily. This might be vodoo, but it doesn't cost much, just a
little bit of temporary space in the database, because when reading we
do the ORDER BY anyway, so the extra DISTINCT isn't more cost.

Author: joto

src/middle-pgsql.cpp

@@ -73,8 +73,8 @@ static void load_id_list(pg_conn_t const &db_connection,
                          std::string const &table,
                          osmium::index::IdSetSmall<osmid_t> *ids)
 {
-    auto const res =
-        db_connection.exec(fmt::format("SELECT id FROM {} ORDER BY id", table));
+    auto const res = db_connection.exec(
+        fmt::format("SELECT DISTINCT id FROM {} ORDER BY id", table));
     for (int n = 0; n < res.num_tuples(); ++n) {
         ids->set(osmium::string_to_object_id(res.get_value(n, 0)));
     }
@@ -836,7 +836,7 @@ WITH changed_buckets AS (
     FROM osm2pgsql_changed_nodes GROUP BY id >> {way_node_index_id_shift}
 )
 INSERT INTO osm2pgsql_changed_ways
-  SELECT DISTINCT w.id
+  SELECT w.id
     FROM {schema}"{prefix}_ways" w, changed_buckets b
     WHERE w.nodes && b.node_ids
       AND {schema}"{prefix}_index_bucket"(w.nodes)
@@ -845,7 +845,7 @@ INSERT INTO osm2pgsql_changed_ways
     } else {
         queries.emplace_back(R"(
 INSERT INTO osm2pgsql_changed_ways
-  SELECT DISTINCT w.id
+  SELECT w.id
     FROM {schema}"{prefix}_ways" w, osm2pgsql_changed_nodes n
     WHERE w.nodes && ARRAY[n.id]
         )");
@@ -854,15 +854,15 @@ INSERT INTO osm2pgsql_changed_ways
     if (m_options->middle_database_format == 1) {
         queries.emplace_back(R"(
 INSERT INTO osm2pgsql_changed_relations
-  SELECT DISTINCT r.id
+  SELECT r.id
     FROM {schema}"{prefix}_rels" r, osm2pgsql_changed_nodes n
     WHERE r.parts && ARRAY[n.id]
       AND r.parts[1:way_off] && ARRAY[n.id]
         )");
     } else {
         queries.emplace_back(R"(
 INSERT INTO osm2pgsql_changed_relations
-  SELECT DISTINCT r.id
+  SELECT r.id
     FROM {schema}"{prefix}_rels" r, osm2pgsql_changed_nodes c
     WHERE {schema}"{prefix}_member_ids"(r.members, 'N'::char) && ARRAY[c.id];
         )");