Use tags = NULL in middle tables if object doesn't have any tags

This doesn't make much of a difference for the ways and rels table, but
if we store all nodes in the database, it does make a huge difference,
because most nodes don't have any tags. For a current planet, disk usage
for the nodes table goes from 476 GB down to 409 GB saving 67 GB or
nearly 15%.

Additionally it makes use of that table simpler. If you want to do any
queries on tags, you need an index on the tags column on the
nodes/ways/rels tables like this:

CREATE INDEX ON planet_osm_ways USING gin (tags);

But that is wasteful, because of the empty tags. We probably want to
generate them as

CREATE INDEX ON planet_osm_ways USING gin (tags) WHERE tags != '{}'::jsonb;

But now all queries on those tables have to include that extra condition
so that the query planner will use the index.

SELECT * FROM planet_osm_ways WHERE tags ? 'highway' AND tags != '{}'::jsonb;

If we use NULLs, the index can be created as:

CREATE INDEX ON planet_osm_ways USING gin (tags) WHERE tags IS NOT NULL;

And now the query becomes simpler, because the NOT NULL is automatically
taken into account by the query planner:

SELECT * FROM planet_osm_ways WHERE tags ? 'highway';

Note that this is an incompatible change to the new format middle
tables, but they are still marked as experimental, so we can do this.

Author: joto

src/middle-pgsql.cpp

@@ -323,6 +323,10 @@ template <typename T>
 void pgsql_parse_json_tags(char const *string, osmium::memory::Buffer *buffer,
                            T *obuilder)
 {
+    if (*string == '\0') { // NULL
+        return;
+    }
+
     auto const tags = nlohmann::json::parse(string);
     if (!tags.is_object()) {
         throw std::runtime_error{"Database format for tags invalid."};
@@ -613,6 +617,10 @@ void middle_pgsql_t::copy_attributes(osmium::OSMObject const &obj)
 void middle_pgsql_t::copy_tags(osmium::OSMObject const &obj)
 {
     if (m_store_options.db_format == 2) {
+        if (obj.tags().empty()) {
+            m_db_copy.add_null_column();
+            return;
+        }
         json_writer_t writer;
         tags_to_json(obj.tags(), &writer);
         m_db_copy.add_column(writer.json());
@@ -1464,7 +1472,7 @@ static table_sql sql_for_nodes_format2(middle_pgsql_options const &options)
             " lat int4 NOT NULL,"
             " lon int4 NOT NULL,"
             "{attribute_columns_definition}"
-            " tags jsonb NOT NULL"
+            " tags jsonb"
             ") {data_tablespace}";
 
         sql.prepare_queries = {
@@ -1530,7 +1538,7 @@ static table_sql sql_for_ways_format2(middle_pgsql_options const &options)
                        " id int8 PRIMARY KEY {using_tablespace},"
                        "{attribute_columns_definition}"
                        " nodes int8[] NOT NULL,"
-                       " tags jsonb NOT NULL"
+                       " tags jsonb"
                        ") {data_tablespace}";
 
     sql.prepare_queries = {"PREPARE get_way(int8) AS"
@@ -1601,7 +1609,7 @@ static table_sql sql_for_relations_format2()
                        " id int8 PRIMARY KEY {using_tablespace},"
                        "{attribute_columns_definition}"
                        " members jsonb NOT NULL,"
-                       " tags jsonb NOT NULL"
+                       " tags jsonb"
                        ") {data_tablespace}";
 
     sql.prepare_queries = {"PREPARE get_rel(int8) AS"