Use far fewer connections in flex output

Most operations are not run in parallel anyway, they only need a single
connection. For the parts running in parallel, we have one connection
per thread. We don't have connections for each table any more.

src/output-flex.cpp

@@ -1075,12 +1075,12 @@ void output_flex_t::after_ways()
 void output_flex_t::stop()
 {
     for (auto &table : m_table_connections) {
-        auto *db_connection = &m_db_connections.at(table.table().num());
-        table.task_set(thread_pool().submit([&, db_connection]() {
-            table.stop(*db_connection,
+        table.task_set(thread_pool().submit([&]() {
+            pg_conn_t const db_connection{get_options()->connection_params,
+                                          "out.flex.stop"};
+            table.stop(db_connection,
                        get_options()->slim && !get_options()->droptemp,
                        get_options()->append);
-            db_connection->close();
         }));
     }
 
@@ -1185,8 +1185,7 @@ void output_flex_t::delete_from_tables(osmium::item_type type, osmid_t osm_id)
 {
     for (auto &table : m_table_connections) {
         if (table.table().matches_type(type) && table.table().has_id_column()) {
-            delete_from_table(&table, m_db_connections.at(table.table().num()),
-                              type, osm_id);
+            delete_from_table(&table, m_db_connection, type, osm_id);
         }
     }
 }
@@ -1230,12 +1229,8 @@ void output_flex_t::relation_modify(osmium::Relation const &rel)
 
 void output_flex_t::start()
 {
-    assert(m_db_connections.empty());
-
     for (auto &table : m_table_connections) {
-        m_db_connections.emplace_back(get_options()->connection_params,
-                                      "out.flex.table");
-        table.start(m_db_connections.back(), get_options()->append);
+        table.start(m_db_connection, get_options()->append);
     }
 }
 
@@ -1263,19 +1258,16 @@ output_flex_t::output_flex_t(output_flex_t const *other,
                              std::shared_ptr<db_copy_thread_t> copy_thread)
 : output_t(other, std::move(mid)), m_tables(other->m_tables),
   m_expire_outputs(other->m_expire_outputs),
+  m_db_connection(get_options()->connection_params, "out.flex.thread"),
   m_stage2_way_ids(other->m_stage2_way_ids),
   m_copy_thread(std::move(copy_thread)), m_lua_state(other->m_lua_state),
   m_process_node(other->m_process_node), m_process_way(other->m_process_way),
   m_process_relation(other->m_process_relation),
   m_select_relation_members(other->m_select_relation_members)
 {
-    assert(m_db_connections.empty());
-
     for (auto &table : *m_tables) {
         auto &tc = m_table_connections.emplace_back(&table, m_copy_thread);
-        m_db_connections.emplace_back(get_options()->connection_params,
-                                      "out.flex.table");
-        tc.prepare(m_db_connections.back());
+        tc.prepare(m_db_connection);
     }
 
     for (auto &expire_output : *m_expire_outputs) {
@@ -1295,6 +1287,7 @@ output_flex_t::output_flex_t(std::shared_ptr<middle_query_t> const &mid,
                              std::shared_ptr<thread_pool_t> thread_pool,
                              options_t const &options)
 : output_t(mid, std::move(thread_pool), options),
+  m_db_connection(get_options()->connection_params, "out.flex.main"),
   m_copy_thread(std::make_shared<db_copy_thread_t>(options.connection_params))
 {
     init_lua(options.style);
@@ -1521,10 +1514,8 @@ void output_flex_t::reprocess_marked()
         for (auto &table : m_table_connections) {
             if (table.table().matches_type(osmium::item_type::way) &&
                 table.table().has_id_column()) {
-                auto const &db_connection =
-                    m_db_connections.at(table.table().num());
-                table.analyze(db_connection);
-                table.create_id_index(db_connection);
+                table.analyze(m_db_connection);
+                table.create_id_index(m_db_connection);
             }
         }
 

src/output-flex.hpp

@@ -290,8 +290,8 @@ class output_flex_t : public output_t
 
     std::vector<table_connection_t> m_table_connections;
 
-    /// The connections to the database server for each table.
-    std::vector<pg_conn_t> m_db_connections;
+    /// The connection to the database server.
+    pg_conn_t m_db_connection;
 
     // This is shared between all clones of the output and must only be
     // accessed while protected using the lua_mutex.