Generalization: Allow setting condition for running SQL commands

If a condition is set with "if_has_rows", the SQL command in
"if_has_rows" will be run and only if that returns any rows, the main
sql command is run. This check happens only in "append" mode, in
"create" mode the command is always run.

To be used with something like "SELECT 1 FROM some_table LIMIT 1" to
check whether the table has any rows (for instance an expire table).

Author: joto

src/gen/osm2pgsql-gen.cpp

@@ -373,6 +373,9 @@ class genproc_t
         bool const transaction = luaX_get_table_bool(lua_state(), "transaction",
                                                      1, "Argument #1", false);
 
+        std::string const if_has_rows = luaX_get_table_string(
+            lua_state(), "if_has_rows", 1, "Argument #1", "");
+
         std::vector<std::string> queries;
         if (transaction) {
             queries.emplace_back("BEGIN");
@@ -403,10 +406,21 @@ class genproc_t
             queries.emplace_back("COMMIT");
         }
 
+        pg_conn_t const db_connection{m_conninfo};
+
+        if (m_append && !if_has_rows.empty()) {
+            auto const result = db_connection.exec(if_has_rows);
+            if (result.num_tuples() == 0) {
+                log_info("Not running SQL command: {} (no rows in "
+                         "condition result).",
+                         description);
+                return 0;
+            }
+        }
+
         log_info("Running SQL commands: {}.", description);
 
         util::timer_t timer_sql;
-        pg_conn_t const db_connection{m_conninfo};
         for (auto const &query : queries) {
             log_debug("Running sql: {}", query);
             db_connection.exec(query);