Disable synchronous commit on all connections

This simplifies the code, because we don't have to decide whether to set
this or not on each connection. And it doesn't matter much when setting
this for read-only connections which don't need this.

Setting synchronous_commit = off speeds up writing to the database,
because commis are not delayed until the WAL is written to disk. It
can not lead to corruption of the database.

https://www.postgresql.org/docs/current/runtime-config-wal.html#GUC-SYNCHRONOUS-COMMIT

src/db-copy.cpp

@@ -130,9 +130,6 @@ void db_copy_thread_t::thread_t::operator()()
     try {
         m_conn = std::make_unique<pg_conn_t>(m_connection_params, "copy");
 
-        // Let commits happen faster by delaying when they actually occur.
-        m_conn->exec("SET synchronous_commit = off");
-
         // Disable sequential scan on database tables in the copy threads.
         // The copy threads only do COPYs (which are unaffected by this
         // setting) and DELETEs which we know benefit from the index. For

src/flex-table.cpp

@@ -247,7 +247,6 @@ void table_connection_t::connect(connection_params_t const &connection_params)
 
     m_db_connection =
         std::make_unique<pg_conn_t>(connection_params, "out.flex.table");
-    m_db_connection->exec("SET synchronous_commit = off");
 }
 
 static void enable_check_trigger(pg_conn_t const &db_connection,

src/pgsql.cpp

@@ -75,6 +75,11 @@ pg_conn_t::pg_conn_t(connection_params_t const &connection_params,
     if (!get_logger().debug_enabled()) {
         exec("SET client_min_messages = WARNING");
     }
+
+    // Disable synchronous_commit on all connections. For some connections it
+    // might not matter, especially if they read only, but then it doesn't
+    // hurt either.
+    exec("SET synchronous_commit = off");
 }
 
 void pg_conn_t::close()

src/table.cpp

@@ -67,8 +67,6 @@ void table_t::sync() { m_copy.sync(); }
 void table_t::connect()
 {
     m_sql_conn = std::make_unique<pg_conn_t>(m_connection_params, "out.pgsql");
-    //let commits happen faster by delaying when they actually occur
-    m_sql_conn->exec("SET synchronous_commit = off");
 }
 
 void table_t::start(connection_params_t const &connection_params,