Function ExecCheckXactReadOnly

src/backend/executor/execMain.c:1657–1743

* Check that the query does not imply any writes to non-temp tables; * unless we're in parallel mode, in which case don't even allow writes * to temp tables. * * This function is used to check if the current statement will perform any writes. * It is used to enforce: * (1) read-only mode (both fts and transaction isolation level read only) * as well as * (2) to keep track of when a

 * ensures that we use two-phase commit for this transaction.
 */
static void
ExecCheckXactReadOnly(PlannedStmt *plannedstmt)
{
	ListCell   *l;
	int         rti;

	/*
	 * CREATE TABLE AS or SELECT INTO?
	 *
	 * XXX should we allow this if the destination is temp?  Considering that
	 * it would still require catalog changes, probably not.
	 */
	if (plannedstmt->intoClause != NULL)
	{
		if ((plannedstmt->intoClause->rel && plannedstmt->intoClause->rel->relpersistence == RELPERSISTENCE_TEMP)
		   || (plannedstmt->intoClause->rel == NULL && plannedstmt->intoClause->ivm))
			ExecutorMarkTransactionDoesWrites();
		else
			PreventCommandIfReadOnly(CreateCommandName((Node *) plannedstmt));
	}

	/*
	 * Refresh matview will write xlog.
	 */
	if (plannedstmt->refreshClause != NULL)
	{
		PreventCommandIfReadOnly(CreateCommandName((Node *) plannedstmt));
	}

    rti = 0;
	/*
	 * Fail if write permissions are requested in parallel mode for table
	 * (temp or non-temp), otherwise fail for any non-temp table.
	 */
	foreach(l, plannedstmt->rtable)
	{
		RangeTblEntry *rte = (RangeTblEntry *) lfirst(l);

		rti++;

		if (rte->rtekind != RTE_RELATION)
			continue;

		if ((rte->requiredPerms & (~ACL_SELECT)) == 0)
			continue;

		/*
		 * External and foreign tables don't need two phase commit which is for
		 * local mpp tables
		 */
		if (get_rel_relkind(rte->relid) == RELKIND_FOREIGN_TABLE)
			continue;

		if (isTempNamespace(get_rel_namespace(rte->relid)))
		{
			ExecutorMarkTransactionDoesWrites();
			continue;
		}