Function ExecSetVariableStmt

src/backend/utils/misc/guc.c:9052–9200

* SET command */

 * SET command
 */
void
ExecSetVariableStmt(VariableSetStmt *stmt, bool isTopLevel)
{
	GucAction	action = stmt->is_local ? GUC_ACTION_LOCAL : GUC_ACTION_SET;

	/*
	 * Workers synchronize these parameters at the start of the parallel
	 * operation; then, we block SET during the operation.
	 */
	if (IsInParallelMode())
		ereport(ERROR,
				(errcode(ERRCODE_INVALID_TRANSACTION_STATE),
				 errmsg("cannot set parameters during a parallel operation")));

	switch (stmt->kind)
	{
		case VAR_SET_VALUE:
		case VAR_SET_CURRENT:
			if (stmt->is_local &&
				Gp_role != GP_ROLE_EXECUTE && !IsBootstrapProcessingMode())
			{
				WarnNoTransactionBlock(isTopLevel, "SET LOCAL");
			}

			SIMPLE_FAULT_INJECTOR("set_variable_fault");
			(void) set_config_option(stmt->name,
									 ExtractSetVariableArgs(stmt),
									 (superuser() ? PGC_SUSET : PGC_USERSET),
									 PGC_S_SESSION,
									 action, true, 0, false);
			DispatchSetPGVariable(stmt->name, stmt->args, stmt->is_local);
			break;
		case VAR_SET_MULTI:

			/*
			 * Special-case SQL syntaxes.  The TRANSACTION and SESSION
			 * CHARACTERISTICS cases effectively set more than one variable
			 * per statement.  TRANSACTION SNAPSHOT only takes one argument,
			 * but we put it here anyway since it's a special case and not
			 * related to any GUC variable.
			 */
			if (strcmp(stmt->name, "TRANSACTION") == 0)
			{
				ListCell   *head;

				if (Gp_role != GP_ROLE_EXECUTE)
					WarnNoTransactionBlock(isTopLevel, "SET TRANSACTION");

				foreach(head, stmt->args)
				{
					DefElem    *item = (DefElem *) lfirst(head);

					if (strcmp(item->defname, "transaction_isolation") == 0)
						SetPGVariable("transaction_isolation",
									  list_make1(item->arg), stmt->is_local);
					else if (strcmp(item->defname, "transaction_read_only") == 0)
						SetPGVariable("transaction_read_only",
									  list_make1(item->arg), stmt->is_local);