The translator does not support select expressions. Query:
SELECT DISTINCT DATAMART_NAME AS c_9, 'constant' AS c_10 FROM (SELECT datamart_name FROM
(
SELECT 'DATACOM Datamart' AS DATAMART_NAME FROM SECURITY_DM_DATACOM_DEV_R140_CA31_CMGRD1.VIEWOBJACCESS
UNION ALL
SELECT 'DB2 D91A Datamart' AS DATAMART_NAME FROM SECURITY_DM_DB2_D91APTIB_CMGRD1.VIEWOBJACCESS
) AS t
) a

Final part of plan:
OPTIMIZATION COMPLETE:
PROCESSOR PLAN:
SortNode(0) output=[datamart_name AS c_9, '_constant_' AS c_10] [DUP_REMOVE]
ProjectNode(1) output=[datamart_name AS c_9, '_constant_' AS c_10] [datamart_name AS c_9, '_constant_' AS c_10]
UnionAllNode(2) output=[datamart_name]
AccessNode(3) output=['DATACOM Datamart' AS DATAMART_NAME] SELECT 'DATACOM Datamart' FROM SECURITY_DM_DATACOM_DEV_R140_CA31_CMGRD1.VIEWOBJACCESS AS g_0
AccessNode(4) output=['DB2 D91A Datamart' AS DATAMART_NAME] SELECT 'DB2 D91A Datamart' FROM SECURITY_DM_DB2_D91APTIB_CMGRD1.VIEWOBJACCESS AS g_0

Expected LIMIT 1 in the access nodes. Comment from Steven: "More than likely we are not checking across a union for this optimization."

See https://community.jboss.org/message/883460 for more details and complete plan