Running the following inner merge query:

SELECT ORDERS.O_ORDERKEY, ORDERS.O_ORDERDATE, ORDERS.O_CLERK, CUSTOMER.C_CUSTKEY, CUSTOMER.C_NAME, CUSTOMER.C_ACCTBAL
FROM TPCR01_PostgreSQLVirt.ORDERS, TPCR01_OracleVirt.CUSTOMER WHERE (CUSTOMER.C_CUSTKEY = ORDERS.O_CUSTKEY)
AND (ORDERS.O_ORDERDATE < '1992-02-06 00:00:00') AND (CUSTOMER.C_ACCTBAL > 5100)

produces exception:
1 [PSQLException]An I/O error occured while sending to the backend.
2 [IOException]Tried to send an out-of-range integer as a 2-byte value: 50000
at org.teiid.translator.jdbc.JDBCQueryExecution.execute(JDBCQueryExecution.java:90)
at org.teiid.dqp.internal.datamgr.ConnectorWorkItem.execute(ConnectorWorkItem.java:272)
at org.teiid.dqp.internal.process.DataTierTupleSource.getResults(DataTierTupleSource.java:342)
at org.teiid.dqp.internal.process.DataTierTupleSource$1.call(DataTierTupleSource.java:138)
at org.teiid.dqp.internal.process.DataTierTupleSource$1.call(DataTierTupleSource.java:135)
at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:303)
at java.util.concurrent.FutureTask.run(FutureTask.java:138)
at org.teiid.dqp.internal.process.DQPCore$FutureWork.run(DQPCore.java:118)
at org.teiid.dqp.internal.process.DQPWorkContext.runInContext(DQPWorkContext.java:232)
at org.teiid.dqp.internal.process.ThreadReuseExecutor$RunnableWrapper.run(ThreadReuseExecutor.java:118)
at org.teiid.dqp.internal.process.ThreadReuseExecutor$3.run(ThreadReuseExecutor.java:288)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
at java.lang.Thread.run(Thread.java:680)

From googling the exception, I found the suggestion (i've yet to find where Postgres specifically states the max allowable) to split the parameters into 30,000 chunks. So I changed the PostgreSQLExecutionFactory by adding the following the CTOR: setMaxDependentInPredicates(30);
and this allowed the query to finish. Not to say this is the final solution, but it's an option that worked.