Answer

When joining an external table to a SPU-based table, you may encounter an error similar to the following if the external table is not the first table to be scanned in the generated plan.
ERROR: 200288 : query does not support complex External Table scan


The following is an example of SQL that will work:
DEV(ADMIN)=> create external table et(i int) using (dataobject('/dev/null'));
 
       CREATE EXTERNAL TABLE
 
       DEV(ADMIN)=> create table t1(i int);
 
       CREATE TABLE
 
       DEV(ADMIN)=> create table t2(j int);
 
       CREATE TABLE
 
        DEV(ADMIN)=> select * from et join t1 on t1.i = et.i join t2 on t2.j = t1.i;
 
               I | I | J
 
               ---+---+---
 
        (0 rows)


However, the following SQL will result in the error: 
DEV(ADMIN)=> select * from t1 join et on t1.i = et.i join t2 on t2.j = t1.i;
 
         ERROR:  200288 : query does not support complex External Table scan


Netezza only supports one external table in an SQL statement.