对于空值和常量，可以执行以下操作：

--load your hive table
A = LOAD 'hive_db.table_name' USING org.apache.hive.hcatalog.pig.HCatLoader();

--this will generate a NULL column with datatype bytearray
B = FOREACH A GENERATE X,Y,NULL,W;

--then you can further process your data
STORE B INTO '/your/hdfs' USING PigStorage(',');