When using table information from the Hive metastore with Pig, add the -useHCatalog option when invoking pig:

$ pig -useHCatalog test.pig

In the script, use HCatLoader to have table schema retrieved automatically:

A = LOAD 'groups' USING org.apache.hive.hcatalog.pig.HCatLoader();
DESCRIBE A;

Output:

A: {name: chararray,placeholder: chararray,id: int}