Change the order of tuple fields - apache-pig

Question

As an example, lets say I load two different files into a pig script
A = LOAD 'file1' USING PigStorage('\t') AS (
day:chararray,
month:chararray,
year:chararray,
message:chararray);
B = LOAD 'file2' USING PigStorage('\t) AS (
month:chararray,
day:chararray,
year:chararry,
message:chararray);
Now, notice the order of the fields is different, so if I combine them into one file C = UNION A, B; I get...
(2,OCT,2013,INFO INVALID USERNAME)
(OCT,3,2013,WARN STACK OVERFLOW)
If for no other reason than to make the data easier to read, I'd like to reorder the fields, so that both of them follow a common format and have the same positional notation for each field.
(2,OCT,2013,INFO INVALID USERNAME)
(3,OCT,2013,WARN STACK OVERFLOW)
This also crops up in a few other places with messages, levels, hosts, etc. It's not just date fields, I'd like to make everything "prettier" all around.
In some weird pseudo-code, I'd be looking for something like:
D = FOREACH B
REORDER (month,day,year) TO (day,month,year);
I haven't been able to find an example of anyone trying to do this and don't see a function that would do it. So maybe it's not possible and I'm alone here, but if anyone has any ideas I'd appreciate some hints.

Answer 1

In general, this is not necessary in Pig because you can just refer to fields by name and not worry about their position in the record. If your goal is to do a UNION of the two relations, you can achieve this by using the ONSCHEMA keyword:
C = UNION ONSCHEMA A, B;
That said, if you do really need to reorder a relation, a simple FOREACH...GENERATE is all you need:
D = FOREACH B GENERATE day, month, year, message;
Note that in your example, you are not actually working with tuples, you are working with entire records. If you did have a tuple, though, you can use the TOTUPLE built-in UDF to get where you need to go:
DESCRIBE E;
E: {t: (month: chararray,day: chararray,year: chararray,message: chararray)}
F = FOREACH E GENERATE TOTUPLE(t.day, t.month, t.year, t.message) AS t;
DESCRIBE F;
F: {t: (day: chararray,month: chararray,year: chararray,message: chararray)}