Runtime detection can be done row by row. This will solve the problem in
your sample, though it suffers a little bit performance.
Require casting before adding is also clean. However, this would break
backward compatibility.
Dmitriy Ryaboy wrote:
How is runtime detection done? I worry that if 1.txt contains:
1, 2
1.1, 2.2
We get into a situation where addition of the fields in the first tuple
produces integers, and adding the fields of the second tuple produces
doubles.
A more invasive but perhaps easier to reason about solution might be to be
stricter about types, and require bytearrays to be cast to whatever type
they are supposed to be if you want to add / delete / do non-byte-things to
them.
This is a problem if UDFs that output tuples or bags don't specify schemas
(and specifying schemas of tuples and bags is fairly onerous right now in
Java). I am not sure what the solution here is, other than finding a clean,
less onerous way of declaring schemas, fixing up everything in builtin and
piggybank to only use the new clean sparkly api and document the heck out of
it.
D
On Thu, Jan 13, 2011 at 8:58 PM, Daniel Dai <jiany...@yahoo-inc.com> wrote:
One goal of semantic cleanup work undergoing is to clarify the usage of
unknown type.
In Pig schema system, user can define output schema for LoadFunc/EvalFunc.
Pig will propagate those schema to the entire script. Defining schema for
LoadFunc/EvalFunc is optional. If user don't define schema, Pig will mark
them bytearray. However, in the run time, user can feed any data type in.
Before, Pig assumes the runtime type for bytearray is DataByteArray, which
arose several issues (PIG-1277, PIG-999, PIG-1016).
In 0.9, Pig will treat bytearray as unknown type. Pig will inspect the
object to figure out what the real type is at runtime. We've done that for
all shuffle keys (PIG-1277). However, there are other cases. One case is
adding two bytearray. For example,
a = load '1.txt' using SomeLoader() as (a0, a1); // Assume SomeLoader does
not define schema, but actually feed Integer
b = foreach a generate a0+a1;
In Pig 0.8, schema system marks a0 and a1 as bytearray. In the case of
a0+a1, Pig cast both a0 and a1 to double (in TypeCheckingVisitor), and mark
the output schema for a0+a1 as double. Here is something interesting,
SomeLoader loads Integer, and we get Double after adding. We can change it
if we do the following:
1. Don't cast bytearray into Double (in TypeCheckingVisitor)
2. Change POAdd(Similarly, all other ExpressionOperators, multply, divide,
etc) to handle bytearray. When the schema for POAdd is bytearray, Pig will
figure out the data type at runtime, and process adding according to the
real type
Pro:
1. Consistent with the goal for unknown type cleanup: treat all bytearray
as unknown type. In the runtime, inspect the object to find the real type
Cons:
1. Slow down the processing since we need to inspect object type at runtime
2. Bring some indeterminism to schema system. Before a0+a1 is double,
downstream schema is more clear.
Any comments?
Daniel
