Cheolsoo Park created PIG-3268:
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Summary: Case statement support
Key: PIG-3268
URL: https://issues.apache.org/jira/browse/PIG-3268
Project: Pig
Issue Type: New Feature
Components: internal-udfs, parser
Affects Versions: 0.11
Reporter: Cheolsoo Park
Assignee: Cheolsoo Park
Fix For: 0.12
Currently, Pig has no support for case statement. To mimic it, users often use
nested bincond operators. However, that easily becomes unreadable when there
are multiple levels of nesting.
For example,
{code}
a = LOAD '1.txt' USING PigStorage(',') AS (i:int);
b = FOREACH a GENERATE (
i % 3 == 0 ? '3n' : (i % 3 == 1 ? '3n + 1' : '3n + 2')
);
{code}
This can be re-written much more nicely using case statement as follows:
{code}
a = LOAD '1.txt' USING PigStorage(',') AS (i:int);
b = FOREACH a GENERATE (
CASE i % 3
WHEN 0 THEN '3n'
WHEN 1 THEN '3n + 1'
ELSE '3n + 2'
END
);
{code}
I propose that we implement case statement in the following manner:
* Add built-in UDFs that take expressions as args. Take for example the
aforementioned case statement, we can define a UDF such as {{builtInUdf(i % 3,
0, '3n', 1, '3n + 1', '3n + 2')}}.
* Add syntactical sugar for these built-in UDFs.
In fact, I burrowed this idea from HIVE-164.
One downside of this approach is that all the possible args schemas of these
UDFs must be pre-computed. Specifically, we need to populate the full list of
possible args schemas in {{EvalFunc.getArgToFuncMapping}}.
In particular, since we obviously cannot support infinitely long args, it is
necessary to impose a limit on level of nesting. For now, I arbitrarily chose
50, but it can be easily changed.
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