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https://issues.apache.org/jira/browse/PIG-3268?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]
Cheolsoo Park updated PIG-3268:
-------------------------------
Description:
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 the size of when branches. For now, I
arbitrarily chose 50, but it can be easily changed.
was:
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.
> 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
>
> Attachments: PIG-3268.patch
>
>
> 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 the size of when branches. For now, I
> arbitrarily chose 50, but it can be easily changed.
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