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https://issues.apache.org/jira/browse/PIG-1434?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12899042#action_12899042
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Aniket Mokashi commented on PIG-1434:
-------------------------------------
Comments on the finalized syntax--
With the above changes Pig now supports -
{code}
Y = foreach X generate $1/(long) C.count, $2-(long) C.max;
{code}
1. Casts are *optional* and the datatype of scalar depends on the schema of C
(ie depending on the schema of C, we add the casts implicitly. So, typically,
count is a long and max is a double). In case of undeclared(null) schema for C,
default type of scalar is *chararray*.
2. Projections are mandatory. For example
{code}
Y = foreach X generate C; // is an *error*
{code}
We need to use-
{code}
Y = foreach X generate C.$0;
{code}
3. Check if C is a scalar or not is not performed until runtime, thus it will
fail at the time of execution of UDF with ExecException("Scalar has more than
one row in the output").
> Allow casting relations to scalars
> ----------------------------------
>
> Key: PIG-1434
> URL: https://issues.apache.org/jira/browse/PIG-1434
> Project: Pig
> Issue Type: Improvement
> Reporter: Olga Natkovich
> Assignee: Aniket Mokashi
> Fix For: 0.8.0
>
> Attachments: scalarImpl.patch, ScalarImpl1.patch, ScalarImpl5.patch,
> ScalarImplFinale.patch, ScalarImplFinale1.patch, ScalarImplFinaleRebase.patch
>
>
> This jira is to implement a simplified version of the functionality described
> in https://issues.apache.org/jira/browse/PIG-801.
> The proposal is to allow casting relations to scalar types in foreach.
> Example:
> A = load 'data' as (x, y, z);
> B = group A all;
> C = foreach B generate COUNT(A);
> .....
> X = ....
> Y = foreach X generate $1/(long) C;
> Couple of additional comments:
> (1) You can only cast relations including a single value or an error will be
> reported
> (2) Name resolution is needed since relation X might have field named C in
> which case that field takes precedence.
> (3) Y will look for C closest to it.
> Implementation thoughts:
> The idea is to store C into a file and then convert it into scalar via a UDF.
> I believe we already have a UDF that Ben Reed contributed for this purpose.
> Most of the work would be to update the logical plan to
> (1) Store C
> (2) convert the cast to the UDF
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