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Dmitriy V. Ryaboy commented on PIG-2713:
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Harsh: "." is an operator that dereferences inside a tuple (or generates a
projection, for bags). "::" is just two colons. If you do a "describe" on the
relation, you will see that the field names after a join are made up of the
name of the original relation, "::", and the original field name.
Pig does some disambiguation when possible, so that if you say "bar" and there
is a field called "foo::bar", it will know what you are talking about -- as
long as there is only one bar, of course.
The issue is getting a bit confused by the fact that pig allows you to treat
relations as scalars if they only have 1 row. That means that while normally a
"foreach" operator only works on one relation, you can actually say "foreach
some_relation generate some_field / some_other_relation.some_other_field". In
that case, pig figures you mean "some_other_relation" is a single-row relation,
pull out "some_other_field" from it, and pull it in here. It's a convenience
syntax since most people don't think of doing a replicated join to do the same
thing. Sadly, after this feature was released, we have discovered that people
often make grammatical mistakes like the one you and Bejoy are making, which
would have failed fast before, quickly leading to identifying a resolution, but
now fail in odd ways since Pig thinks you are trying to treat some other
relation as a scalar -- rather than realizing you are just referencing fields
incorrectly. We have a ticket open to fix this problem and revert to a
fail-fast mode.
> Pig query planner throwing parse error on Joins
> ------------------------------------------------
>
> Key: PIG-2713
> URL: https://issues.apache.org/jira/browse/PIG-2713
> Project: Pig
> Issue Type: Bug
> Components: parser
> Affects Versions: 0.8.1, 0.9.2
> Environment: CentOS 6
> Reporter: Bejoy KS
>
> Pig parser is throwing an exception when two columns in a table has the same
> name and when they are used as part of some projection operation after join.
> Error message
> ERROR 1103: Merge join/Cogroup only supports Filter, Foreach, filter and Load
> as its predecessor. Found :
> Error would be thrown for common join as well.
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