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From "Santhosh Srinivasan (JIRA)" <j...@apache.org>
Subject [jira] Commented: (PIG-449) Schemas for bags should contain tuples all the time
Date Tue, 11 Nov 2008 22:51:44 GMT

    [ https://issues.apache.org/jira/browse/PIG-449?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12646712#action_12646712
] 

Santhosh Srinivasan commented on PIG-449:
-----------------------------------------

Currently, bags in Pig are containers of tuples. Accessing elements inside a bag should translate
to accessing elements inside the tuple contained in the bag. In addition, accessing tuples
inside a bag should be restricted to the FLATTEN keyword in a FOREACH statement. A few examples
shown below will demonstrate the point.

{code}
a = load '/user/pig/data/student.data' using PigStorage(' ') as (name, age, gpa);
b = foreach a generate {(16, 4.0e-2, 'hello')} as b:{t:(i: int, d: double, c: chararray)};
c = foreach b generate b.i; -- Here b.i should generate a bag of integers by accessing the
column called 'i' inside each tuple
d = foeach b generate b.t; -- This should be outlawed as the tuple inside the bag does not
have a column called 't' although the tuple inside the bag are named 't'
{code}

Summary:

1. The frontend should translate access to columns in a bag to columns inside the tuple in
the bag
2. The frontend should prevent access to tuples inside the bag via projections and allow access
only via the FLATTEN keyword

Thoughts/suggestions/comments are welcome.

> Schemas for bags should contain tuples all the time
> ---------------------------------------------------
>
>                 Key: PIG-449
>                 URL: https://issues.apache.org/jira/browse/PIG-449
>             Project: Pig
>          Issue Type: Bug
>    Affects Versions: types_branch
>            Reporter: Santhosh Srinivasan
>            Assignee: Santhosh Srinivasan
>             Fix For: types_branch
>
>
> The front end treats relations as operators that return bags.  When the schema of a load
statement is specified, the bag is associated with the schema specified by the user. Ideally,
the schema corresponds to the tuple contained in the bag. 
> With PIG-380, the schema for bag constants are computed by the front end. The schema
for the bag contains the tuple which in turn contains the schema of the columns. This results
in errors when columns are accessed directly just like the load statements.
> The front end should then treat access to the columns as a double dereference, i.e.,
access the tuple inside the bag and then the column inside the tuple.
> {code}
> grunt> a = load '/user/sms/data/student.data' using PigStorage(' ') as (name, age,
gpa);
> grunt> b = foreach a generate {(16, 4.0e-2, 'hello')} as b:{t:(i: int, d: double,
c: chararray)};
> grunt> describe b;
> b: {b: {t: (i: integer,d: double,c: chararray)}}
> grunt> c = foreach b generate b.i;
> 111064 [main] ERROR org.apache.pig.tools.grunt.GruntParser  - java.io.IOException: Invalid
alias: i in {t: (i: integer,d: double,c: chararray)}
>         at org.apache.pig.PigServer.parseQuery(PigServer.java:293)
>         at org.apache.pig.PigServer.registerQuery(PigServer.java:258)
>         at org.apache.pig.tools.grunt.GruntParser.processPig(GruntParser.java:432)
>         at org.apache.pig.tools.pigscript.parser.PigScriptParser.parse(PigScriptParser.java:242)
>         at org.apache.pig.tools.grunt.GruntParser.parseContOnError(GruntParser.java:93)
>         at org.apache.pig.tools.grunt.Grunt.run(Grunt.java:58)
>         at org.apache.pig.Main.main(Main.java:282)
> Caused by: org.apache.pig.impl.logicalLayer.parser.ParseException: Invalid alias: i in
{t: (i: integer,d: double,c: chararray)}
>         at org.apache.pig.impl.logicalLayer.parser.QueryParser.AliasFieldOrSpec(QueryParser.java:5851)
>         at org.apache.pig.impl.logicalLayer.parser.QueryParser.ColOrSpec(QueryParser.java:5709)
>         at org.apache.pig.impl.logicalLayer.parser.QueryParser.BracketedSimpleProj(QueryParser.java:5242)
>         at org.apache.pig.impl.logicalLayer.parser.QueryParser.BaseEvalSpec(QueryParser.java:4040)
>         at org.apache.pig.impl.logicalLayer.parser.QueryParser.UnaryExpr(QueryParser.java:3909)
>         at org.apache.pig.impl.logicalLayer.parser.QueryParser.CastExpr(QueryParser.java:3863)
>         at org.apache.pig.impl.logicalLayer.parser.QueryParser.MultiplicativeExpr(QueryParser.java:3772)
>         at org.apache.pig.impl.logicalLayer.parser.QueryParser.AdditiveExpr(QueryParser.java:3698)
>         at org.apache.pig.impl.logicalLayer.parser.QueryParser.InfixExpr(QueryParser.java:3664)
>         at org.apache.pig.impl.logicalLayer.parser.QueryParser.FlattenedGenerateItem(QueryParser.java:3590)
>         at org.apache.pig.impl.logicalLayer.parser.QueryParser.FlattenedGenerateItemList(QueryParser.java:3500)
>         at org.apache.pig.impl.logicalLayer.parser.QueryParser.GenerateStatement(QueryParser.java:3457)
>         at org.apache.pig.impl.logicalLayer.parser.QueryParser.NestedBlock(QueryParser.java:2933)
>         at org.apache.pig.impl.logicalLayer.parser.QueryParser.ForEachClause(QueryParser.java:2336)
>         at org.apache.pig.impl.logicalLayer.parser.QueryParser.BaseExpr(QueryParser.java:973)
>         at org.apache.pig.impl.logicalLayer.parser.QueryParser.Expr(QueryParser.java:748)
>         at org.apache.pig.impl.logicalLayer.parser.QueryParser.Parse(QueryParser.java:549)
>         at org.apache.pig.impl.logicalLayer.LogicalPlanBuilder.parse(LogicalPlanBuilder.java:60)
>         at org.apache.pig.PigServer.parseQuery(PigServer.java:290)
>         ... 6 more
> 111064 [main] ERROR org.apache.pig.tools.grunt.GruntParser  - Invalid alias: i in {t:
(i: integer,d: double,c: chararray)}
> 111064 [main] ERROR org.apache.pig.tools.grunt.GruntParser  - java.io.IOException: Invalid
alias: i in {t: (i: integer,d: double,c: chararray)}
> grunt> c = foreach b generate b.t;
> grunt> describe c;
> c: {t: {i: integer,d: double,c: chararray}}
> {code}

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