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https://issues.apache.org/jira/browse/PIG-859?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12885011#action_12885011
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Philip (flip) Kromer commented on PIG-859:
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There are many use cases for a self join -- graph explorations especially.
Would it work to say something like
{code:sql|title=bfs.pig}
-- Enumerate paths of length two
Edges = LOAD 'a' AS (src, dest);
E2Paths = Join Edges AS InLinks BY dest, Edges AS Outlinks BY src;
{code}
> Optimizer throw error on self-joins
> -----------------------------------
>
> Key: PIG-859
> URL: https://issues.apache.org/jira/browse/PIG-859
> Project: Pig
> Issue Type: Bug
> Components: impl
> Affects Versions: 0.3.0
> Reporter: Ashutosh Chauhan
>
> Doing self-join results in exception thrown by Optimizer. Consider the
> following query
> {code}
> grunt> A = load 'a';
> grunt> B = Join A by $0, A by $0;
> grunt> explain B;
> 2009-06-20 15:51:38,303 [main] ERROR org.apache.pig.tools.grunt.Grunt -
> ERROR 1094: Attempt to insert between two nodes that were not connected.
> Details at logfile: pig_1245538027026.log
> {code}
> Relevant stack-trace from log-file:
> {code}
> Caused by: org.apache.pig.impl.plan.optimizer.OptimizerException: ERROR
> 2047: Internal error. Unable to introduce split operators.
> at
> org.apache.pig.impl.logicalLayer.optimizer.ImplicitSplitInserter.transform(ImplicitSplitInserter.java:163)
> at
> org.apache.pig.impl.logicalLayer.optimizer.LogicalOptimizer.optimize(LogicalOptimizer.java:163)
> at org.apache.pig.PigServer.compileLp(PigServer.java:844)
> at org.apache.pig.PigServer.compileLp(PigServer.java:781)
> at org.apache.pig.PigServer.getStorePlan(PigServer.java:723)
> at org.apache.pig.PigServer.explain(PigServer.java:566)
> ... 8 more
> Caused by: org.apache.pig.impl.plan.PlanException: ERROR 1094: Attempt
> to insert between two nodes that were not connected.
> at
> org.apache.pig.impl.plan.OperatorPlan.doInsertBetween(OperatorPlan.java:500)
> at
> org.apache.pig.impl.plan.OperatorPlan.insertBetween(OperatorPlan.java:480)
> at
> org.apache.pig.impl.logicalLayer.optimizer.ImplicitSplitInserter.transform(ImplicitSplitInserter.java:139)
> ... 13 more
> {code}
> A possible workaround is:
> {code}
> grunt> A = load 'a';
> grunt> B = load 'a';
> grunt> C = join A by $0, B by $0;
> grunt> explain C;
> {code}
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