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From "David Morel" <>
Subject Cartesian product detection in the query plan?
Date Mon, 28 Jan 2013 09:39:57 GMT
Hi everyone,

I had to kill some queries that were taking forever, and it turns out
they were doing cartesian products (missing ON clause on a JOIN).

I wonder how I could see that in the EXPLAIN output (which I still find
a bit cryptic). Specifically, the stage that it was stuck in was this:

   Stage: Stage-7
     Map Reduce
       Alias -> Map Operator Tree:
             Reduce Output Operator
               sort order:
               tag: 1
               value expressions:
                     expr: _col1
                     type: int
             Reduce Output Operator
               sort order:
               tag: 0
               value expressions:
                     expr: _col0
                     type: bigint
                     expr: _col1
                     type: string
       Reduce Operator Tree:
         Join Operator
           condition map:
                Inner Join 0 to 1
           condition expressions:
             0 {VALUE._col0} {VALUE._col1}
             1 {VALUE._col1}
           handleSkewJoin: false
           outputColumnNames: _col0, _col1, _col3
           File Output Operator
             compressed: true
             GlobalTableId: 0
                 input format: 
                 output format:

Is there anything in there that should have alerted me?

I found out by looking at the query, but I wonder if the query plan (if
I could read it) would have given me that information.

Thanks a lot

David Morel

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