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  1. IMPALA
  2. IMPALA-8021

Add estimated cardinality to EXPLAIN output

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    Details

    • Type: Improvement
    • Status: Resolved
    • Priority: Minor
    • Resolution: Fixed
    • Affects Version/s: Impala 3.1.0
    • Fix Version/s: Impala 3.2.0
    • Component/s: Frontend
    • Labels:
      None
    • Epic Color:
      ghx-label-2

      Description

      The EXPLAIN output provides much useful information in the plan tree. All our planning decisions are based on cardinality; but it appears in the EXPLAIN output only for the EXTENDED level. The profile only contains the plan from the STANDARD level. This change proposes to include row size and cardinality even in the STANDARD level.

      The nodes that have the information call it "cardinality", so continue to use that term.

      Add cardinality to each node so it appears something like this:

      HASH JOIN [INNER JOIN, BROADCAST]
      |  row-size=89B cardinality=1.23G
      |
      |--SCAN HDFS [db.table]
      |     partitions=2/123 files=2 size=4.56MB row-size=89B cardinality=7.89M
      

      Cardinality should appear in all levels above MINIMAL. Cardinality is not needed for EXCHANGE since it can be inferred from other nodes.

      Also, the existing code prints large cardinalities in detail: 1234567890, which is hard to read. Use the abbreviated output, using metric (power of 1000) units, so 1.23G instead.

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              • Assignee:
                Paul.Rogers Paul Rogers
                Reporter:
                Paul.Rogers Paul Rogers
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                • Created:
                  Updated:
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