Hive
  1. Hive
  2. HIVE-836

Add syntax to force a new mapreduce job / transform subquery in mapper

    Details

    • Type: Wish Wish
    • Status: Resolved
    • Priority: Major Major
    • Resolution: Won't Fix
    • Affects Version/s: None
    • Fix Version/s: None
    • Component/s: None
    • Labels:
      None
    • Release Note:
      See comments for workarounds.

      Description

      Hive currently does a lot of awesome work to figure out when my transformers should be used in the mapper and when they should be used in the reducer. However, sometimes I have a different plan.

      For example, consider this:

      foo.sql
      SELECT TRANSFORM(a.val1, a.val2)
      USING './niftyscript'
      AS part1, part2, part3
      FROM (
          SELECT b.val AS val1, c.val AS val2
          FROM tblb b JOIN tblc c on (b.key=c.key)
      ) a
      

      ...now, assume that the join step is very easy and 'niftyscript' is really processor intensive. The ideal format for this is a MR task with few mappers and few reducers, and then a second MR task with lots of mappers.

      Currently, there is no way to even require the outer TRANSFORM statement occur in a separate map phase. Implementing a "hint" such as /* +MAP /, akin to / +MAPJOIN */, would be awesome.

      Current workaround is to dump everything to a temporary table and then start over, but that is not an easy to scale--the subquery structure effectively (and easily) "locks" the mid-points so no other job can touch the table.

        Activity

        Hide
        Adam Kramer added a comment -

        Oh hey there five year old task.

        Workaround: Use CLUSTER BY to force a reduce phase, and a staging table to force a map phase. Hive writes all the data to disk in every phase anyway so the staging table isn't actually a performance hit.

        Also protip: DON'T get distracted by the Hive keywords "MAP" and "REDUCE", they are just synonyms for TRANSFORM and do not do what anybody expects.

        Show
        Adam Kramer added a comment - Oh hey there five year old task. Workaround: Use CLUSTER BY to force a reduce phase, and a staging table to force a map phase. Hive writes all the data to disk in every phase anyway so the staging table isn't actually a performance hit. Also protip: DON'T get distracted by the Hive keywords "MAP" and "REDUCE", they are just synonyms for TRANSFORM and do not do what anybody expects.

          People

          • Assignee:
            Unassigned
            Reporter:
            Adam Kramer
          • Votes:
            0 Vote for this issue
            Watchers:
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            Dates

            • Created:
              Updated:
              Resolved:

              Development