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  1. Apache Drill
  2. DRILL-4378

CONVERT_FROM in View results in table scan of MapR-DB and perhaps HBASE

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      Description

      I created a view to avoid forcing users to write queries that always included the CONVERT_FROM statements. (I am a huge advocate of making things easy for the the users and writing queries with CONVERT_FROM statements isn't easy).

      I ran a query the other day on one of these views and noticed that a query that took 30 seconds really shouldn't take 30 seconds. What do I mean? well I wanted to get part of a record by looking up the MapR-DB Row key (equiv. to HBASE row key) That should be an instant lookup. Sure enough, when I tried it in the hbase shell that returns instantly. So why did Drill take 30 seconds? I shot an email to Ted and Jim at MapR to ask this very question. Ted suggested that I try the query without a view. Sure enough, If I use the convert_from in a direct query, it's an instant (sub second) return. Thus it appears something in the view is not allowing the query to short circuit the read.

      Ted suggests I post here (I am curious if anyone who has HBASE setup is seeing this same issue with views) but also include the EXPLAIN plan. Basically, using my very limited ability to read EXPLAIN plans (If someone has a pointer to a blog post or docs on how to read EXPLAIN I would love that!) it looks like in the view the startRow and stopRow in the hbaseScanSpec are not set, seeming to cause a scan. Is there any away to assist the planner when running this through a view so that we can get the performance of the query without the view but with the easy of use/readability of using the view?

      Thanks!!!

      John

      View Creation

      CREATE VIEW view_testpaste as
      SELECT
      CONVERT_FROM(row_key, 'UTF8') AS pasteid,
      CONVERT_FROM(pastes.pdata.lang, 'UTF8') AS lang,
      CONVERT_FROM(pastes.raw.paste, 'UTF8') AS paste
      FROM dfs.`pastes`.`/pastes` pastes;

      Select from view takes 32 seconds (seems to be a scan)

      > select paste from view_testpaste where pasteid = 'djHEHcPM'

      1 row selected (32.302 seconds)

      Just a direct select returns very fast (0.486 seconds)

      > select CONVERT_FROM(pastes.raw.paste, 'UTF8') AS paste
      FROM dfs.`pastes`.`/pastes` pastes where
      CONVERT_FROM(row_key, 'UTF8') = 'djHEHcPM';

      1 row selected (0.486 seconds)

      EXPLAIN PLAN FOR select paste from view_testpaste where pasteid = 'djHEHcPM'

      ----------+

      text json

      ----------+

      00-00 Screen
      00-01 UnionExchange
      01-01 Project(paste=[CONVERT_FROMUTF8($1)])
      01-02 SelectionVectorRemover
      01-03 Filter(condition=[=(CONVERT_FROMUTF8($0), 'djHEHcPM')])
      01-04 Project(row_key=[$1], ITEM=[ITEM($0, 'paste')])
      01-05 Scan(groupscan=[MapRDBGroupScan [HBaseScanSpec=HBaseScanSpec [tableName=maprfs:///data/pastebiner/pastes, startRow=null, stopRow=null, filter=null], columns=[`row_key`, `raw`.`paste`]]])
      {
      "head" :
      Unknown macro: { "version" }

      ,
      "graph" : [ {
      "pop" : "maprdb-scan",
      "@id" : 65541,
      "userName" : "darkness",
      "hbaseScanSpec" :

      { "tableName" : "maprfs:///data/pastebiner/pastes", "startRow" : "", "stopRow" : "", "serializedFilter" : null }

      ,
      "storage" : {
      "type" : "file",
      "enabled" : true,
      "connection" : "maprfs:///",
      "workspaces" :

      Unknown macro: { "root" }

      ,
      "formats" :

      Unknown macro: { "psv" }

      },
      "columns" : [ "`row_key`", "`raw`.`paste`" ],
      "cost" : 573950.0
      },

      Unknown macro: { "pop" }

      ,

      { "pop" : "filter", "@id" : 65539, "child" : 65540, "expr" : "equal(convert_fromutf8(`row_key`) , 'djHEHcPM') ", "initialAllocation" : 1000000, "maxAllocation" : 10000000000, "cost" : 86092.5 }

      ,

      { "pop" : "selection-vector-remover", "@id" : 65538, "child" : 65539, "initialAllocation" : 1000000, "maxAllocation" : 10000000000, "cost" : 86092.5 }

      ,

      Unknown macro: { "pop" }

      ,

      { "pop" : "union-exchange", "@id" : 1, "child" : 65537, "initialAllocation" : 1000000, "maxAllocation" : 10000000000, "cost" : 86092.5 }

      ,

      { "pop" : "screen", "@id" : 0, "child" : 1, "initialAllocation" : 1000000, "maxAllocation" : 10000000000, "cost" : 86092.5 }

      ]
      }

      ----------+
      1 row selected (0.42 seconds)

      EXPLAIN PLAN FOR select CONVERT_FROM(pastes.raw.paste, 'UTF8') AS paste
      FROM dfs.`pastes`.`/pastes` pastes where
      CONVERT_FROM(row_key, 'UTF8') = 'djHEHcPM';

      ----------+

      text json

      ----------+

      00-00 Screen
      00-01 Project(paste=[CONVERT_FROMUTF8($1)])
      00-02 Project(row_key=[$1], ITEM=[ITEM($0, 'paste')])
      00-03 Scan(groupscan=[MapRDBGroupScan [HBaseScanSpec=HBaseScanSpec [tableName=maprfs:///data/pastebiner/pastes, startRow=djHEHcPM, stopRow=djHEHcPM\x00, filter=RowFilter (EQUAL, djHEHcPM)], columns=[`row_key`, `raw`.`paste`]]])
      {
      "head" :
      Unknown macro: { "version" }

      ,
      "graph" : [ {
      "pop" : "maprdb-scan",
      "@id" : 3,
      "userName" : "darkness",
      "hbaseScanSpec" :

      { "tableName" : "maprfs:///data/pastebiner/pastes", "startRow" : "ZGpIRUhjUE0=", "stopRow" : "ZGpIRUhjUE0A", "serializedFilter" : "CihvcmcuYXBhY2hlLmhhZG9vcC5oYmFzZS5maWx0ZXIuUm93RmlsdGVyEkUKQwgCEj8KL29yZy5hcGFjaGUuaGFkb29wLmhiYXNlLmZpbHRlci5CaW5hcnlDb21wYXJhdG9yEgwKCgoIZGpIRUhjUE0=" }

      ,
      "storage" : {
      "type" : "file",
      "enabled" : true,
      "connection" : "maprfs:///",
      "workspaces" :

      Unknown macro: { "root" }

      ,
      "formats" :

      Unknown macro: { "psv" }

      },
      "columns" : [ "`row_key`", "`raw`.`paste`" ],
      "cost" : 286975.0
      },

      Unknown macro: { "pop" }

      ,

      Unknown macro: { "pop" }

      ,

      { "pop" : "screen", "@id" : 0, "child" : 1, "initialAllocation" : 1000000, "maxAllocation" : 10000000000, "cost" : 286975.0 }

      ]
      }

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            • Assignee:
              jni Jinfeng Ni
              Reporter:
              mandoskippy John Omernik
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              • Created:
                Updated: