The row_number operation seems to be skewed. Dudu
From: @Sanjiv Singh [mailto:sanjiv.is...@gmail.com] Sent: Tuesday, June 28, 2016 8:54 PM To: user@hive.apache.org Subject: Query Performance Issue : Group By and Distinct and load on reducer Hi All, I am having performance issue with data skew of the distinct statement in Hive<http://stackoverflow.com/questions/37894023/understanding-the-data-skew-of-the-countdistinct-statement-in-hive>. See below query with DISTINCT operator. Original Query : SELECT DISTINCT SD.REGION ,SD.HEADEND ,SD.NETWORK ,SD.RETAILUNITCODE ,SD.LOGTIMEDATE ,SD.SPOTKEY ,SD.CRE_DT ,CASE WHEN SD.LOGTIMEDATE IS NULL THEN 'Y' ELSE 'N' END AS DROP_REASON ,ROW_NUMBER() OVER ( ORDER BY NULL ) AS ETL_ROW_ID FROM INTER_ETL AS SD; Table INTER_ETL used for query is big enough. From the logs , it seems that data skew for specific set of values , causing one of reducer have to do all the job. I tried to achieve the same through GROUP BY still having the same issue. Help me to understand the issue and resolution. Query with Distinct V2 : CREATE TEMPORARY TABLE ETL_TMP AS SELECT DISTINCT dt.* FROM ( SELECT SD.REGION ,SD.HEADEND ,SD.NETWORK ,SD.RETAILUNITCODE ,SD.LOGTIMEDATE ,SD.SPOTKEY ,SD.CRE_DT ,CASE WHEN SD.LOGTIMEDATE IS NULL THEN 'Y' ELSE 'N' END AS DROP_REASON ,ROW_NUMBER() OVER ( ORDER BY NULL ) AS ETL_ROW_ID FROM INTER_ETL AS SD ) AS dt; Logs: INFO : Map 1: 107/107 Reducer 2: 417(+1)/418 Reducer 3: 0(+56)/418 INFO : Map 1: 107/107 Reducer 2: 417(+1)/418 Reducer 3: 0(+56)/418 INFO : Map 1: 107/107 Reducer 2: 417(+1)/418 Reducer 3: 0(+56)/418 INFO : Map 1: 107/107 Reducer 2: 417(+1)/418 Reducer 3: 0(+56)/418 INFO : Map 1: 107/107 Reducer 2: 417(+1)/418 Reducer 3: 0(+418)/418 Query With Group By: CREATE TEMPORARY TABLE ETL_TMP AS SELECT REGION ,HEADEND ,NETWORK ,RETAILUNITCODE ,LOGTIMEDATE ,SPOTKEY ,CRE_DT ,DROP_REASON ,ETL_ROW_ID FROM ( SELECT SD.REGION ,SD.HEADEND ,SD.NETWORK ,SD.RETAILUNITCODE ,SD.LOGTIMEDATE ,SD.SPOTKEY ,SD.CRE_DT ,CASE WHEN SD.LOGTIMEDATE IS NULL THEN 'Y' ELSE 'N' END AS DROP_REASON ,ROW_NUMBER() OVER ( ORDER BY NULL ) AS ETL_ROW_ID FROM INTER_ETL AS SD ) AS dt GROUP BY REGION ,HEADEND ,NETWORK ,RETAILUNITCODE ,LOGTIMEDATE ,SPOTKEY ,CRE_DT ,DROP_REASON ,ETL_ROW_ID; Logs: INFO : Map 1: 818/818 Reducer 2: 417(+1)/418 Reducer 3: 0(+418)/418 INFO : Map 1: 818/818 Reducer 2: 417(+1)/418 Reducer 3: 0(+418)/418 INFO : Map 1: 818/818 Reducer 2: 417(+1)/418 Reducer 3: 0(+418)/418 INFO : Map 1: 818/818 Reducer 2: 417(+1)/418 Reducer 3: 0(+418)/418 INFO : Map 1: 818/818 Reducer 2: 417(+1)/418 Reducer 3: 0(+418)/418 Table details : Beeline > dfs -ls /apps/hive/warehouse/PRD_DB.db/INTER_ETL ; +--------------------------------------------------------------------------------------------------------------------------------------------+--+ | DFS Output | +--------------------------------------------------------------------------------------------------------------------------------------------+--+ | Found 15 items | | -rwxrwxrwx 3 Z56034 hdfs 2075244899 2016-06-28 10:24 /apps/hive/warehouse/PRD_DB.db/INTER_ETL/000000_0 | | -rwxrwxrwx 3 Z56034 hdfs 2090030620 2016-06-28 10:24 /apps/hive/warehouse/PRD_DB.db/INTER_ETL/000001_0 | | -rwxrwxrwx 3 Z56034 hdfs 2025516774 2016-06-28 10:24 /apps/hive/warehouse/PRD_DB.db/INTER_ETL/000002_0 | | -rwxrwxrwx 3 Z56034 hdfs 1986848213 2016-06-28 10:24 /apps/hive/warehouse/PRD_DB.db/INTER_ETL/000003_0 | | -rwxrwxrwx 3 Z56034 hdfs 2018883723 2016-06-28 10:24 /apps/hive/warehouse/PRD_DB.db/INTER_ETL/000004_0 | | -rwxrwxrwx 3 Z56034 hdfs 1984690335 2016-06-28 10:24 /apps/hive/warehouse/PRD_DB.db/INTER_ETL/000005_0 | | -rwxrwxrwx 3 Z56034 hdfs 1987494444 2016-06-28 10:24 /apps/hive/warehouse/PRD_DB.db/INTER_ETL/000006_0 | | -rwxrwxrwx 3 Z56034 hdfs 1974674515 2016-06-28 10:24 /apps/hive/warehouse/PRD_DB.db/INTER_ETL/000007_0 | | -rwxrwxrwx 3 Z56034 hdfs 1963720218 2016-06-28 10:24 /apps/hive/warehouse/PRD_DB.db/INTER_ETL/000008_0 | | -rwxrwxrwx 3 Z56034 hdfs 1965892384 2016-06-28 10:24 /apps/hive/warehouse/PRD_DB.db/INTER_ETL/000009_0 | | -rwxrwxrwx 3 Z56034 hdfs 1974272622 2016-06-28 10:24 /apps/hive/warehouse/PRD_DB.db/INTER_ETL/000010_0 | | -rwxrwxrwx 3 Z56034 hdfs 1971948208 2016-06-28 10:24 /apps/hive/warehouse/PRD_DB.db/INTER_ETL/000011_0 | | -rwxrwxrwx 3 Z56034 hdfs 1968141886 2016-06-28 10:24 /apps/hive/warehouse/PRD_DB.db/INTER_ETL/000012_0 | | -rwxrwxrwx 3 Z56034 hdfs 1970930771 2016-06-28 10:24 /apps/hive/warehouse/PRD_DB.db/INTER_ETL/000013_0 | | -rwxrwxrwx 3 Z56034 hdfs 192820628 2016-06-28 10:23 /apps/hive/warehouse/PRD_DB.db/INTER_ETL/000014_0 | +--------------------------------------------------------------------------------------------------------------------------------------------+--+ 'numFiles'='15', 'numRows'='108363614', Regards Sanjiv Singh Mob : +091 9990-447-339
