[ https://issues.apache.org/jira/browse/SPARK-27792?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]
Jason Guo updated SPARK-27792: ------------------------------ Shepherd: Liang-Chi Hsieh > SkewJoin--handle only skewed keys with broadcastjoin and other keys with > normal join > ------------------------------------------------------------------------------------ > > Key: SPARK-27792 > URL: https://issues.apache.org/jira/browse/SPARK-27792 > Project: Spark > Issue Type: New Feature > Components: SQL > Affects Versions: 2.4.3 > Reporter: Jason Guo > Priority: Major > Attachments: SMJ DAG.png, SMJ tasks.png, skew join DAG.png, sql.png, > time.png > > > This feature is designed to handle data skew in Join > > *Senario* > * A big table (big_skewed) which contains a a few skewed key > * A small table (small_even) which has no skewed key and is larger than the > broadcast threshold > * When big_skewed.join(small_even), a few tasks will be much slower than > other tasks because they need to handle the skewed key > *Effect* > This feature reduce the join time from 5.7 minutes to 2.1 minutes > !time.png! > !sql.png! > > *Experiment* > *Without this feature, the whole job took 5.7 minutes* > tableA has 2 skewed keys 9500048 and 9500096 > {code:java} > INSERT OVERWRITE TABLE big_skewed > SELECT CAST(CASE WHEN id < 908000000 THEN (9500000 + (CAST (RAND() * 2 AS > INT) + 1) * 48 ) > ELSE CAST(id/100 AS INT) END AS STRING), 'A' > name > FROM ids > WHERE id BETWEEN 900000000 AND 1050000000;{code} > tableB has no skewed keys > {code:java} > INSERT OVERWRITE TABLE small_even > SELECT CAST(CAST(id/100 AS INT) AS STRING), 'B' > name > FROM ids > WHERE id BETWEEN 950000000 AND 950500000;{code} > > Join them with setting spark.sql.autoBroadcastJoinThreshold to 3000 > {code:java} > insert overwrite table result_with_skew > select big_skewed.id, tabig_skewed.value, small_even.value > from big_skewed > join small_even > on small_even.id=big_skewed.id; > {code} > > The sort merge join is slow with 2 straggle tasks > !SMJ DAG.png! > !SMJ tasks.png! > > *With this feature, the job took only 2.1 minutes* > The skewed keys are joint with broadcast join and the non-skewed keys are > joint with sort merge join > !skew join DAG.png! > > -- This message was sent by Atlassian JIRA (v7.6.3#76005) --------------------------------------------------------------------- To unsubscribe, e-mail: issues-unsubscr...@spark.apache.org For additional commands, e-mail: issues-h...@spark.apache.org