Importantly, if your query does use ORDER BY Hive's implementation only supports a single reducer at the moment for this operation. Group by, aggregation functions and joins take place in the reducer by default whereas filter operations happen in the mapper; Use the hive.map.aggr=true option to perform the first level aggregation directly in the map task; Set the number of mappers/reducers depending on the type of task being performed. Problem statement : Find total amount purchased along with number of transaction for each customer. Hadoop job information for Stage-1: number of mappers: 1; number of reducers: 1. Set hive.map.aggr=true Set hive.exec.parallel=true Set mapred.tasks.reuse.num.tasks=-1 Set hive.mapred.map.speculative.execution=false Set hive.mapred.reduce.speculative.execution=false By using this map join hint set hive.auto.convert.join = true; and increasing the small table file size the job initiated but it was map 0 % -- reduce 0% It is comparatively simple and easier to implement than the map side join as the sorting and shuffling phase sends the values having identical keys to the same reducer and therefore, by default, the data is organized for us. I have downloaded mapr sandbox and when I try to run a simple hive query the map reduce job is failing. In order to manually set the number of mappers in a Hive query when TEZ is the execution engine, the configuration `tez.grouping.split-count` can be used by either: Setting it when logged into the HIVE CLI. A nice feature in Hive is the automatic merging of small files, this solves the problem of generating small files in HDFS as a result of the number of mappers and reducers in the task. In order to set a constant number of reducers: 16. of nodes> * set mapreduce.reduce.memory.mb=5120; SET hive.exec.parallel=true. of Reducers per slave: It is same as No of Mappers per slave (2) No. Hive; HIVE-16666; Set hive.exec.stagingdir a relative directory or a sub directory of distination data directory will cause Hive to delete the intermediate query results SET hive.groupby.skewindata=true; Hive will first trigger an additional MapReduce job whose map output will randomly distribute to the reducer to avoid data skew. Set the number of reducers relatively high, since the mappers will forward almost all their data to the reducers. Ignored when mapred.job.tracker is "local". set hive.exec.reducers.bytes.per.reducer=1000000. of reducers. #hadoop #sqoop #defaultmapper #defaultreducer #hadoopinterviewquestion. It also sets the number of map tasks to be equal to the number of buckets. // Ideally The number of Reducers in a Map-Reduce must be set to: 0.95 or 1.75 multiplied by (
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