Sage-Tips

Why MapReduce will not run if you run select * from table in hive?

Originally Answered: why mapreduce will not run if you run select * from table in hive? However, while using “select from “, Hive requires a map-reduce job since it needs to extract the ‘column’ from each row by parsing it from the file it loads.

How many mappers will run for Hive query?

It depends on how many cores and how much memory you have on each slave. Generally, one mapper should get 1 to 1.5 cores of processors. So if you have 15 cores then one can run 10 Mappers per Node. So if you have 100 data nodes in Hadoop Cluster then one can run 1000 Mappers in a Cluster.

How do you set mappers in hive?

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:

1. Setting it when logged into the HIVE CLI. In other words, `set tez. grouping.
2. An entry in the `hive-site. xml` can be added through Ambari.

How mapper and reducer works in hive?

Map Reduce talk in terms of key value pair , which means mapper will get input in the form of key and value pair, they will do the required processing then they will produce intermediate result in the form of key value pair ,which would be input for reducer to further work on that and finally reducer will also write …

What is reducer in hive?

Hadoop Reducer Tutorial – Objective In Hadoop, Reducer takes the output of the Mapper (intermediate key-value pair) process each of them to generate the output. The output of the reducer is the final output, which is stored in HDFS. Usually, in the Hadoop Reducer, we do aggregation or summation sort of computation.

How do I know how many mappers I have?

of Mappers per MapReduce job:The number of mappers depends on the amount of InputSplit generated by trong>InputFormat (getInputSplits method). If you have 640MB file and Data Block size is 128 MB then we need to run 5 Mappers per MapReduce job.

How do you increase mappers in hive?

from my_hbase_table select col1, count(1) group by col1; The map reduce job spawns only 2 mappers and I’d like to increase that. With a plain map reduce job I would configure the yarn and mapper memory to increase the number of mappers.

How do you control the number of mappers?

You cannot set number of mappers explicitly to a certain number which is less than the number of mappers calculated by Hadoop. This is decided by the number of Input Splits created by hadoop for your given set of input. You may control this by setting mapred.

How many mappers would be running in an application?

Usually, 1 to 1.5 cores of processor should be given to each mapper. So for a 15 core processor, 10 mappers can run.

What does Mapper do in hive?

Hadoop Mapper is a function or task which is used to process all input records from a file and generate the output which works as input for Reducer. It produces the output by returning new key-value pairs.

Does Hive use reducers or mappers?

I’m looking for Hive query scenarios, where it uses only mappers or only reducers. in hive if you do simple query like select * from table there will be no map reduce job is going to run as we are just dumping the data.

How to use mapjoin in hive?

Hive# select /*+MAPJOIN (..)*/… //this kind of joins will loads small table to memory and does the join on map phase only. When ever we do insert values into table and loading the data should be used only map phase. When we does Create table as simple select then only mapper phase will be initialized.

How do I set MapReduce as the execution engine for hive?

The following snippet shows the configuration classification and property to use to set MapReduce as the execution engine for Hive: Connect to the master node. For more information, see Connect to the master node using SSH in the Amazon EMR Management Guide . At the command prompt for the current master node, type hive .

What happens if you don’t map non-primary keys in hive?

If you do not map a non-primary key attribute, no error is generated, but you won’t see the data in the Hive table. If the data types do not match, the value is null. Then you can start running Hive operations on hivetable1.