What is fetch task in Hive?
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What is fetch task in Hive?
task. conversion. This parameter controls which kind of simple query can be converted to a single fetch task. It was added in Hive 0.10 per HIVE-2925.
How can Hive avoid MapReduce?
conversion property can (FETCH task) minimize latency of mapreduce overhead. When queried SELECT, FILTER, LIMIT queries, this property skip mapreduce and using FETCH task. As a result Hive can execute query without run mapreduce task.
How does Hive process a query?
Interface of the Hive such as Command Line or Web user interface delivers query to the driver to execute. In this, UI calls the execute interface to the driver such as ODBC or JDBC. Driver designs a session handle for the query and transfer the query to the compiler to make execution plan.
What is MapReduce in Hive?
MapReduce: It is a parallel programming model for processing large amounts of structured, semi-structured, and unstructured data on large clusters of commodity hardware. HDFS:Hadoop Distributed File System is a part of Hadoop framework, used to store and process the datasets.
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 …
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.
What are stages in hive?
A Hive query gets converted into a sequence (it is more a Directed Acyclic Graph) of stages. These stages may be map/reduce stages or they may even be stages that do metastore or file system operations like move and rename.
How do I know how many mappers I have?
It depends on the no of files and file size of all the files individually. Calculate the no of Block by splitting the files on 128Mb (default). Two files with 130MB will have four input split not 3. According to this rule calculate the no of blocks, it would be the number of Mappers in Hadoop for the job.
What is Mapper function and reducer function?
The Map function takes input from the disk as pairs, processes them, and produces another set of intermediate pairs as output. The Reduce function also takes inputs as pairs, and produces pairs as output.
How do you increase 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:
- Setting it when logged into the HIVE CLI. In other words, `set tez. grouping.
- An entry in the `hive-site. xml` can be added through Ambari.