Table of Contents
What is Bucket map side join in Hive?
How does Bucket Map Join work? In Hive, Bucket map join is used when the joining tables are large and are bucketed on the join column. In this kind of join, one table should have buckets in multiples of the number of buckets in another table.
What is Bucket map side join?
Introduction to Bucket Map Join For suppose if one table has 2 buckets then the other table must have either 2 buckets or a multiple of 2 buckets (2, 4, 6, and so on). Further, since the preceding condition is satisfied then the joining can be done on the mapper side only. Else a normal inner join is performed.
How does bucketing works in Hive?
Bucketing in hive is the concept of breaking data down into ranges, which are known as buckets, to give extra structure to the data so it may be used for more efficient queries. The range for a bucket is determined by the hash value of one or more columns in the dataset (or Hive metastore table).
How does map join work?
Map join is a feature used in Hive queries to increase its efficiency in terms of speed. Join is a condition used to combine the data from 2 tables. So, when we perform a normal join, the job is sent to a Map-Reduce task which splits the main task into 2 stages – “Map stage” and “Reduce stage”.
What is map join and SMB join in Hive?
In SMB join in Hive, each mapper reads a bucket from the first table and the corresponding bucket from the second table and then a merge sort join is performed. Sort Merge Bucket (SMB) join in hive is mainly used as there is no limit on file or partition or table join.
How do you determine the number of buckets in Hive?
In general, the bucket number is determined by the expression hash_function(bucketing_column) mod num_buckets. (There’s a ‘0x7FFFFFFF in there too, but that’s not that important). The hash_function depends on the type of the bucketing column.
How do you bucket data in Hive?
bucketing = true, so that Hive knows to create the number of buckets declared in the table definition to populate the bucketed table.
- set hive. enforce. bucketing = true;
- INSERT OVERWRITE TABLE bucketed_user PARTITION (country)
- set hive. enforce. bucketing = true;
- INSERT OVERWRITE TABLE bucketed_user PARTITION (country)
How many buckets can be created in Hive?
Buckets can help with the predicate pushdown since every value belonging to one value will end up in one bucket. So if you bucket by 31 days and filter for one day Hive will be able to more or less disregard 30 buckets.
How do I join a map in hive?
Configuring Map Join Options in Hive
- hive. auto. convert. join : By default, this option is set to true . When it is enabled, during joins, when a table with a size less than 25 MB (hive. mapjoin.
- hive. auto. convert. join. noconditionaltask : When three or more tables are involved in the join condition. Using hive.
How does Hive join work?
First, let’s discuss how join works in Hive. A common join operation will be compiled to a MapReduce task, as shown in figure 1. A common join task involves a map stage and a reduce stage. A mapper reads from join tables and emits the join key and join value pair into an intermediate file.
What is SMB join?
SMB is a join performed on bucket tables that have the same sorted, bucket, and join condition columns. It reads data from both bucket tables and performs common joins (map and reduce triggered) on the bucket tables.
Can we create buckets without partition in Hive?
Generally, in the table directory, each bucket is just a file, and Bucket numbering is 1-based. Along with Partitioning on Hive tables bucketing can be done and even without partitioning. vi. Moreover, Bucketed tables will create almost equally distributed data file parts.
Why we use hive bucket map join in MapReduce?
In normal join, if the tables are large, reducer gets overloaded in MapReduce framework as it receives all the data from the join key and value basis, and the performance also degrades as more data is shuffled. So we use the Hive Bucket Map Join feature when we are joining tables that are bucketed and joined on the bucketing column.
How do I reduce the number of tasks in hive bucket?
hive.enforce.bucketing =true several reduce tasks is set equal to the number of buckets that are mentioned in the table. Set hive.optimize.bucketmapjoin = True This enables the bucket to join operation, leading to reduced scan cycles while executing queries on bucketed tables.
What is the difference between left and right join in hive?
A left join is possible to be done to a map join only when the right table size is small. A right join can be done to a map join only when the left table size is small. We have tried to include the best possible points of Map Join in Hive.
How to insert values or data in bucketed table in hive?
To insert values or data in a bucketed table, we have to specify below property in Hive, This property is used to enable dynamic bucketing in Hive, while data is being loaded in the same way as dynamic partitioning is set using this: several reduce tasks is set equal to the number of buckets that are mentioned in the table.