what is map join in hive

What Is Map Join In Hive? Map join is a Hive feature that is used to speed up Hive queries. It lets a table to be loaded into memory so that a join could be performed within a mapper without using a Map/Reduce step. If queries frequently depend on small table joins, using map joins speed up queries’ execution.

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.

What is map in Hive?

map: It is an unordered collection of key-value pairs. Keys must be of primitive types. Values can be of any type. struct: It is a collection of elements of different types. Examples: complex datatypes.

What is map side join in MapReduce?

There are two types of join operations in MapReduce: Map Side Join: As the name implies, the join operation is performed in the map phase itself. Therefore, in the map side join, the mapper performs the join and it is mandatory that the input to each map is partitioned and sorted according to the keys.

What is indexing in Hive?

Introduction to Indexes in Hive. Indexes are a pointer or reference to a record in a table as in relational databases. Indexing is a relatively new feature in Hive. In Hive, the index table is different than the main table. Indexes facilitate in making query execution or search operation faster.

What is Bucket side join in Hive?

In Hive, Bucket map join is used when the joining tables are large and are bucketed on the join column. … Here, only the required buckets are fetched onto the mapper side but not the complete data of the table. It means that only the matching buckets of small tables are replicated onto each mapper while joining.

How does sort merge join work?

The sort-merge join is a common join algorithm in database systems using sorting. The join predicate needs to be an equality join predicate. The algorithm sorts both relations on the join attribute and then merges the sorted relations by scanning them sequentially and looking for qualifying tuples.

What is the difference between left join and left outer join?

There really is no difference between a LEFT JOIN and a LEFT OUTER JOIN. Both versions of the syntax will produce the exact same result in PL/SQL. Some people do recommend including outer in a LEFT JOIN clause so it’s clear that you’re creating an outer join, but that’s entirely optional.

What is the default join in Hive?

Hive supports equi joins by default. You can optimize your join by using Map-side Join or a Merge Join depending upon the size and sort order of your tables.

What is explode in Hive?

The explode function explodes an array to multiple rows. Returns a row-set with a single column (col), one row for each element from the array.

Which is faster map side join or reduce side join Why?

Map side join is usually used when one data set is large and the other data set is small. Whereas the Reduce side join can join both the large data sets. The Map side join is faster as it does not have to wait for all mappers to complete as in case of reducer. Hence reduce side join is slower.

What join is also called as map side only join?

Apache Hive Map Join is also known as Auto Map Join, or Map Side Join, or Broadcast Join. There is one more join available that is Common Join or Sort Merge Join. However, there is a major issue with that it there is too much activity spending on shuffling data around. So, as a result, that slows the Hive Queries.

Which join is not supported by Hive?

Only equality joins, outer joins, and left semi joins are supported in Hive. Hive does not support join conditions that are not equality conditions as it is very difficult to express such conditions as a map/reduce job. Also, more than two tables can be joined in Hive.

What is view in HQL?

A view allows a query to be saved and treated like a table. It is a logical construct, as it does not store data like a table. In other words, materialized views are not currently supported by Hive.

How can I see views in Hive?

There is nothing like SHOW VIEWS in Hive. DESCRIBE and DESCRIBE EXTENDED statements can be used for views like for tables, however, for DESCRIBE EXTENDED, the detailed table information has a variable named typeable which has value = ‘virtual view’ for views. EXTERNAL and LOCATION clause also works for views.

What is deferred rebuild in Hive?

If WITH DEFERRED REBUILD is specified on CREATE INDEX, then the newly created index is initially empty (regardless of whether the table contains any data). The ALTER INDEX … REBUILD command can be used to build the index structure for all partitions or a single partition.

What is skew join in Hive?

A skew join is used when there is a table with skew data in the joining column. A skew table is a table that is having values that are present in large numbers in the table compared to other data. Skew data is stored in a separate file while the rest of the data is stored in a separate file.

What is dynamic partitioning in Hive?

Dynamic partitioning is the strategic approach to load the data from the non-partitioned table where the single insert to the partition table is called a dynamic partition.

What is Bucket map?

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.

Is merge join faster than nested loop?

Nested loop is generally chosen for smaller tables and if it is possible to do Index seek on the inner table to ensure better performance. Merge Join is considered to be more efficient for large tables with the keys columns sorted. It only need to read the keys once, and does not require lot of CPU cycles.

Why is sort merge join important?

The key idea of the sort-merge algorithm is to first sort the relations by the join attribute, so that interleaved linear scans will encounter these sets at the same time. In practice, the most expensive part of performing a sort-merge join is arranging for both inputs to the algorithm to be presented in sorted order.

What is the difference between left outer join and right outer join?

The main difference between the Left Join and Right Join lies in the inclusion of non-matched rows. Left outer join includes the unmatched rows from the table which is on the left of the join clause whereas a Right outer join includes the unmatched rows from the table which is on the right of the join clause.

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