Table of Contents
How do I get a map side join in Hive?
The syntax for Map Join in Hive. If we want to perform a join query using map-join then we have to specify a keyword “/*+ MAPJOIN(b) */” in the statement as below: SELECT /*+ MAPJOIN(c) */ * FROM tablename1 t1 JOIN tablename2 t2 ON (t1. emp_id = t2. emp_id);
Does Hive support non equi join?
Hive does not support non equi joins: The common work around is to move the join condition to the where clause, which work fine when you want an inner join.
Does Hive support left anti join?
Here is a citation from Hive manual: “LEFT SEMI JOIN implements the uncorrelated IN/EXISTS subquery semantics in an efficient way. As of Hive 0.13 the IN/NOT IN/EXISTS/NOT EXISTS operators are supported using subqueries so most of these JOINs don’t have to be performed manually anymore.
How do I disable map side join in Hive?
convert. join=false we can disable this feature. However, common join can convert to map join automatically, when hive. auto.
What is Mapjoin 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.
Which is faster map side join or reduce side join?
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 is Mapjoin?
What is the difference between left join and left anti join?
So, the Left Anti Semi Join is the opposite of a Left Semi Join. However, that does not make it a right semi join. Instead “Anti” affects which rows are returned and which aren’t. Like the Left Semi Join, the Left Anti Semi Join returns only rows from the left row source.
Where is map side join done?
Map join is a type of join where a smaller table is loaded in memory and the join is done in the map phase of the MapReduce job. As no reducers are necessary, map joins are way faster than the regular joins.
Why do you use a non equi join?
A non-equi join can be used to solve some interesting query problems. You can use a non-equi join to check for duplicate value or when you need to compare one value in a table falls within a range of values within another.
How do you use anti join?
Anti joins are a type of filtering join, since they return the contents of the first table, but with their rows filtered depending upon the match conditions. The syntax for an anti join is more or less the same as for a left join: simply swap left_join() for anti_join() .
How does left anti join work?
Like the Left Semi Join, the Left Anti Semi Join returns only rows from the left row source. Each row is also returned at most once. And duplicates are also not eliminated. However, other than the Left Semi Join, the Left Anti Semi Join returns only rows for which no match on the right side exists.
What is the maximum number of partitions in Hive?
Current Hive versions with RDBMS metastore backend should be able to handle 10000+ partitions.
How do you increase parallelism in Hive?
Below are the list of practices that we can follow to optimize Hive Queries.
- Enable Compression in Hive.
- Optimize Joins.
- Avoid Global Sorting in Hive.
- Enable Tez Execution Engine.
- Optimize LIMIT operator.
- Enable Parallel Execution.
- Enable Mapreduce Strict Mode.
- Single Reduce for Multi Group BY.
How do I use mapjoins in hive?
Simply set hive.auto.convert.join to true in your config, and Hive will automatically use mapjoins for any tables smaller than hive.mapjoin.smalltable.filesize (default is 25MB). Mapjoins have a limitation in that the same table or alias cannot be used to join on different columns in the same query.
Can joins be automatically converted to bucket map joins in hive?
Whether joins can be automatically converted to bucket map joins in Hive when Tez is used as the execution engine ( Configuration Properties#hive.execution.engine is set to ” tez “). The log level to use for tasks executing as part of the DAG.
Where can I find a list of configuration properties for hive?
The canonical list of configuration properties is managed in the HiveConf Java class, so refer to the HiveConf.java file for a complete list of configuration properties available in your Hive release. For information about how to use these configuration properties, see Configuring Hive.