Scenario: Reducer is slow in Azure HDInsight

This article describes troubleshooting steps and possible resolutions for issues when using Interactive Query components in Azure HDInsight clusters.

When running a query such as insert into table1 partition(a,b) select a,b,c from table2 the query plan starts a bunch of reducers but the data from each partition goes to a single reducer. This causes the query to be as slow as the time taken by the largest partition's reducer.

Open beeline and verify the value of set hive.optimize.sort.dynamic.partition.

The value of this variable is meant to be set to true/false based on the nature of the data.

If the partitions in the input table are less(say less than 10), and so is the number of output partitions, and the variable is set to true, this causes data to be globally sorted and written using a single reducer per partition. Even if the number of reducers available is larger, a few reducers may be lagging behind due to data skew and the max parallelism can't be attained. When changed to false, more than one reducer may handle a single partition and multiple smaller files are written out, resulting in faster insert. This might affect further queries though because of the presence of smaller files.

A value of true makes sense when the number of partitions is larger and data isn't skewed. In such cases the result of the map phase are written out such that each partition will be handled by a single reducer resulting in better subsequent query performance.

1. Try to repartition the data to normalize into multiple partitions.

2. If #1 isn't possible, set the value of the config to false in beeline session and try the query again. set hive.optimize.sort.dynamic.partition=false. Setting the value to false at a cluster level is not recommended. The value of true is optimal and set the parameter as necessary based on nature of data and query.

If you didn't see your problem or are unable to solve your issue, visit one of the following channels for more support:

• If you need more help, you can submit a support request from the Azure portal. Select Support from the menu bar or open the Help + support hub.