How to force MR execution when running simple Hive query?

Question

There is Hive 2.1.1 over MR, table test_table stored as sequencefile and the following ad-hoc query:

select t.*
  from test_table t
 where t.test_column = 100

Although this query can be executed without starting MR (fetch task), sometimes it takes longer to scan HDFS files rather than triggering a single map job.

When I want to enforce MR execution, I make the query more complex: e.g., using distinct. The significant drawbacks of this approach are:

1. Query results may differ from the original query's
2. Brings meaningless calculation load on the cluster

Is there a recommended way to force MR execution when using Hive-on-MR?

Answer 1

The hive executor decides either to execute map task or fetch task depending on the following settings (with defaults):

• hive.fetch.task.conversion ("more") — the strategy for converting MR tasks into fetch tasks
• hive.fetch.task.conversion.threshold (1 GB) — max size of input data that can be fed to a fetch task
• hive.fetch.task.aggr (false) — when set to true, queries like select count(*) from src also can be executed in a fetch task

It prompts me the following two options:

1. set hive.fetch.task.conversion.threshold to a lower value, e.g. 512 Mb
2. set hive.fetch.task.conversion to "none"

For some reason lowering the threshold did not change anything in my case, so I stood with the second option: seems fine for ad-hoc queries.

More details regarding these settings can be found in Cloudera forum and Hive wiki.