I have a blog post up on configuring Polybase:
Microsoft’s next recommendation is to make sure that predicate pushdown is enabled. To do that, we’re going to go back to the Hadoop VM and grab our yarn.application.classpath from there. To do that, cd to /etc/hadoop/conf/ and vi yarn-site.xml (or use whatever other text reader you want). Copy the value for yarn.application.classpath, which should be a pretty long string. Mine looks like:
<value>$HADOOP_CONF_DIR,/usr/hdp/current/hadoop-client/*,/usr/hdp/current/hadoop-client/lib/*,/usr/hdp/current/hadoop-hdfs-client/*,/usr/hdp/current/hadoop-hdfs-client/lib/*,/usr/hdp/current/hadoop-yarn-client/*,/usr/hdp/current/hadoop-yarn-client/lib/*</value>Now that you have a copy of that value, go to your SQL Server installation directory (by default, C:\Program Files\Microsoft SQL Server\MSSQL13.MSSQLSERVER\MSSQL\Binn\Polybase\Hadoop\conf) and open up yarn-site.xml. Paste the value into the corresponding yarn.application.classpath setting and you’re good to go.
This is part one of a series on using Polybase.