| <?xml version="1.0" encoding="UTF-8"?> |
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| <!DOCTYPE concept PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd"> |
| <concept id="schedule_random_replica" rev="2.5.0"> |
| |
| <title>SCHEDULE_RANDOM_REPLICA Query Option (<keyword keyref="impala25"/> or higher only)</title> |
| |
| <titlealts audience="PDF"> |
| |
| <navtitle>SCHEDULE RANDOM REPLICA</navtitle> |
| |
| </titlealts> |
| |
| <prolog> |
| <metadata> |
| <data name="Category" value="Impala"/> |
| <data name="Category" value="Impala Query Options"/> |
| <data name="Category" value="Performance"/> |
| <data name="Category" value="Developers"/> |
| <data name="Category" value="Data Analysts"/> |
| </metadata> |
| </prolog> |
| |
| <conbody> |
| |
| <p> |
| The <codeph>SCHEDULE_RANDOM_REPLICA</codeph> query option fine-tunes the scheduling |
| algorithm for deciding which host processes each HDFS data block or Kudu tablet to reduce |
| the chance of CPU hotspots. |
| </p> |
| |
| <p> |
| By default, Impala estimates how much work each host has done for the query, and selects |
| the host that has the lowest workload. This algorithm is intended to reduce CPU hotspots |
| arising when the same host is selected to process multiple data blocks / tablets. Use the |
| <codeph>SCHEDULE_RANDOM_REPLICA</codeph> query option if hotspots still arise for some |
| combinations of queries and data layout. |
| </p> |
| |
| <p> |
| The <codeph>SCHEDULE_RANDOM_REPLICA</codeph> query option only applies to tables and |
| partitions that are not enabled for the HDFS caching. |
| </p> |
| |
| <p conref="../shared/impala_common.xml#common/type_boolean"/> |
| |
| <p conref="../shared/impala_common.xml#common/default_false"/> |
| |
| <p conref="../shared/impala_common.xml#common/added_in_250"/> |
| |
| <p conref="../shared/impala_common.xml#common/related_info"/> |
| |
| <p> |
| <xref href="impala_perf_hdfs_caching.xml#hdfs_caching"/>, |
| <xref |
| href="impala_scalability.xml#scalability_hotspots"/> , |
| <xref |
| href="impala_replica_preference.xml#replica_preference"/> |
| </p> |
| |
| </conbody> |
| |
| </concept> |