| <?xml version="1.0" encoding="UTF-8"?> |
| <!DOCTYPE html |
| PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> |
| <html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"> |
| <head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /> |
| |
| <meta name="copyright" content="(C) Copyright 2023" /> |
| <meta name="DC.rights.owner" content="(C) Copyright 2023" /> |
| <meta name="DC.Type" content="concept" /> |
| <meta name="DC.Title" content="SCHEDULE_RANDOM_REPLICA Query Option (Impala 2.5 or higher only)" /> |
| <meta name="DC.Relation" scheme="URI" content="../topics/impala_set.html" /> |
| <meta name="prodname" content="Impala" /> |
| <meta name="prodname" content="Impala" /> |
| <meta name="version" content="Impala 3.4.x" /> |
| <meta name="version" content="Impala 3.4.x" /> |
| <meta name="DC.Format" content="XHTML" /> |
| <meta name="DC.Identifier" content="schedule_random_replica" /> |
| <link rel="stylesheet" type="text/css" href="../commonltr.css" /> |
| <title>SCHEDULE_RANDOM_REPLICA Query Option (Impala 2.5 or higher only)</title> |
| </head> |
| <body id="schedule_random_replica"> |
| |
| |
| <h1 class="title topictitle1" id="ariaid-title1">SCHEDULE_RANDOM_REPLICA Query Option (<span class="keyword">Impala 2.5</span> or higher only)</h1> |
| |
| |
| |
| |
| |
| |
| <div class="body conbody"> |
| |
| <p class="p"> |
| The <code class="ph codeph">SCHEDULE_RANDOM_REPLICA</code> 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 class="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 |
| <code class="ph codeph">SCHEDULE_RANDOM_REPLICA</code> query option if hotspots still arise for some |
| combinations of queries and data layout. |
| </p> |
| |
| |
| <p class="p"> |
| The <code class="ph codeph">SCHEDULE_RANDOM_REPLICA</code> query option only applies to tables and |
| partitions that are not enabled for the HDFS caching. |
| </p> |
| |
| |
| <p class="p"> |
| <strong class="ph b">Type:</strong> Boolean; recognized values are 1 and 0, or <code class="ph codeph">true</code> and |
| <code class="ph codeph">false</code>; any other value interpreted as <code class="ph codeph">false</code> |
| </p> |
| |
| |
| <p class="p"> |
| <strong class="ph b">Default:</strong> <code class="ph codeph">false</code> |
| </p> |
| |
| |
| <p class="p"> |
| <strong class="ph b">Added in:</strong> <span class="keyword">Impala 2.5.0</span> |
| </p> |
| |
| |
| <p class="p"> |
| <strong class="ph b">Related information:</strong> |
| </p> |
| |
| |
| <p class="p"> |
| <a class="xref" href="impala_perf_hdfs_caching.html#hdfs_caching">Using HDFS Caching with Impala (Impala 2.1 or higher only)</a>, |
| <a class="xref" href="impala_scalability.html#scalability_hotspots">Avoiding CPU Hotspots for HDFS Cached Data</a> , |
| <a class="xref" href="impala_replica_preference.html#replica_preference">REPLICA_PREFERENCE Query Option (Impala 2.7 or higher only)</a> |
| </p> |
| |
| |
| </div> |
| |
| |
| <div class="related-links"> |
| <div class="familylinks"> |
| <div class="parentlink"><strong>Parent topic:</strong> <a class="link" href="../topics/impala_set.html">SET Statement</a></div> |
| </div> |
| </div></body> |
| </html> |
