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| <!DOCTYPE concept PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd"> |
| <concept id="replica_preference" rev="2.7.0"> |
| |
| <title>REPLICA_PREFERENCE Query Option (<keyword keyref="impala27"/> or higher only)</title> |
| <titlealts audience="PDF"><navtitle>REPLICA_PREFERENCE</navtitle></titlealts> |
| <prolog> |
| <metadata> |
| <data name="Category" value="Impala"/> |
| <data name="Category" value="Impala Query Options"/> |
| <data name="Category" value="Developers"/> |
| <data name="Category" value="Data Analysts"/> |
| </metadata> |
| </prolog> |
| |
| <conbody> |
| |
| <p rev="2.7.0"> |
| <indexterm audience="hidden">REPLICA_PREFERENCE query option</indexterm> |
| </p> |
| |
| <p> |
| The <codeph>REPLICA_PREFERENCE</codeph> query option |
| lets you spread the load more evenly if hotspots and bottlenecks persist, by allowing hosts to do local reads, |
| or even remote reads, to retrieve the data for cached blocks if Impala can determine that it would be |
| too expensive to do all such processing on a particular host. |
| </p> |
| |
| <p> |
| <b>Type:</b> numeric (0, 3, 5) |
| or corresponding mnemonic strings (<codeph>CACHE_LOCAL</codeph>, <codeph>DISK_LOCAL</codeph>, <codeph>REMOTE</codeph>). |
| The gaps in the numeric sequence are to accomodate other intermediate |
| values that might be added in the future. |
| </p> |
| |
| <p> |
| <b>Default:</b> 0 (equivalent to <codeph>CACHE_LOCAL</codeph>) |
| </p> |
| |
| <p conref="../shared/impala_common.xml#common/added_in_270"/> |
| |
| <p conref="../shared/impala_common.xml#common/related_info"/> |
| <p> |
| <xref href="impala_perf_hdfs_caching.xml#hdfs_caching"/>, <xref href="impala_schedule_random_replica.xml#schedule_random_replica"/> |
| </p> |
| |
| </conbody> |
| </concept> |