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| <!DOCTYPE concept PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd"> |
| <concept rev="3.1.0" id="topn_bytes_limit"> |
| |
| <title>TOPN_BYTES_LIMIT Query Option (<keyword keyref="impala31_full"/> or higher only)</title> |
| |
| <titlealts audience="PDF"> |
| |
| <navtitle>TOPN_BYTES_LIMIT</navtitle> |
| |
| </titlealts> |
| |
| <prolog> |
| <metadata> |
| <data name="Category" value="Impala"/> |
| <data name="Category" value="Impala Query Options"/> |
| <data name="Category" value="Querying"/> |
| <data name="Category" value="Developers"/> |
| <data name="Category" value="Data Analysts"/> |
| </metadata> |
| </prolog> |
| |
| <conbody> |
| |
| <p> |
| The <codeph>TOPN_BYTES_LIMIT</codeph> query option places a limit on the amount of |
| estimated memory that Impala can process for <i>top-N</i> queries. |
| </p> |
| |
| <p> |
| <i>Top-N</i> queries are the queries that include both <codeph>ORDER BY</codeph> and |
| <codeph>LIMIT</codeph> clauses. <i>Top-N</i> queries don't spill to disk so they have to |
| keep all rows they process in memory, and those queries can cause out-of-memory issues |
| when running with a large limit and an offset. If the Impala planner estimates that a |
| <i>top-N</i> operator will process more bytes than the <codeph>TOPN_BYTES_LIMIT</codeph> |
| value, it will replace the <i>top-N</i> operator with the <i>sort</i> operator. Switching |
| to the <i>sort</i> operator allows Impala to spill to disk, thus requiring less memory |
| than <i>top-N</i>, but potentially with performance penalties. |
| </p> |
| |
| <p> |
| The option has no effect when set to 0 or -1. |
| </p> |
| |
| <p> |
| <b>Syntax:</b> |
| </p> |
| |
| <codeblock>SET TOPN_BYTES_LIMIT=<varname>limit</varname></codeblock> |
| |
| <p> |
| <b>Type:</b> Number |
| </p> |
| |
| <p> |
| <b>Default:</b> 536870912 (512 MB) |
| </p> |
| |
| <p> |
| <b>Added in:</b> <keyword keyref="impala31"/> |
| </p> |
| |
| </conbody> |
| |
| </concept> |