docs/build/plain-html/topics/impala_disable_codegen_rows_threshold.html - impala - Git at Google

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   <h1 class="title topictitle1" id="ariaid-title1">DISABLE_CODEGEN_ROWS_THRESHOLD Query Option (<span class="keyword">Impala 2.10</span> or higher only)</h1>


   <div class="body conbody">

     <p class="p"> This setting controls the cutoff point (in terms of number
       of rows processed per Impala daemon) below which Impala disables native
       code generation for the whole query. Native code generation is very
       beneficial for queries that process many rows because it reduces the time
       taken to process of each row. However, generating the native code adds
       latency to query startup. Therefore, automatically disabling codegen for
       queries that process relatively small amounts of data can improve query
       response time. </p>


     <p class="p">
         <strong class="ph b">Syntax:</strong>
       </p>


 <pre class="pre codeblock"><code>SET DISABLE_CODEGEN_ROWS_THRESHOLD=<var class="keyword varname">number_of_rows</var></code></pre>

     <p class="p">
       <strong class="ph b">Type:</strong> numeric
     </p>


     <p class="p">
       <strong class="ph b">Default:</strong> 50000
     </p>


     <p class="p">
       <strong class="ph b">Usage notes:</strong> Typically, you increase the default value to make this optimization apply to more queries.
       If incorrect or corrupted table and column statistics cause Impala to apply this optimization incorrectly to
       queries that actually involve substantial work, you might see the queries being slower as a result of codegen
       being disabled. In that case, recompute statistics with the <code class="ph codeph">COMPUTE STATS</code> or
       <code class="ph codeph">COMPUTE INCREMENTAL STATS</code> statement. If there is a problem collecting accurate statistics,
       you can turn this feature off by setting the value to 0.
     </p>


     <p class="p">
         <strong class="ph b">Internal details:</strong>
       </p>


     <p class="p">
       This setting applies to queries where the number of rows processed can be accurately
       determined, either through table and column statistics, or by the presence of a
       <code class="ph codeph">LIMIT</code> clause. If Impala cannot accurately estimate the number of rows,
       then this setting does not apply.
     </p>


     <p class="p">
       If a query uses the complex data types <code class="ph codeph">STRUCT</code>, <code class="ph codeph">ARRAY</code>,
       or <code class="ph codeph">MAP</code>, then codegen is never automatically disabled regardless of the
       <code class="ph codeph">DISABLE_CODEGEN_ROWS_THRESHOLD</code> setting.
     </p>


     <p class="p">
         <strong class="ph b">Added in:</strong> <span class="keyword">Impala 2.10.0</span>
       </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>
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	<meta name="DC.Relation" scheme="URI" content="../topics/impala_set.html" />
	<meta name="prodname" content="Impala" />
	<meta name="prodname" content="Impala" />
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	<meta name="version" content="Impala 3.4.x" />
	<meta name="DC.Format" content="XHTML" />
	<meta name="DC.Identifier" content="disable_codegen_rows_threshold" />
	<link rel="stylesheet" type="text/css" href="../commonltr.css" />
	<title>DISABLE_CODEGEN_ROWS_THRESHOLD Query Option (Impala 2.10 or higher only)</title>
	</head>
	<body id="disable_codegen_rows_threshold">


	<h1 class="title topictitle1" id="ariaid-title1">DISABLE_CODEGEN_ROWS_THRESHOLD Query Option (<span class="keyword">Impala 2.10</span> or higher only)</h1>




	<div class="body conbody">

	<p class="p"> This setting controls the cutoff point (in terms of number
	of rows processed per Impala daemon) below which Impala disables native
	code generation for the whole query. Native code generation is very
	beneficial for queries that process many rows because it reduces the time
	taken to process of each row. However, generating the native code adds
	latency to query startup. Therefore, automatically disabling codegen for
	queries that process relatively small amounts of data can improve query
	response time. </p>


	<p class="p">
	<strong class="ph b">Syntax:</strong>
	</p>


	<pre class="pre codeblock"><code>SET DISABLE_CODEGEN_ROWS_THRESHOLD=<var class="keyword varname">number_of_rows</var></code></pre>

	<p class="p">
	<strong class="ph b">Type:</strong> numeric
	</p>


	<p class="p">
	<strong class="ph b">Default:</strong> 50000
	</p>


	<p class="p">
	<strong class="ph b">Usage notes:</strong> Typically, you increase the default value to make this optimization apply to more queries.
	If incorrect or corrupted table and column statistics cause Impala to apply this optimization incorrectly to
	queries that actually involve substantial work, you might see the queries being slower as a result of codegen
	being disabled. In that case, recompute statistics with the <code class="ph codeph">COMPUTE STATS</code> or
	<code class="ph codeph">COMPUTE INCREMENTAL STATS</code> statement. If there is a problem collecting accurate statistics,
	you can turn this feature off by setting the value to 0.
	</p>


	<p class="p">
	<strong class="ph b">Internal details:</strong>
	</p>


	<p class="p">
	This setting applies to queries where the number of rows processed can be accurately
	determined, either through table and column statistics, or by the presence of a
	<code class="ph codeph">LIMIT</code> clause. If Impala cannot accurately estimate the number of rows,
	then this setting does not apply.
	</p>


	<p class="p">
	If a query uses the complex data types <code class="ph codeph">STRUCT</code>, <code class="ph codeph">ARRAY</code>,
	or <code class="ph codeph">MAP</code>, then codegen is never automatically disabled regardless of the
	<code class="ph codeph">DISABLE_CODEGEN_ROWS_THRESHOLD</code> setting.
	</p>


	<p class="p">
	<strong class="ph b">Added in:</strong> <span class="keyword">Impala 2.10.0</span>
	</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>