| <?xml version="1.0" encoding="UTF-8"?> |
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| <!DOCTYPE concept PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd"> |
| <concept rev="2.10.0" id="disable_codegen_rows_threshold"> |
| |
| <title>DISABLE_CODEGEN_ROWS_THRESHOLD Query Option (<keyword keyref="impala210_full"/> or higher only)</title> |
| <titlealts audience="PDF"><navtitle>DISABLE CODEGEN ROWS THRESHOLD</navtitle></titlealts> |
| <prolog> |
| <metadata> |
| <data name="Category" value="Impala"/> |
| <data name="Category" value="Impala Query Options"/> |
| <data name="Category" value="Scalability"/> |
| <data name="Category" value="Performance"/> |
| <data name="Category" value="Developers"/> |
| <data name="Category" value="Data Analysts"/> |
| </metadata> |
| </prolog> |
| |
| <conbody> |
| |
| <p rev="2.0.0"> 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 conref="../shared/impala_common.xml#common/syntax_blurb"/> |
| |
| <codeblock>SET DISABLE_CODEGEN_ROWS_THRESHOLD=<varname>number_of_rows</varname></codeblock> |
| |
| <p> |
| <b>Type:</b> numeric |
| </p> |
| |
| <p> |
| <b>Default:</b> 50000 |
| </p> |
| |
| <p> |
| <b>Usage notes:</b> 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 <codeph>COMPUTE STATS</codeph> or |
| <codeph>COMPUTE INCREMENTAL STATS</codeph> statement. If there is a problem collecting accurate statistics, |
| you can turn this feature off by setting the value to 0. |
| </p> |
| |
| <p conref="../shared/impala_common.xml#common/internals_blurb"/> |
| |
| <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 |
| <codeph>LIMIT</codeph> clause. If Impala cannot accurately estimate the number of rows, |
| then this setting does not apply. |
| </p> |
| |
| <p rev="2.3.0"> |
| If a query uses the complex data types <codeph>STRUCT</codeph>, <codeph>ARRAY</codeph>, |
| or <codeph>MAP</codeph>, then codegen is never automatically disabled regardless of the |
| <codeph>DISABLE_CODEGEN_ROWS_THRESHOLD</codeph> setting. |
| </p> |
| |
| <p conref="../shared/impala_common.xml#common/added_in_2100"/> |
| |
| <!-- Don't have any other places that tie into this particular optimization technique yet. |
| Potentially: conceptual topics about code generation, distributed queries |
| |
| <p conref="../shared/impala_common.xml#common/related_info"/> |
| <p> |
| </p> |
| --> |
| |
| </conbody> |
| |
| </concept> |