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| <!DOCTYPE concept PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd"> |
| <concept id="max_scan_range_length"> |
| |
| <title>MAX_SCAN_RANGE_LENGTH Query Option</title> |
| <titlealts audience="PDF"><navtitle>MAX_SCAN_RANGE_LENGTH</navtitle></titlealts> |
| <prolog> |
| <metadata> |
| <data name="Category" value="Impala"/> |
| <data name="Category" value="Impala Query Options"/> |
| <data name="Category" value="Performance"/> |
| <data name="Category" value="Developers"/> |
| <data name="Category" value="Data Analysts"/> |
| </metadata> |
| </prolog> |
| |
| <conbody> |
| |
| <p> |
| <indexterm audience="hidden">MAX_SCAN_RANGE_LENGTH query option</indexterm> |
| Maximum length of the scan range. Interacts with the number of HDFS blocks in the table to determine how many |
| CPU cores across the cluster are involved with the processing for a query. (Each core processes one scan |
| range.) |
| </p> |
| |
| <p> |
| Lowering the value can sometimes increase parallelism if you have unused CPU capacity, but a too-small value |
| can limit query performance because each scan range involves extra overhead. |
| </p> |
| |
| <p> |
| Only applicable to HDFS tables. Has no effect on Parquet tables. Unspecified or 0 indicates backend default, |
| which is the same as the HDFS block size for each table. |
| </p> |
| |
| <p> |
| Although the scan range can be arbitrarily long, Impala internally uses an 8 MB read buffer so that it can |
| query tables with huge block sizes without allocating equivalent blocks of memory. |
| </p> |
| |
| <p> |
| <b>Type:</b> numeric |
| </p> |
| |
| <p rev="IMPALA-3799"> |
| In <keyword keyref="impala27_full"/> and higher, the argument value can include unit specifiers, |
| such as <codeph>100m</codeph> or <codeph>100mb</codeph>. In previous versions, |
| Impala interpreted such formatted values as 0, leading to query failures. |
| </p> |
| |
| <p> |
| <b>Default:</b> 0 |
| </p> |
| </conbody> |
| </concept> |