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| <!DOCTYPE concept PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd"> |
| <concept id="use_null_slots_cache"> |
| |
| <title>USE_NULL_SLOTS_CACHE Query Option</title> |
| <titlealts audience="PDF"><navtitle>USE NULL SLOTS CACHE</navtitle></titlealts> |
| <prolog> |
| <metadata> |
| <data name="Category" value="Impala"/> |
| <data name="Category" value="Impala Query Options"/> |
| <data name="Category" value="Troubleshooting"/> |
| <data name="Category" value="Querying"/> |
| <data name="Category" value="Developers"/> |
| <data name="Category" value="Data Analysts"/> |
| </metadata> |
| </prolog> |
| |
| <conbody> |
| <p> |
| <indexterm audience="hidden">USE_NULL_SLOTS_CACHE Query Option</indexterm> Impala |
| optimizes expressions that contain null-rejecting conjuncts - expressions that are |
| false if all "slots" (values returned from a table) are <codeph>NULL</codeph>. |
| Evaluating whether a conjunct is null-rejecting can be expensive when performed over a |
| lot of expressions. The "null slots cache" speeds analysis of generated queries with |
| many similar expressions by caching the result of evaluating an expression after slots |
| have been replaced with <codeph>NULL</codeph>s. |
| </p> |
| |
| <p> |
| If query analysis uses a lot of memory, performance of the "null slots cache" can be |
| reviewed by enabling logging <codeph>org.apache.impala.analysis.Analyzer=DEBUG</codeph> |
| and searching coordinator logs for messages like |
| </p> |
| |
| <codeblock> |
| null slots cache size: 286, median entry: 4416.0, 99th entry: 252688.88, hit rate: 0.99456364 |
| </codeblock> |
| |
| <p><b>Type: </b>BOOLEAN</p> |
| <p><b>Default: </b>TRUE</p> |
| <p><b>Added in: </b>Impala 4.5</p> |
| </conbody> |
| </concept> |