| <?xml version="1.0" encoding="UTF-8"?> |
| <!-- |
| Licensed to the Apache Software Foundation (ASF) under one |
| or more contributor license agreements. See the NOTICE file |
| distributed with this work for additional information |
| regarding copyright ownership. The ASF licenses this file |
| to you under the Apache License, Version 2.0 (the |
| "License"); you may not use this file except in compliance |
| with the License. You may obtain a copy of the License at |
| |
| http://www.apache.org/licenses/LICENSE-2.0 |
| |
| Unless required by applicable law or agreed to in writing, |
| software distributed under the License is distributed on an |
| "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY |
| KIND, either express or implied. See the License for the |
| specific language governing permissions and limitations |
| under the License. |
| --> |
| <!DOCTYPE concept PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd"> |
| <concept rev="2.0.0" id="appx_count_distinct"> |
| |
| <title>APPX_COUNT_DISTINCT Query Option (<keyword keyref="impala20"/> or higher only)</title> |
| |
| <titlealts audience="PDF"> |
| |
| <navtitle>APPX COUNT DISTINCT</navtitle> |
| |
| </titlealts> |
| |
| <prolog> |
| <metadata> |
| <data name="Category" value="Impala"/> |
| <data name="Category" value="Impala Query Options"/> |
| <data name="Category" value="Aggregate Functions"/> |
| <data name="Category" value="Developers"/> |
| <data name="Category" value="Data Analysts"/> |
| </metadata> |
| </prolog> |
| |
| <conbody> |
| |
| <p rev="2.0.0"> |
| When the <codeph>APPX_COUNT_DISTINCT</codeph> query option is set to |
| <codeph>TRUE</codeph>, Impala implicitly converts <codeph>COUNT(DISTINCT)</codeph> |
| operations to the <codeph>NDV()</codeph> function calls. The resulting count is |
| approximate rather than precise. Enable the query option when a tolerable amount of error |
| is acceptable in order to obtain faster query results than with a <codeph>COUNT |
| (DISTINCT)</codeph> queries. |
| </p> |
| |
| <p conref="../shared/impala_common.xml#common/type_boolean"/> |
| |
| <p conref="../shared/impala_common.xml#common/default_false_0"/> |
| |
| <p conref="../shared/impala_common.xml#common/related_info"/> |
| |
| <p> |
| <xref href="impala_count.xml#count"/>, |
| <xref |
| href="impala_distinct.xml#distinct"/>, <xref href="impala_ndv.xml#ndv"/> |
| </p> |
| |
| </conbody> |
| |
| </concept> |