| <?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 id="disable_hbase_num_rows_estimate"> |
| |
| <title>DISABLE_HBASE_NUM_ROWS_ESTIMATE Query Option</title> |
| |
| <titlealts audience="PDF"> |
| |
| <navtitle>DISABLE HBASE NUM ROWS ESTIMATE</navtitle> |
| |
| </titlealts> |
| |
| <prolog> |
| <metadata> |
| <data name="Category" value="Impala"/> |
| <data name="Category" value="Impala Query Options"/> |
| <data name="Category" value="Performance"/> |
| </metadata> |
| </prolog> |
| |
| <conbody> |
| |
| <p> |
| Use the <codeph>DISABLE_HBASE_NUM_ROWS_ESTIMATE</codeph> query option to disable key |
| sampling of HBase tables in row count and row size estimation. |
| </p> |
| |
| <p> |
| While generating a plan for an HBase query, the planner samples the underlying HBase |
| tables to estimate their row count and row size, and the sampling can negatively impact |
| the planning time. When the HBase table stats does not change much in short time, disable |
| the sampling by setting the <codeph>DISABLE_HBASE_NUM_ROWS_ESTIMATE</codeph> query option |
| to <codeph>TRUE</codeph>. And Impala planner will fall back to using Hive Metastore (HMS) |
| table stats instead. |
| </p> |
| |
| <p> |
| When <codeph>DISABLE_HBASE_NUM_ROWS_ESTIMATE</codeph> query option is set to |
| <codeph>TRUE</codeph>, you need to update the HMS table stats by running <codeph>COMPUTE |
| STATS</codeph>. Alternatively, you can manually set table statistics by running |
| <codeph>ALTER TABLE</codeph>. See <xref |
| href="impala_perf_stats.xml#perf_stats"/> |
| for details. |
| </p> |
| |
| <p> |
| The following values are supported: |
| <ul> |
| <li> |
| <codeph>TRUE</codeph> or <codeph>1</codeph>: Disables the normal key sampling of HBase |
| tables and uses HMS table stats for estimation. |
| </li> |
| |
| <li> |
| <codeph>FALSE</codeph> or <codeph>0</codeph>: Enables the normal sampling of HBase |
| tables. |
| </li> |
| </ul> |
| </p> |
| |
| <p> |
| <b>Type:</b> <codeph>BOOLEAN</codeph> |
| </p> |
| |
| <p> |
| <b>Default:</b> <codeph>FALSE</codeph> |
| </p> |
| |
| </conbody> |
| |
| </concept> |