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* Refresh query docs. Larger changes: - New doc: querying/datasource.md describes the various kinds of datasources you can use, and has examples for both SQL and native. - New doc: querying/query-execution.md describes how native queries are executed at a high level. It doesn't go into the details of specific query engines or how queries run at a per-segment level. But I think it would be good to add or link that content here in the future. - Refreshed doc: querying/sql.md updated to refer to joins, reformatted a bit, added a new "Query translation" section that explains how queries are translated from SQL to native, and removed configuration details (moved to configuration/index.md). - Refreshed doc: querying/joins.md updated to refer to join datasources. Smaller changes: - Add helpful banners to the top of query documentation pages telling people whether a given page describes SQL, native, or both. - Add SQL metrics to operations/metrics.md. - Add some color and cross-links in various places. - Add native query component docs to the sidebar, and renamed them so they look nicer. - Remove Select query from the sidebar. - Fix Broker SQL configs in configuration/index.md. Remove them from querying/sql.md. - Combined querying/searchquery.md and querying/searchqueryspec.md. * Updates. * Fix numbering. * Fix glitches. * Add new words to spellcheck file. * Assorted changes. * Further adjustments. * Add missing punctuation.
84 lines
2.6 KiB
Markdown
84 lines
2.6 KiB
Markdown
---
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id: virtual-columns
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title: "Virtual columns"
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---
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<!--
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~ Licensed to the Apache Software Foundation (ASF) under one
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~ or more contributor license agreements. See the NOTICE file
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~ distributed with this work for additional information
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~ regarding copyright ownership. The ASF licenses this file
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~ to you under the Apache License, Version 2.0 (the
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~ "License"); you may not use this file except in compliance
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~ with the License. You may obtain a copy of the License at
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~
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~ http://www.apache.org/licenses/LICENSE-2.0
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~
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~ Unless required by applicable law or agreed to in writing,
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~ software distributed under the License is distributed on an
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~ "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
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~ KIND, either express or implied. See the License for the
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~ specific language governing permissions and limitations
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~ under the License.
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-->
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> Apache Druid supports two query languages: [Druid SQL](sql.md) and [native queries](querying.md).
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> This document describes the native
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> language. For information about functions available in SQL, refer to the
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> [SQL documentation](sql.md#scalar-functions).
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Virtual columns are queryable column "views" created from a set of columns during a query.
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A virtual column can potentially draw from multiple underlying columns, although a virtual column always presents itself as a single column.
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Virtual columns can be used as dimensions or as inputs to aggregators.
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Each Apache Druid query can accept a list of virtual columns as a parameter. The following scan query is provided as an example:
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```
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{
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"queryType": "scan",
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"dataSource": "page_data",
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"columns":[],
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"virtualColumns": [
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{
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"type": "expression",
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"name": "fooPage",
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"expression": "concat('foo' + page)",
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"outputType": "STRING"
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},
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{
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"type": "expression",
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"name": "tripleWordCount",
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"expression": "wordCount * 3",
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"outputType": "LONG"
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}
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],
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"intervals": [
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"2013-01-01/2019-01-02"
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]
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}
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```
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## Virtual column types
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### Expression virtual column
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The expression virtual column has the following syntax:
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```
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{
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"type": "expression",
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"name": <name of the virtual column>,
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"expression": <row expression>,
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"outputType": <output value type of expression>
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}
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```
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|property|description|required?|
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|--------|-----------|---------|
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|name|The name of the virtual column.|yes|
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|expression|An [expression](../misc/math-expr.md) that takes a row as input and outputs a value for the virtual column.|yes|
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|outputType|The expression's output will be coerced to this type. Can be LONG, FLOAT, DOUBLE, or STRING.|no, default is FLOAT|
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