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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.
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| id | title |
|---|---|
| virtual-columns | Virtual columns |
Apache Druid supports two query languages: Druid SQL and native queries. This document describes the native language. For information about functions available in SQL, refer to the SQL documentation.
Virtual columns are queryable column "views" created from a set of columns during a query.
A virtual column can potentially draw from multiple underlying columns, although a virtual column always presents itself as a single column.
Virtual columns can be used as dimensions or as inputs to aggregators.
Each Apache Druid query can accept a list of virtual columns as a parameter. The following scan query is provided as an example:
{
"queryType": "scan",
"dataSource": "page_data",
"columns":[],
"virtualColumns": [
{
"type": "expression",
"name": "fooPage",
"expression": "concat('foo' + page)",
"outputType": "STRING"
},
{
"type": "expression",
"name": "tripleWordCount",
"expression": "wordCount * 3",
"outputType": "LONG"
}
],
"intervals": [
"2013-01-01/2019-01-02"
]
}
Virtual column types
Expression virtual column
The expression virtual column has the following syntax:
{
"type": "expression",
"name": <name of the virtual column>,
"expression": <row expression>,
"outputType": <output value type of expression>
}
| property | description | required? |
|---|---|---|
| name | The name of the virtual column. | yes |
| expression | An expression that takes a row as input and outputs a value for the virtual column. | yes |
| outputType | The expression's output will be coerced to this type. Can be LONG, FLOAT, DOUBLE, or STRING. | no, default is FLOAT |