mirror of https://github.com/apache/druid.git
81 lines
2.4 KiB
Markdown
81 lines
2.4 KiB
Markdown
---
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layout: doc_page
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title: "Virtual Columns"
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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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~ http://www.apache.org/licenses/LICENSE-2.0
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~ Unless required by applicable law or agreed to in writing,
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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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# Virtual Columns
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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 (incubating) 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.html) 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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