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2.3 KiB
2.3 KiB
| id | title |
|---|---|
| virtual-columns | Virtual Columns |
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 (incubating) 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 |