jon-wei commented on a change in pull request #6119: Add docs for virtual
columns and transform specs
URL: https://github.com/apache/incubator-druid/pull/6119#discussion_r208762761
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File path: docs/content/querying/virtual-columns.md
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@@ -0,0 +1,59 @@
+---
+layout: doc_page
+---
+
+# 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 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"
+ ]
+}
+```
+
+## Expression virtual column
+
+Druid currently has one type of virtual column, the expression virtual column.
Review comment:
I updated this part to have a "virtual column types" section header and put
"expression" under there without additional comments
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