io.druid.segment

Interface VirtualColumn

All Superinterfaces:

Cacheable

All Known Implementing Classes:

ExpressionVirtualColumn, MapVirtualColumn

public interface VirtualColumn
extends Cacheable

Virtual columns are "views" created over a ColumnSelectorFactory. They can potentially draw from multiple underlying columns, although they always present themselves as if they were a single column. A virtual column object will be shared amongst threads and must be thread safe. The selectors returned from the various makeXXXSelector methods need not be thread safe.

Method Summary

Modifier and Type	Method and Description
ColumnCapabilities	capabilities(String columnName) Returns the capabilities of this virtual column, which includes a type that corresponds to the best performing base selector supertype (e.
String	getOutputName() Output name of this column.
ColumnValueSelector<?>	makeColumnValueSelector(String columnName, ColumnSelectorFactory factory) Build a selector corresponding to this virtual column.
DimensionSelector	makeDimensionSelector(DimensionSpec dimensionSpec, ColumnSelectorFactory factory) Build a selector corresponding to this virtual column.
List<String>	requiredColumns() Returns a list of columns that this virtual column will access.
boolean	usesDotNotation() Indicates that this virtual column can be referenced with dot notation.

Methods inherited from interface io.druid.java.util.common.Cacheable

getCacheKey

Method Detail

getOutputName

String getOutputName()

Output name of this column.

Returns:

name

makeDimensionSelector

DimensionSelector makeDimensionSelector(DimensionSpec dimensionSpec,
                                        ColumnSelectorFactory factory)

Build a selector corresponding to this virtual column. Also provides the name that the virtual column was referenced with (through DimensionSpec.getDimension(), which is useful if this column uses dot notation. The virtual column is expected to apply any necessary decoration from the dimensionSpec.

Parameters:

dimensionSpec - the dimensionSpec this column was referenced with

factory - column selector factory

Returns:

the selector, must not be null

makeColumnValueSelector

ColumnValueSelector<?> makeColumnValueSelector(String columnName,
                                               ColumnSelectorFactory factory)

Build a selector corresponding to this virtual column. Also provides the name that the virtual column was referenced with, which is useful if this column uses dot notation.

Parameters:

columnName - the name this virtual column was referenced with

factory - column selector factory

Returns:

the selector, must not be null

capabilities

ColumnCapabilities capabilities(String columnName)

Returns the capabilities of this virtual column, which includes a type that corresponds to the best performing base selector supertype (e. g. BaseLongColumnValueSelector) of the object, returned from makeColumnValueSelector(String, ColumnSelectorFactory). May vary based on columnName if this column uses dot notation.

Parameters:

columnName - the name this virtual column was referenced with

Returns:

capabilities, must not be null

requiredColumns

List<String> requiredColumns()

Returns a list of columns that this virtual column will access. This may include the names of other virtual columns. May be empty if a virtual column doesn't access any underlying columns. Does not pass columnName because there is an assumption that the list of columns needed by a dot-notation supporting virtual column will not vary based on the columnName.

Returns:

column names

usesDotNotation

boolean usesDotNotation()

Indicates that this virtual column can be referenced with dot notation. For example, a virtual column named "foo" could be referred to as "foo.bar" with the Cursor it is registered with. In that case, init will be called with columnName "foo.bar" rather than "foo".

Returns:

whether to use dot notation