org.apache.druid.segment

Interface VirtualColumn

All Superinterfaces:

Cacheable

All Known Implementing Classes:

ExpressionVirtualColumn

public interface VirtualColumn
extends Cacheable

Virtual columns are "views" created over a ColumnSelectorFactory or ColumnSelector. 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.
default BitmapIndex	getBitmapIndex(String columnName, ColumnSelector selector) Returns the BitmapIndex for efficient filtering on columns that support it.
String	getOutputName() Output name of this column.
ColumnValueSelector<?>	makeColumnValueSelector(String columnName, ColumnSelectorFactory factory) Build a selector corresponding to this virtual column.
default ColumnValueSelector<?>	makeColumnValueSelector(String columnName, ColumnSelector columnSelector, ReadableOffset offset) Returns similar ColumnValueSelector object as returned by makeColumnValueSelector(String, ColumnSelectorFactory) except this method has full access to underlying column and can potentially provide a more efficient implementation.
DimensionSelector	makeDimensionSelector(DimensionSpec dimensionSpec, ColumnSelectorFactory factory) Build a selector corresponding to this virtual column.
default DimensionSelector	makeDimensionSelector(DimensionSpec dimensionSpec, ColumnSelector columnSelector, ReadableOffset offset) Returns similar DimensionSelector object as returned by makeDimensionSelector(DimensionSpec, ColumnSelectorFactory) except this method has full access to underlying column and can potentially provide a more efficient implementation.
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 org.apache.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

makeDimensionSelector

@Nullable
default DimensionSelector makeDimensionSelector(DimensionSpec dimensionSpec,
                                                          ColumnSelector columnSelector,
                                                          ReadableOffset offset)

Returns similar DimensionSelector object as returned by makeDimensionSelector(DimensionSpec, ColumnSelectorFactory) except this method has full access to underlying column and can potentially provide a more efficient implementation. Users of this interface must ensure to first call this method whenever possible. Typically this can not be called in query paths on top of IncrementalIndex which doesn't have columns as in persisted segments.

Parameters:

dimensionSpec -

columnSelector -

offset -

Returns:

the selector

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

makeColumnValueSelector

@Nullable
default ColumnValueSelector<?> makeColumnValueSelector(String columnName,
                                                                 ColumnSelector columnSelector,
                                                                 ReadableOffset offset)

Returns similar ColumnValueSelector object as returned by makeColumnValueSelector(String, ColumnSelectorFactory) except this method has full access to underlying column and can potentially provide a more efficient implementation. Users of this interface must ensure to first call this method whenever possible. Typically this can not be called in query paths on top of IncrementalIndex which doesn't have columns as in persisted segments.

Parameters:

columnName -

columnSelector -

offset -

Returns:

the selector

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

getBitmapIndex

default BitmapIndex getBitmapIndex(String columnName,
                                   ColumnSelector selector)

Returns the BitmapIndex for efficient filtering on columns that support it. This method is only used if ColumnCapabilities returned from capabilities(String) has flag for BitmapIndex support.

Parameters:

columnName -

selector -

Returns:

BitmapIndex