Package org.apache.drill.exec.physical.impl.scan.project

Class NullColumnLoader

java.lang.Object

org.apache.drill.exec.physical.impl.scan.project.StaticColumnLoader

org.apache.drill.exec.physical.impl.scan.project.NullColumnLoader

public class NullColumnLoader
extends StaticColumnLoader

Create and populate null columns for the case in which a SELECT statement refers to columns that do not exist in the actual table. Nullable and array types are suitable for null columns. (Drill defines an empty array as the same as a null array: not true, but the best we have at present.) Required types cannot be used as we don't know what value to set into the column values.

Seeks to preserve "vector continuity" by reusing vectors when possible. Cases:

• A column a was available in a prior reader (or batch), but is no longer available, and is thus null. Reuses the type and vector of the prior reader (or batch) to prevent trivial schema changes.
• A column has an implied type (specified in the metadata about the column provided by the reader.) That type information is used instead of the defined null column type.
• A column has no type information. The type becomes the null column type defined by the reader (or nullable int by default.
• Required columns are not suitable. If any of the above found a required type, convert the type to nullable.
• The resulting column and type, whatever it turned out to be, is placed into the vector cache so that it can be reused by the next reader or batch, to again preserve vector continuity.

The above rules eliminate "trivia" schema changes, but can still result in "hard" schema changes if a required type is replaced by a nullable type.

Nested Class Summary

Nested Classes

Modifier and Type	Class	Description
static interface	NullColumnLoader.NullColumnSpec

Field Summary

Fields

Modifier and Type	Field	Description
static final TypeProtos.MajorType	DEFAULT_NULL_TYPE

Fields inherited from class org.apache.drill.exec.physical.impl.scan.project.StaticColumnLoader

loader, vectorCache

Constructor Summary

Constructors

Constructor	Description
NullColumnLoader(ResultVectorCache vectorCache, List<? extends NullColumnLoader.NullColumnSpec> defns, TypeProtos.MajorType nullType, boolean allowRequired)

Method Summary

Modifier and Type	Method	Description
VectorContainer	load(int rowCount)	Populate static vectors with the defined static values.
VectorContainer	output()

Methods inherited from class org.apache.drill.exec.physical.impl.scan.project.StaticColumnLoader

close

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Field Details

DEFAULT_NULL_TYPE

public static final TypeProtos.MajorType DEFAULT_NULL_TYPE

Constructor Details

NullColumnLoader

public NullColumnLoader(ResultVectorCache vectorCache,
 List<? extends NullColumnLoader.NullColumnSpec> defns,
 TypeProtos.MajorType nullType,
 boolean allowRequired)

Method Details

output

public VectorContainer output()

load

public VectorContainer load(int rowCount)

Description copied from class: StaticColumnLoader

Populate static vectors with the defined static values.

Specified by:

load in class StaticColumnLoader

Parameters:

rowCount - number of rows to generate. Must match the row count in the batch returned by the reader