org.apache.drill.exec.physical.impl.scan.columns

Class ColumnsArrayParser

java.lang.Object

org.apache.drill.exec.physical.impl.scan.columns.ColumnsArrayParser

All Implemented Interfaces:

ScanLevelProjection.ScanProjectionParser

public class ColumnsArrayParser
extends Object
implements ScanLevelProjection.ScanProjectionParser

Parses the `columns` array. Doing so is surprisingly complex.

• Depending on what is known about the input file, the `columns` array may be required or optional.
• If the columns array is required, then the wildcard (`*`) expands to `columns`.
• If the columns array appears, then no other table columns can appear.
• Both 'columns' and the wildcard can appear for queries such as:

   select * from dfs.`multilevel/csv`
   where columns[1] < 1000

• The query can select specific elements such as `columns`[2]. In this case, only array elements can appear, not the unindexed `columns` column.
• If is possible for `columns` to appear twice. In this case, the project operator will make a copy.

To handle these cases, the general rule is: allow any number of wildcard or `columns` appearances in the input projection, but collapse them all down to a single occurrence of `columns` in the output projection. (Upstream code will prevent `columns` from appearing twice in its non-indexed form.)

It falls to this parser to detect a not-uncommon user error, a query such as the following:

 SELECT max(columns[1]) AS col1
 FROM cp.`textinput/input1.csv`
 WHERE col1 IS NOT NULL
 

In standard SQL, column aliases are not allowed in the WHERE clause. So, Drill will push two columns down to the scan operator: `columns`[1] and `col1`. This parser will detect the "extra" columns and must provide a message that helps the user identify the likely original problem.

Constructor Summary

Constructors

Constructor and Description
ColumnsArrayParser(boolean requireColumnsArray)
ColumnsArrayParser(boolean requireColumnsArray, boolean allowOtherCols)

Method Summary

Modifier and Type	Method and Description
void	bind(ScanLevelProjection builder)
void	build()
UnresolvedColumnsArrayColumn	columnsArrayCol()
boolean	parse(RequestedColumn inCol)
void	validate()
void	validateColumn(ColumnProjection col)

Methods inherited from class java.lang.Object

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

Constructor Detail

ColumnsArrayParser

public ColumnsArrayParser(boolean requireColumnsArray,
                          boolean allowOtherCols)

ColumnsArrayParser

public ColumnsArrayParser(boolean requireColumnsArray)

Method Detail

bind

public void bind(ScanLevelProjection builder)

Specified by:

bind in interface ScanLevelProjection.ScanProjectionParser

parse

public boolean parse(RequestedColumn inCol)

Specified by:

parse in interface ScanLevelProjection.ScanProjectionParser

validate

public void validate()

Specified by:

validate in interface ScanLevelProjection.ScanProjectionParser

validateColumn

public void validateColumn(ColumnProjection col)

Specified by:

validateColumn in interface ScanLevelProjection.ScanProjectionParser

build

public void build()

Specified by:

build in interface ScanLevelProjection.ScanProjectionParser

columnsArrayCol

public UnresolvedColumnsArrayColumn columnsArrayCol()