SupportsReadingMetadata (Flink : Table : Common 1.12.5 API)

```
@PublicEvolving
public interface SupportsReadingMetadata
```
Interface for ScanTableSources that support reading metadata columns.
Metadata columns add additional columns to the table's schema. A table source is responsible for adding requested metadata columns at the end of produced rows. This includes potentially forwarding metadata columns from contained formats.
Examples in SQL look like:
```
 // reads the column from corresponding metadata key `timestamp`
 CREATE TABLE t1 (i INT, s STRING, timestamp TIMESTAMP(3) WITH LOCAL TIME ZONE METADATA, d DOUBLE)

 // reads the column from metadata key `timestamp` and casts to INT
 CREATE TABLE t2 (i INT, s STRING, myTimestamp INT METADATA FROM 'timestamp', d DOUBLE)
 
```
By default, if this interface is not implemented, the statements above would fail because the table source does not provide a metadata key called `timestamp`.
If this interface is implemented, listReadableMetadata() lists all metadata keys and their corresponding data types that the source exposes to the planner. The planner will use this information for validation and insertion of explicit casts if necessary.
The planner will select required metadata columns (i.e. perform projection push down) and will call applyReadableMetadata(List, DataType) with a list of metadata keys. An implementation must ensure that metadata columns are appended at the end of the physical row in the order of the provided list after the apply method has been called.
Note: The final output data type emitted by a source changes from the physically produced data type to a data type with metadata columns. applyReadableMetadata(List, DataType) will pass the updated data type for convenience. If a source implements SupportsProjectionPushDown, the projection must be applied to the physical data in the first step. The passed updated data type will have considered information from SupportsProjectionPushDown already.
The metadata column's data type must match with listReadableMetadata(). For the examples above, this means that a table source for `t2` returns a TIMESTAMP and not INT. The casting to INT will be performed by the planner in a subsequent operation:
```
 // for t1 and t2
 ROW < i INT, s STRING, d DOUBLE >                                              // physical output
 ROW < i INT, s STRING, d DOUBLE, timestamp TIMESTAMP(3) WITH LOCAL TIME ZONE > // final output
 
```

方法概要

所有方法实例方法抽象方法
限定符和类型	方法和说明
`void`	`applyReadableMetadata(List<String> metadataKeys, DataType producedDataType)` Provides a list of metadata keys that the produced `RowData` must contain as appended metadata columns.
`Map<String,DataType>`	`listReadableMetadata()` Returns the map of metadata keys and their corresponding data types that can be produced by this table source for reading.

- 方法详细资料
  - listReadableMetadata
```
Map<String,DataType> listReadableMetadata()
```
    Returns the map of metadata keys and their corresponding data types that can be produced by this table source for reading.
    The returned map will be used by the planner for validation and insertion of explicit casts (see LogicalTypeCasts.supportsExplicitCast(LogicalType, LogicalType)) if necessary.
    The iteration order of the returned map determines the order of metadata keys in the list passed in applyReadableMetadata(List, DataType). Therefore, it might be beneficial to return a LinkedHashMap if a strict metadata column order is required.
    If a source forwards metadata from one or more formats, we recommend the following column order for consistency:
```
 KEY FORMAT METADATA COLUMNS + VALUE FORMAT METADATA COLUMNS + SOURCE METADATA COLUMNS
 
```
    Metadata key names follow the same pattern as mentioned in Factory. In case of duplicate names in format and source keys, format keys shall have higher precedence.
    Regardless of the returned DataTypes, a metadata column is always represented using internal data structures (see RowData).
    另请参阅:
    
    DecodingFormat.listReadableMetadata()
  - applyReadableMetadata
```
void applyReadableMetadata(List<String> metadataKeys,
                           DataType producedDataType)
```
    Provides a list of metadata keys that the produced RowData must contain as appended metadata columns.
    Note: Use the passed data type instead of TableSchema.toPhysicalRowDataType() for describing the final output data type when creating TypeInformation. If the source implements SupportsProjectionPushDown, the projection is already considered in the given output data type.
    
    参数:
    
    metadataKeys - a subset of the keys returned by listReadableMetadata(), ordered by the iteration order of returned map
    
    producedDataType - the final output type of the source
    
    另请参阅:
    
    DecodingFormat.applyReadableMetadata(List)

接口 SupportsReadingMetadata

方法概要

方法详细资料

listReadableMetadata

applyReadableMetadata