@PublicEvolving public interface SupportsWritingMetadata
DynamicTableSinks that support writing metadata columns.
Metadata columns add additional columns to the table's schema. A table sink is responsible for accepting requested metadata columns at the end of consumed rows and persist them. This includes potentially forwarding metadata columns to contained formats.
Examples in SQL look like:
// writes data to the corresponding metadata key `timestamp` CREATE TABLE t1 (i INT, s STRING, timestamp TIMESTAMP(3) WITH LOCAL TIME ZONE METADATA, d DOUBLE) // casts data from INT and writes to metadata key `timestamp` CREATE TABLE t2 (i INT, s STRING, myTimestamp INT METADATA FROM 'timestamp', d DOUBLE) // metadata is not persisted because metadata column is virtual CREATE TABLE t3 (i INT, s STRING, timestamp TIMESTAMP(3) WITH LOCAL TIME ZONE METADATA VIRTUAL, d DOUBLE)
By default, if this interface is not implemented, the statements above would fail because the table sink does not provide a metadata key called `timestamp`.
If this interface is implemented,
listWritableMetadata() lists all metadata keys and
their corresponding data types that the sink 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 and will call
applyWritableMetadata(List, DataType) with a list of metadata keys. An implementation must
ensure that metadata columns are accepted at the end of the physical row in the order of the
provided list after the apply method has been called.
The metadata column's data type must match with
listWritableMetadata(). For the
examples above, this means that a table sink for `t2` accepts a TIMESTAMP and not INT. The
casting from INT will be performed by the planner in a preceding operation:
// for t1 and t2 ROW < i INT, s STRING, d DOUBLE > // physical input ROW < i INT, s STRING, d DOUBLE, timestamp TIMESTAMP(3) WITH LOCAL TIME ZONE > // final input // for t3 ROW < i INT, s STRING, d DOUBLE > // physical input ROW < i INT, s STRING, d DOUBLE > // final input
|Modifier and Type||Method and Description|
Provides a list of metadata keys that the consumed
Returns the map of metadata keys and their corresponding data types that can be consumed by this table sink for writing.
The returned map will be used by the planner for validation and insertion of explicit
LogicalTypeCasts.supportsExplicitCast(LogicalType, LogicalType)) if
The iteration order of the returned map determines the order of metadata keys in the list
applyWritableMetadata(List, DataType). Therefore, it might be beneficial
to return a
LinkedHashMap if a strict metadata column order is required.
If a sink forwards metadata to one or more formats, we recommend the following column order for consistency:
KEY FORMAT METADATA COLUMNS + VALUE FORMAT METADATA COLUMNS + SINK METADATA COLUMNS
Metadata key names follow the same pattern as mentioned in
Factory. In case of
duplicate names in format and sink keys, format keys shall have higher precedence.
RowDatawill contain as appended metadata columns which must be persisted.
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