org.apache.flink.table.connector.source.abilities

Interface SupportsProjectionPushDown

@PublicEvolving
public interface SupportsProjectionPushDown

Enables to push down a (possibly nested) projection into a ScanTableSource.

Given the following SQL:

   CREATE TABLE t (i INT, r ROW < d DOUBLE, b BOOLEAN>, s STRING);
   SELECT s, r.d FROM t;
 

In the above example, r.d and s are required fields. Other fields can be skipped in a projection. Compared to table's schema, fields are reordered.

By default, if this interface is not implemented, a projection is applied in a subsequent operation after the source.

For efficiency, a source can push a projection further down in order to be close to the actual data generation. A projection is only selecting fields that are used by a query (possibly in a different field order). It does not contain any computation. A projection can either be performed on the fields of the top-level row only or consider nested fields as well (see supportsNestedProjection()).

Note: If a source implements SupportsComputedColumnPushDown, the projection must be applied to the physical data in the first step. The SupportsComputedColumnPushDown (already aware of the projection) will then use the projected physical data and insert computed columns into the result.

Method Summary

Modifier and Type	Method and Description
void	applyProjection(int[][] projectedFields) Provides the field index paths that should be used for a projection.
boolean	supportsNestedProjection() Returns whether this source supports nested projection.

Method Detail

supportsNestedProjection

boolean supportsNestedProjection()

Returns whether this source supports nested projection.

applyProjection

void applyProjection(int[][] projectedFields)

Provides the field index paths that should be used for a projection. The indices are 0-based and support fields within (possibly nested) structures if this is enabled via supportsNestedProjection().

In the example mentioned in SupportsProjectionPushDown, this method would receive:

• [[2], [1]] which is equivalent to [["s"], ["r"]] if supportsNestedProjection() returns false.
• [[2], [1, 0]] which is equivalent to [["s"], ["r", "d"]]] if supportsNestedProjection() returns true.

Parameters:

projectedFields - field index paths of all fields that must be present in the physically produced data