@PublicEvolving public interface SupportsProjectionPushDown
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,
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
|Modifier and Type||Method and Description|
Provides the field index paths that should be used for a projection.
Returns whether this source supports nested projection.
void applyProjection(int projectedFields)
In the example mentioned in
SupportsProjectionPushDown, this method would receive:
[, ]which is equivalent to
[, [1, 0]]which is equivalent to
[["s"], ["r", "d"]]]if
projectedFields- field index paths of all fields that must be present in the physically produced data
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