protected StructProjection lazyStructProjection() {
        if (structProjection == null) {
            structProjection = StructProjection.create(schema, deleteSchema);
        }
        return structProjection;
    }
Construct the {@link StructProjection} lazily because it is not serializable.
inlong-sort/sort-flink/sort-flink-v1.15/sort-connectors/iceberg/src/main/java/org/apache/inlong/sort/iceberg/sink/EqualityFieldKeySelector.java