[GitHub] [beam] ibzib commented on a change in pull request #15712: [BEAM-12976] Add Java interfaces for projection pushdown.

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ibzib commented on a change in pull request #15712:
URL: https://github.com/apache/beam/pull/15712#discussion_r741454282



##########
File path: sdks/java/core/src/main/java/org/apache/beam/sdk/schemas/ProjectionProducer.java
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@@ -0,0 +1,57 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.beam.sdk.schemas;
+
+import org.apache.beam.sdk.annotations.Experimental;
+
+/**
+ * A factory for operations that execute a projection on a {@link Schema}-aware {@link
+ * org.apache.beam.sdk.values.PCollection}.
+ *
+ * <p>Typically this interface will be implemented by a reader {@link
+ * org.apache.beam.sdk.transforms.PTransform} that is capable of pushing down a projection to an
+ * external source.
+ */
+@Experimental
+public interface ProjectionProducer<T> {
+  /** What kinds of projection support (if any) an operation offers. */
+  enum ProjectSupport {
+    /**
+     * No projections are supported. In other words, the operation can only return a fixed set of
+     * transforms from its input.
+     */
+    NONE,
+    /**
+     * Projections are supported as long as fields are projected in the same order as the data
+     * source.
+     */
+    WITHOUT_FIELD_REORDERING,

Review comment:
       I removed WITHOUT_FIELD_REORDERING, since as you pointed out, we can implement field reordering on any source by appending a DoFn at the end. 




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