[GitHub] [beam] apilloud commented on a change in pull request #15216: [BEAM-12609] Enable projection pushdown in SchemaIO.

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apilloud commented on a change in pull request #15216:
URL: https://github.com/apache/beam/pull/15216#discussion_r676983861



##########
File path: sdks/java/core/src/main/java/org/apache/beam/sdk/schemas/io/PushdownProjector.java
##########
@@ -0,0 +1,48 @@
+/*
+ * 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.io;
+
+import org.apache.beam.sdk.annotations.Experimental;
+import org.apache.beam.sdk.schemas.FieldAccessDescriptor;
+import org.apache.beam.sdk.transforms.PTransform;
+import org.apache.beam.sdk.values.PCollection;
+import org.apache.beam.sdk.values.PInput;
+import org.apache.beam.sdk.values.Row;
+
+/**
+ * Factory for creating a {@link PTransform} that can execute a projection.
+ *
+ * <p>Typically this interface will be implemented by a reader {@link PTransform} that is capable of
+ * pushing down projection to an external source. For example, {@link SchemaIO#buildReader()} may
+ * return a {@link PushdownProjector} to which a projection may be applied later.
+ */
+@Experimental
+public interface PushdownProjector {
+  /**
+   * Returns a {@link PTransform} that will execute the projection specified by the {@link
+   * FieldAccessDescriptor}.
+   */
+  PTransform<? extends PInput, PCollection<Row>> withProjectionPushdown(

Review comment:
       Awesome.

##########
File path: sdks/java/extensions/sql/src/main/java/org/apache/beam/sdk/extensions/sql/meta/provider/SchemaIOTableProviderWrapper.java
##########
@@ -118,6 +123,34 @@ public Schema getSchema() {
       return begin.apply(readerTransform);
     }
 
+    @Override
+    public PCollection<Row> buildIOReader(
+        PBegin begin, BeamSqlTableFilter filters, List<String> fieldNames) {
+      PTransform<PBegin, PCollection<Row>> readerTransform = schemaIO.buildReader();
+      if (readerTransform instanceof PushdownProjector) {
+        PushdownProjector pushdownProjector = (PushdownProjector) readerTransform;
+        FieldAccessDescriptor fieldAccessDescriptor =
+            FieldAccessDescriptor.withFieldNames(fieldNames);
+        // The pushdown must return a PTransform that can be applied to a PBegin, or this cast will
+        // fail.
+        readerTransform =
+            (PTransform<PBegin, PCollection<Row>>)
+                pushdownProjector.withProjectionPushdown(fieldAccessDescriptor);
+      }

Review comment:
       Shouldn't this have an `else` that looks something like `BaseBeamTable` and reject attempts to push down if it isn't supported? You should probably also reject filter pushdown (the `if (!(filters instanceof DefaultTableFilter)) {` block).
   
   https://github.com/apache/beam/blob/243128a8fc52798e1b58b0cf1a271d95ee7aa241/sdks/java/extensions/sql/src/main/java/org/apache/beam/sdk/extensions/sql/meta/BaseBeamTable.java#L30




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