[GitHub] [beam] nielm commented on a change in pull request #12611: [BEAM-10139][BEAM-10140] Add cross-language support for Java SpannerIO with python wrapper

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nielm commented on a change in pull request #12611:
URL: https://github.com/apache/beam/pull/12611#discussion_r501066733



##########
File path: sdks/java/io/google-cloud-platform/src/main/java/org/apache/beam/sdk/io/gcp/spanner/SpannerIO.java
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@@ -678,6 +703,42 @@ public Read withPartitionOptions(PartitionOptions partitionOptions) {
               .withTransaction(getTransaction());
       return input.apply(Create.of(getReadOperation())).apply("Execute query", readAll);
     }
+
+    SerializableFunction<Struct, Row> getFormatFn() {
+      return (SerializableFunction<Struct, Row>)
+          input ->
+              Row.withSchema(Schema.builder().addInt64Field("Key").build())
+                  .withFieldValue("Key", 3L)
+                  .build();
+    }
+  }
+
+  public static class ReadRows extends PTransform<PBegin, PCollection<Row>> {
+    Read read;
+    Schema schema;
+
+    public ReadRows(Read read, Schema schema) {
+      super("Read rows");
+      this.read = read;
+      this.schema = schema;

Review comment:
       I don't see any good solution here...
   When reading an entire table, it could be possible to read the table's schema first, and determine what types the columns are, but this does not work for a query as the query output columns may not correspond to table columns. 
   
   Adding `LIMIT 1` would only work for simple queries, anything with joins, `GROUP BY`, `ORDER BY` will require the majority of the query to be executed before a single row is returned. 
   
   So the only solution I can see is for the caller to specify the row Schema as you do here..




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