wjones127 commented on code in PR #13088:
URL: https://github.com/apache/arrow/pull/13088#discussion_r868114504
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cpp/src/arrow/dataset/dataset_test.cc:
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@@ -107,6 +111,38 @@ TEST_F(TestInMemoryDataset, InMemoryFragment) {
AssertSchemaEqual(batch->schema(), schema);
}
+TEST_F(TestInMemoryDataset, HandlesDifferingSchemas) {
+ constexpr int64_t kBatchSize = 1024;
+
+ // These schemas can be merged
+ SetSchema({field("i32", int32()), field("f64", float64())});
+ auto batch1 = ConstantArrayGenerator::Zeroes(kBatchSize, schema_);
+ SetSchema({field("i32", int64())});
+ auto batch2 = ConstantArrayGenerator::Zeroes(kBatchSize, schema_);
+ RecordBatchVector batches{batch1, batch2};
+
+ auto dataset = std::make_shared<InMemoryDataset>(schema_, batches);
Review Comment:
In file fragments, it's totally normal to have a physical schema that is
different from the dataset schema.
This came up when I realized we could create a union dataset out of
filesystem ones but not in-memory ones if the schemas differed.
> The other way (arguably) would be to have ReplaceSchema project the
batches (though that is a lot more work).
I thought about that, but then are we materializing the projected batches
before any scan is started? It seems more efficient for the projection to
happen as part of the scan.
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