lidavidm commented on a change in pull request #9810:
URL: https://github.com/apache/arrow/pull/9810#discussion_r604880136
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File path: cpp/examples/arrow/dataset-documentation-example.cc
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@@ -217,24 +229,29 @@ std::shared_ptr<arrow::Table> SelectAndProjectDataset(
auto scan_builder = dataset->NewScan().ValueOrDie();
std::vector<std::string> names;
std::vector<ds::Expression> exprs;
+ // Read all the original columns.
for (const auto& field : dataset->schema()->fields()) {
names.push_back(field->name());
exprs.push_back(ds::field_ref(field->name()));
}
+ // Also derive a new column.
names.push_back("b_large");
exprs.push_back(ds::greater(ds::field_ref("b"), ds::literal(1)));
ABORT_ON_FAILURE(scan_builder->Project(exprs, names));
Review comment:
Ah, the example here is using Project to define new virtual columns from
the physical columns, while your snippet is simply selecting a subset of the
physical columns to read. The former API is a superset of the latter. There's
an example of what you currently do a little bit prior to this in the file.
I've added explanatory comments for these examples, though in the prose I think
it should also be clear.
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