You might pose your question on [email protected], more of the people with closer Gandiva knowledge are present there
On Sat, Apr 4, 2020 at 10:45 PM Yue Ni <[email protected]> wrote: > > Thanks so much. This is exactly what I was looking for, and I will give it a > try. > > BTW, as a follow up question, I wonder if there is anyone has any idea that > if I want to apply both selection and projection to a record batch, is there > any performance difference between these two ways: > 1) use filter to get a selection vector and convert it to an array ==> use > arrow::compute::Take to filter the record batch ==> construct a projector to > do projection for the filtered record batch > 2) use filter to get a selection vector ==> construct projector to do > projection and apply the selection vector at the same time > > The first approach allows me to process a query step by step (first selection > then projection) and the second approach is more concise but seems not > clearly separated compared to the first approach. I prefer the first approach > since it is easier to handle a filtering-only query, but would like to > confirm if it will degrade the performance if both selection and projection > are needed. > > On Sun, Apr 5, 2020 at 11:08 AM Wes McKinney <[email protected]> wrote: >> >> Try the arrow::compute::Take function >> >> https://github.com/apache/arrow/blob/master/cpp/src/arrow/compute/kernels/take.h#L121 >> >> On Sat, Apr 4, 2020 at 8:19 PM Yue Ni <[email protected]> wrote: >> > >> > Hi Wes, >> > >> > Thanks for the reply, but I don't think this is what I am looking for. >> > >> > It seems to me this `result.to_array()` will only return the array for the >> > selection vector >> > (https://github.com/apache/arrow/blob/b07c2626cb3cdd3498b41da9feedf7c8319baa27/python/pyarrow/gandiva.pyx#L130), >> > but it is not clear to me how I can use the selection vector to filter >> > the original record batch. >> > >> > If I understand correctly, in this test case, >> > `result.to_array().equals(pa.array(range(1000), type=pa.uint32()))` is >> > asserting that the selection vector has integer index values from [0, >> > 1000), but I am looking for to obtain an array in the filtered record >> > batch which should be an array of floats here. I know I can iterate >> > indices in the selection vector and use it to retrieve each row in >> > original record batch columns, but I am not certain if this is the right >> > way to do it. For example, if I have multiple columns in the original >> > record batch, do I need to iterate the selection vector multiple times to >> > filter each of the column? Since this is a common task, I expect there is >> > an easy/efficient API to do this. >> > >> > Basically, I am looking for something like: >> > selection_vector = filter.evaluate(record_batch, pa.default_memory_pool()) >> > filtered_column_arrays_in_record_batch = >> > record_batch.filter(selection_vector) # what is the API for doing this? >> > >> > In the C++ test cases, the closest thing I find is to construct a gandiva >> > projector to use the selection vector, but every test case there requires >> > client to construct a gandiva expression to build the projector (for >> > example, in this test case, a {sum_expr} is used for constructing the >> > projector, >> > https://github.com/apache/arrow/blob/967728fe4654e5d53bc0789e64e5a9ba7f27f263/cpp/src/gandiva/tests/filter_project_test.cc#L121 >> > ). >> > >> > I wonder if the filtering can be done without involving creating a >> > projection expression. At the same time, if projector is expected to be >> > used for doing this, what projector expressions should be used if I want >> > to keep all the columns as they are but just with some rows filtered based >> > on the criteria given? >> > >> > On Sun, Apr 5, 2020 at 7:27 AM Wes McKinney <[email protected]> wrote: >> >> >> >> You can see an example of filtering via the Python bindings >> >> >> >> https://github.com/apache/arrow/blob/master/python/pyarrow/tests/test_gandiva.py#L89 >> >> >> >> This creates a gandiva::Filter using gandiva::Filter::Make, which can >> >> be used to filter a RecordBatch >> >> >> >> Is this what you need? >> >> >> >> On Fri, Apr 3, 2020 at 7:12 PM Yue Ni <[email protected]> wrote: >> >> > >> >> > Hi there, >> >> > >> >> > I am using the gandiva C++ library for processing RecordBatch. I would >> >> > like to know how I can apply gandiva::Filter for a RecordBatch so that >> >> > I can do some filtering without using the projector. >> >> > >> >> > Since I don't find any documentation for it, I read some source code >> >> > about its usage, and here are the test cases I found about its usage: >> >> > 1) >> >> > https://github.com/apache/arrow/blob/967728fe4654e5d53bc0789e64e5a9ba7f27f263/cpp/src/gandiva/tests/filter_test.cc >> >> > 2) >> >> > https://github.com/apache/arrow/blob/967728fe4654e5d53bc0789e64e5a9ba7f27f263/cpp/src/gandiva/tests/filter_project_test.cc >> >> > >> >> > From my reading, I find it is possible to get a SelectionVector by >> >> > using the gandiva::Filter, at the same time, you can use the >> >> > SelectionVector with the gandiva::Projector to filter RecordBatch when >> >> > doing projection. My questions are: >> >> > 1) if I don't want to do any projection but simply filtering, what is >> >> > the recommended way to do it? >> >> > 2) I am trying to handle the case like "SELECT * FROM table WHERE >> >> > blah", is it recommended to apply filtering without projection in this >> >> > case or is there any alternative approach doing it? >> >> > >> >> > Thanks. >> >> > >> >> > Regards, >> >> > Yue >> >> >
