I have a Parquet file and I don't want to read the whole file into memory. I want to read the metadata and then read the rest of the file on demand. That is, for example, I want to read the second page of the first column in the third-row group. How would I do that using Apache Parquet cpp library? I have the offset of the part that I want to read from the metadata and can read it directly from the disk. Is there any way to pass that buffer to Apache Parquet library to uncompress, decode and iterate through the values? How about the same thing for column chunk or row groups? Basically, I want to read the file partially and then pass it to the parquet APIs to process it as opposes to give the file handler to the API and let it go through the file. Is it possible?
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1 Answers
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Behind the scences this is what the Apache Parquet C++ library actually does. When you pass in a file handle, it will only read the parts it needs to. As it requires the file footer (the main metadata) to know where to find the segments of data, this will always be read. The data segments will only be read once you request them.
No need to write special code for this, the library already has it built-in. Thus, if you want to know in fine detail on how this is working, you only need to read the source of the library: https://github.com/apache/arrow/tree/master/cpp/src/parquet
