I am trying to read and write a trivial dataset into Julia. The dataset is mtcars
, taken from R, with an arbitrarily added column bt
with random Boolean values. The file/folder structure (below) was written out using the R arrow
package.
The files are laid out as follows:
arr
|-- bt=false
| `-- part-1.arrow
`-- bt=true
`-- part-0.arrow
How can I faithfully reproduce the original table in Julia?
What I've tried so far:
Using the Parquet.jl
package. Documentation suggests that it should automatically detect partitioning folder structure for columns of bool/string/date type. When I read the data in, using read_parquet(path; kwargs)
, the resulting data structure does not have the bt
column. I've tried setting the column_generator
keyword argument to the default Parquet.dataset_column_generator
but this did not work.
Using Arrow.jl
- I cannot find a documented way (unless I misunderstood) to directly read in a partitioned data structure.
R does not generate additional metadata files to store the schema, but I understand this is optional and not part of the arrow spec?
We can always read the parquet file to a dataframe in Spark and see the content. They are of columnar formats and are more suitable for analytical environments,write once and read many. Parquet files are more suitable for Read intensive applications.
Apache Parquet is a file format designed to support fast data processing for complex data. Unlike row-based formats like CSV, Parquet is column-oriented – meaning the values of each table column are stored next to each other rather than those of each record.
As you can learn more in the Apache Parquet format, a Parquet file consists of multiple row groups. read_table will read all of the row groups and concatenate them into a single table.
In practice, a Parquet dataset may consist of many files in many directories. We can read a single file back with read_table: You can pass a subset of columns to read, which can be much faster than reading the whole file (due to the columnar layout):
As you can learn more in the Apache Parquet format, a Parquet file consists of multiple row groups. read_table will read all of the row groups and concatenate them into a single table. You can read individual row groups with read_row_group:
Try this. They have listed a method as this
Partitions in a parquet file or dataset can also be iterated over using an iterator returned by the Tables.partitions
method.
using Parquet, DataFrames
for partition in Tables.partitions(read_parquet(path))
df = DataFrame(partition)
...
end
For further reference: https://github.com/JuliaIO/Parquet.jl
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