Store dependency table in different format

[hagenw]

For tables we support CSV to provide them in a human readable format, but this is not necessary for the dependency table. In addition, the dependency table is frequently accessed to gather basic information about a database.

I think it would make sense to switch to another format when storing it for new databases. It should be fast to read, and maybe support reading only parts like columns or rows of it to make sure it will always fit in memory.

[hagenw]

As a side note, pyarrow will become a dependency of pandas anyway: https://github.com/pandas-dev/pandas/blob/main/web/pandas/pdeps/0010-required-pyarrow-dependency.md
So, it should be fine if we starting integrating pyarrow based approaches here as well, e.g., storing dependencies as parquet files.

[hagenw]

We have now benchmark results for comparing using csv, pickle, parquet files to store the dependency table available at https://github.com/audeering/audb/tree/a8bb3367a37fae79601e189ccac76a1a12105bae/benchmarks#audbdependencies-loadingwriting-to-file.

We first focus on the results for reading as this will be performed more often than writing.

• The fastest results can be achieved when using pyarrow dtypes for the internal dataframe representation of the dependency table, together with pyarrow.Table for reading from csv and parquet files
• Reading speed is nearly identical from csv, pickle, and parquet files, and takes around 0.1 s for a dependency table with 1,000,000 entries
• The parquet file needs only around 20% of space on disk (20 MB for 1,000,000 entries)

When looking at writing performance we get:

• Writing speed is nearly identical for csv, pickle, parquet, and takes around 0.25 s for a dependency table with 1,000,000 entries

Having those results in mind it seems to be reasonable to switch storing the dependency table directly as parquet files, both on the server and in cache.

[hagenw]

Solved by #372.