Efficient Querying

In general, Kartothek should know as early as possible which data to exclude, so Kartothek doesn’t have to read and transfer it. If you’re going to query a dataset a lot, you should consider storing it in an optimized layout. See Writing.

Background

To understand how to query efficiently, you should understand Kartothek’s querying process:

When querying data, Kartothek tries to apply the given predicates as early as possible, so it has to read and parse as few parquet files as possible:

  • During partition pruning, Kartothek looks if there exists an index for the filtered column. This may either be if we partitioned the column (primary index) or if we generated a secondary index for it. Kartothek then combines the results for multiple columns.
  • In the resulting set of Parquet files, Kartothek looks at the Parquet metadata, which states the range of values for each column in this file. If there are multiple row groups, each one specifies the range of values inside it. Kartothek checks if the queried data is in the range of a row group and therefore if this row group is loaded. This process is called Predicate Pushdown.

Kartothek then fetches these row groups and filters the resulting Pandas dataframes for the conditions that could not be (completely) resolved by then.

You can find more information on Parquet in the Parquet Documentation.

Reading

Following the dataset’s Storage Layout and querying process you should include the following in your query:

  • restrictions on indexed columns (columns on which the dataset is partitioned or secondary indices)
  • specific selection of columns

Through restrictions on partitioned columns and indexed columns, Kartothek can significantly reduce the number of parquet files to look at. The selection of columns allows it to transfer only a subset of the data.

Warning

When using the in operation to filter your data, avoid passing a long list of values (i.e. larger than 100 elements in the list), as this might slow down the read performance.

Example

In this example, our data has three integer columns A, B and C. The dataset is partitioned by A and a secondary index for B was generated.

df = kartothek.io.eager.read_table(
    store=some_store,
    predicates=[[("A", ">=", 5), ("A", "<", 10), ("B", "==", 100), ("C", "<", 0)]],
)
  • Kartothek looks into the index for B to find the relevant Parquet files.
  • It filters the Parquet files for the ones, that lie in the partitions A=5 to A=9.
  • The Parquet files are filtered by looking at their footer. Kartothek can see there if the range of values for column C match the searched values.
  • The relevant files are loaded and filtered with Pandas for C < 0.

Details on the parallel querying process can be found in Parallel Execution with Dask.

Writing

When writing datasets, you should think about the following points to optimize for better querying speed. When in doubt, test your chosen parameters and refine them based on your results.

  • Generate partitions and indices appropriate for your use case.
  • Look for the right sizing of the parquet files.
  • Check your row group sizing. They allow for partial loading of a Parquet file.
  • Sort column contents to produce compact and disjoint value ranges between row groups. This allows for better selections of row groups.
  • Avoid string columns. Try to use a more specific datatype. See Table type system for more information on possible column types.

See kartothek.io.dask.dataframe.update_dataset_from_ddf() for more information.