xarray-sql

Query Xarray with SQL

pip install xarray-sql

What is this?

This is an experiment to provide a SQL interface for array datasets.

import xarray as xr
import xarray_sql as xql

ds = xr.tutorial.open_dataset('air_temperature')

# The same as a dask-sql Context; i.e. an Apache DataFusion Context.
ctx = xql.XarrayContext()
ctx.from_dataset('air', ds, chunks=dict(time=24))  # the dataset needs to be chunked!
# data is only materialized when we make a query.

result = ctx.sql('''
  SELECT
    "lat", "lon", AVG("air") as air_avg
  FROM
    "air"
  GROUP BY
   "lat", "lon"
''')
# DataFrame()
# +------+-------+--------------------+
# | lat  | lon   | air_avg            |
# +------+-------+--------------------+
# | 75.0 | 205.0 | 259.88662671232834 |
# | 75.0 | 207.5 | 259.48268150684896 |
# | 75.0 | 230.0 | 258.9192123287667  |
# | 75.0 | 275.0 | 257.07574315068456 |
# | 75.0 | 322.5 | 250.11792123287654 |
# | 75.0 | 325.0 | 250.81590068493134 |
# | 72.5 | 205.0 | 262.74933904109537 |
# | 72.5 | 207.5 | 262.5384315068488  |
# | 72.5 | 230.0 | 260.82879452054743 |
# | 72.5 | 275.0 | 257.3063321917804  |
# +------+-------+--------------------+
# Data truncated.

# The full query is only made when we call `collect()`, or, in this case,
# `to_pandas()`.
df = result.to_pandas()
df.head()
#     lat    lon     air_avg
# 0  75.0  232.5  258.836188
# 1  75.0  247.5  257.716171
# 2  75.0  262.5  257.347959
# 3  75.0  277.5  257.671308
# 4  72.5  232.5  260.654401

Succinctly, we "pivot" Xarray Datasets (with consistent dimensions) to treat them like tables so we can run SQL queries against them.

Why build this?

A few reasons:

• Even though SQL is the lingua franca of data, scientific datasets are often inaccessible to non-scientists (SQL users).
• Joining tabular data with raster data is common yet difficult. It could be easy.
• There are many cloud-native, Xarray-openable datasets, from Google Earth Engine to the Source Cooperative. Wouldn’t it be great if these were also SQL-accessible? How can the bridge be built with minimal effort?

This is a light-weight way to prove the value of the interface.

The larger goal is to explore the hypothesis that the Pangeo ecosystem is a scientific database. Here, xarray-sql can be thought of as a missing DB front end.

How does it work?

All chunks in a Xarray Dataset are transformed into a Dask DataFrame via from_map() and to_dataframe(). For SQL support, we just use dask-sql. That's it!

2025 update: This library now implements a Dask-like from_map interface in pure DataFusion and PyArrow, but works with the same principle!

2026 update: Instead of from_map(), we make factory functions from blocks of Xarray datasets that return RecordBatchReaders. These feed into a Rust-based DataFusion TableProvider. Every chunk is uses the Arrow in memory format to translate between Python and Rust. Even still, the core of what makes this idea work is the core pivot() operation from where this project began!

Why does this work?

Underneath Xarray, Dask, and Pandas, there are NumPy arrays. These are paged in chunks and represented contiguously in memory. It is only a matter of metadata that breaks them up into ndarrays. pivot(), which uses to_dataframe(), just changes this metadata (via a ravel()/reshape()), back into a column amenable to a DataFrame. We take advantage of this light weight metadata change to make chunked information scannable by a DB engine (DataFusion).

What are the current limitations?

TBD, DataFusion provides a whole new world! Currently, we're looking for early users – "tire kickers", if you will. We'd love your input to shape the direction of this project! Please, give this a try and file issues as you see fit. Check out our contributing guide, too 😉.

I can say that for now, the library is oriented towards making whole scans of Xarray Datasets. Common filter optimizations (even basic ones like an .sel() on core dimensions, let alone predicate push downs) are not fully implemented yet. However, these operations and more are on our roadmap.

What would a deeper integration look like?

I have a few ideas so far. One approach involves applying operations directly on Xarray Datasets. This approach is being pursued here, as xql.

Deeper still: I was thinking we could make a virtual filesystem for parquet that would internally map to Zarr. Raster-backed virtual parquet would open up integrations to numerous tools like dask, pyarrow, duckdb, and BigQuery. More thoughts on this in #4.

2025 update: Something like this is being built across a few projects! The ones I know about are:

• CartoDB's Raquet
• The DataFusion community's arrow-zarr

As of writing, this project is amid integrating a rust-based DataFusion backend provided by arrow-zarr.

Roadmap

• Lazy evaluation via the pyarrow Dataset interface #93. Implemented in #100
• Support proper parallelism via proper partition handling on the rust/datafusion side. #106
• Support core datafusion optimizations to scan less data, like 104, ...
• Translate a single Zarr to a collection of tables via DataFusion's catalog interface #85.
• Distributed beyond a single node through the DataFusion integration with Ray Datasets #68 or Apache Ballista #98.
• Demo: calculate Sea Surface Temperature from 1940 - Present in SQL #36.
• Provide an option to integrate DataFusion directly to Zarr via Rust #4.
• (To be formally announced eventually): The 100 Trillion Row Challenge #34.

Sponsors & Contributors

I want to give a special thanks to the following folks and institutions:

• Pramod Gupta and the Anthromet Team at Google Research for the problem formation and design inspiration.
• Jake Wall and AI2/Ecoscope for compute resources and key use cases.
• Charles Stern, Stephan Hoyer, Alexander Kmoch, Wei Ji, and Qiusheng Wu for the early review and discussion of this project.
• Tom Nichols, Kyle Barron, Tom White, and Maxime Dion for the Array Working Group and DataFusion-specific collaboration.
• The gracious volunteer data science students at UCSD's DS3 org, who are working to make this library better.

License

Copyright 2024 Alexander Merose

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    https://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.

All vendored code has proper license attribution.