Docs: Learn xarray-sql in 15 minutes by alxmrs · Pull Request #277 · xqlsystems/xarray-sql

alxmrs · 2026-10-21T17:03:41Z

A new docs page, Learn xarray-sql in 15 minutes, in the style of Learn X in Y Minutes, a site that teaches a language in one commented source file. It's for readers who know Python, NumPy or Xarray but not SQL, and for readers who know SQL but not Xarray. It sits first in the nav, after Home.

The page

Two short introductions, one per audience:
- for SQL readers, what a Dataset is and how it becomes a table;
- for Xarray readers, a table of Xarray operations and their SQL spelling (sel → WHERE, mean → GROUP BY, cumsum → window functions, alignment → JOIN, NaN → NULL).
The tour: one Python program of about 280 lines, built on a 12-cell Dataset defined inline, so it needs no downloads. In order:
0. the pivot from a Dataset to a table;
1. registering a Dataset and querying it on DataFusion;
2. SQL for Xarray users: WHERE, GROUP BY, NULLs, CASE, window functions, CTEs and JOINs;
3. to_dataset, and when it needs dims=;
4. mixed-dimension Datasets as schemas, with table_names;
5. the same table on DuckDB, Polars and ADBC (SQLite in memory);
6. geometry and H3 in DuckDB: a geometry column with ST_Within and bbox_conjuncts, and a table of stations joined to the grid by H3 hexagon ID;
7. xql.serve, queried in the same program through ADBC's Flight SQL driver;
8. gotchas: quoting, lazy DDL on DataFusion, which WHEREs skip chunks, and chunking.
Further reading links to Examples, Engines, Geospatial, Performance and Known issues. The introduction also points to the browser demo.

The test

tests/test_learn_page.py runs the page's program. Each # => comment is checked against the value of the line it annotates: the test parses the program, wraps each annotated expression in a check, and confirms by tokenizing that no # => comment is left unchecked. Changing one expected value makes the test fail and name the line. All the packages the page uses are already in the test extra. The tour installs DuckDB's spatial and h3 extensions, so the test skips when they can't be downloaded.

Checks

The test passes with -W error.
Ruff format and mypy are clean.
zensical build with 0.0.71 (the version CI deploys with) builds the page with no warnings, and every link resolves.

Filed while writing it

#276: without ORDER BY, to_dataset gives coordinates in the engine's nondeterministic row order. On DuckDB, the same query returned lat as [20, 10] on some runs. The page uses ORDER BY and explains why.

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Summary by CodeRabbit

Documentation
- Added a 15-minute, runnable tutorial introducing xarray-sql for Python, NumPy, Xarray, and SQL users.
- Documented dataset-to-table mapping, lazy chunked reads, SQL equivalents, grouping, filtering, aggregates, joins, NULL handling, and conversion back to Xarray.
- Added guidance on integrations, geospatial and H3 queries, Flight SQL, and common usage considerations.
- Added the tutorial to the documentation navigation.
Tests
- Added automated validation that the tutorial’s executable examples produce the documented results.

A docs page in the style of Learn X in Y Minutes: one commented Python program that teaches xarray-sql to readers who know Xarray or NumPy but not SQL, or SQL but not Xarray. It opens with the pivot from a Dataset to a table and a table of Xarray operations and their SQL, then tours registering and querying, SQL for Xarray users, the round trip back to a Dataset, mixed-dimension Datasets, DuckDB, Polars, ADBC and serving over Flight SQL, and ends with the usual gotchas. A test runs the page and checks every "# =>" comment against the value of the line it annotates, so the page can't drift from the library. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01NyN7idACeDMh94qrnFMu6h

coderabbitai · 2026-10-21T17:03:59Z

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📝 Walkthrough

Adds a 15-minute xarray-sql tutorial, includes it in the documentation navigation, and adds a test that executes the tutorial’s Python examples and checks their annotated results.

Changes: Learn tutorial

Layer / File(s)	Summary
Tutorial content and navigation `docs/learn.md`, `zensical.toml`	Adds Dataset-to-table concepts, SQL examples, Dataset reconstruction, integrations, geospatial and H3 queries, Flight SQL examples, usage guidance, and the navigation entry.
Executable example validation `tests/test_learn_page.py`	Extracts annotated Python code, transforms annotations into runtime checks, validates results, and skips when required dependencies or extensions are unavailable.

Priority: ⬇️ Low

Estimated code review effort: 2 (Simple) | ~15 minutes

Change: Other

Possibly related issues

DuckDB: spatial join on a GEOMETRY column of an Arrow scan fails (EXPRESSION_FILTER pushdown) — draft for upstream #278: The tutorial documents the spatial join pattern and related CRS handling described by this issue, but does not fix DuckDB behavior.
to_dataset: without ORDER BY, coordinates come back in the engine's nondeterministic row order #276: The tutorial documents using ORDER BY before to_dataset, matching the coordinate-order workaround described by this issue.

Merge Risk: ⚪ Minimal · up to 99aff

No merge-blocking issue is identified for the tutorial and its example test; proceed with normal checks.

Pre-merge checks: 4 · 1

Failed checks: 1 warning

Check name	Status	Explanation	Resolution
Docstring Coverage		Docstring coverage is 33.33% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 9 functions across 1 files. (1 skipped: 1…	Write docstrings for the functions missing them to satisfy the coverage threshold.

Passed checks: 4 passed

Check name	Status	Explanation
Description Check		Check skipped - CodeRabbit’s high-level summary is enabled.
Title check		The title clearly and concisely describes the primary change: adding a documentation tutorial for learning xarray-sql in 15 minutes.
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.

Full details: Docstring Coverage

Explanation

Docstring coverage is 33.33% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 9 functions across 1 files. (1 skipped: 1 unsupported.)

Resolution

Write docstrings for the functions missing them to satisfy the coverage threshold.

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alxmrs

Wonderful! LGTM

A new section of the tour shows DuckDB's spatial extension on a registered Dataset (a geometry column, ST_Within paired with bbox_conjuncts) and its community h3 extension joining a table of stations to the grid by hexagon ID. It ends with the two things to know when the polygons come from a file: match ST_Read's EPSG:4326 tag with geometry_crs, and join polygon tables through ST_Point for now. The page test skips when DuckDB can't download the extensions. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01NyN7idACeDMh94qrnFMu6h

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Actionable comments posted: 2

🪄 Fix comments: Fix CodeRabbit comments on this PR

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Inline comments:
Review comments at @docs/learn.md:
- Around line 331-334: Update the documentation around the
geometry_crs="EPSG:4326" example to clarify that the coordinates remain
longitude-first (lon, lat), despite the CRS metadata. Explain that DuckDB’s
ST_Transform uses EPSG:4326’s declared latitude-longitude order by default and
transformations must use always_xy := true, or coordinates must be reordered
beforehand.

Review comments at @tests/test_learn_page.py:
- Around line 86-89: Update the tutorial test around the DuckDB extension
preflight so unavailable spatial or H3 extensions do not skip the entire test.
Preserve execution of extension-independent examples, and conditionally skip
only the geometry and H3 examples that require those extensions.

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alxmrs

Thank you! LGTM! :)

alxmrs · 2026-10-21T18:08:55Z

+# ST_Within tests each point against a polygon. A function can't skip
+# chunks, but BETWEEN on a dim can, so pair the polygon with its bounding
+# box; bbox_conjuncts writes that part.
+bbox = xql.bbox_conjuncts((95, 5, 115, 15), x="lon", y="lat")


TIL we had this function!

tbh, I don't think we should.

We can handle this issue separately.

Sounds good. I left bbox_conjuncts as it is in this PR. Want me to open an issue for it? If so, tell me what you'd want instead (for example, xarray-sql deriving the bounding box from ST_Within itself), and I'll write it up.

Filed as #280: move bbox_conjuncts into the geo extra as sugar, keeping xql.bbox_conjuncts as a deprecated alias for a release. It also asks whether xarray-sql could derive the box from ST_Within itself.

alxmrs · 2026-10-21T18:11:57Z

+""").fetchall()
+# => [(14.5,)]
+
+# H3 joins data of different shapes. Give every row the ID of the hexagon


Later on, it would be cool to show how to incorporate other generic data science indexes, like KD trees.

Agreed, that would make a good follow-up section. A KD tree (or a ball tree) over a grid's coordinates answers nearest-neighbour and radius queries, the same kind of job H3 does here. I can draft an issue for it if you like.

Filed as #281, starting with a docs section: build a k-d tree in Python, register the matches as a table, join it in SQL. Engine-native nearest-neighbour indexes and a helper are listed as follow-ups.

ST_Read tags GeoJSON EPSG:4326, which officially puts latitude first, while GeoJSON and xarray-sql's points put longitude first, as OGC:CRS84 says. So the page now retags the polygons with ST_SetCRS(geom, 'OGC:CRS84') instead of tagging the points EPSG:4326, which could mislead a later ST_Transform. When DuckDB can't install its spatial and h3 extensions, the page test now runs the rest of the tour, blanking only the geometry section, and then reports a skip, instead of skipping the whole page. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01NyN7idACeDMh94qrnFMu6h

alxmrs commented Oct 21, 2026

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alxmrs mentioned this pull request Oct 21, 2026

Known issues: DuckDB spatial joins and CRS tags on geometry columns #279

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alxmrs commented Oct 21, 2026

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alxmrs merged commit 9509f21 into main Oct 21, 2026
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alxmrs deleted the learn-in-y-minutes branch October 21, 2026 18:47

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alxmrs commented Oct 21, 2026 •

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