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sql-transform

SQLProjection — projections over __THIS__, fit once, serve row-at-a-time.

The fit half works today: window aggregates over __THIS__ are marginalized — computed once over training data, materialized into params tables, and the SQL rewritten to join them instead of recomputing:

from sql_transform import SQLProjection

p = SQLProjection(
    "SELECT (age - avg(age) OVER (PARTITION BY country))"
    " / stddev_samp(age) OVER (PARTITION BY country) AS age_z FROM __THIS__"
).fit(train)   # train: pyarrow.Table

p.serving_sql
# SELECT (__cf_t.age - __cf_p0.__cf_a0) / __cf_p0.__cf_a1 AS age_z
# FROM __THIS__ AS __cf_t
# LEFT JOIN __CF_PARAMS_0__ AS __cf_p0
#   ON (__cf_t.country IS NOT DISTINCT FROM __cf_p0.country)
p.params
# {"__CF_PARAMS_0__": <pyarrow.Table: country, __cf_a0, __cf_a1>}
p.plan
# (FitStep(name='__CF_LEVEL_0__', sql='SELECT ...', reads=('__THIS__',)),
#  FitStep(name='__CF_PARAMS_0__', sql='SELECT DISTINCT ...', reads=('__CF_LEVEL_0__',)))

Fitting is an explicit plan — a topologically ordered DAG of named SQL steps (nested aggregations across CTE levels genuinely depend on each other). Every intermediate is inspectable by name and every step is plain SQL you can run by hand.

The serving half (infer/infer_batch through Confit) is a later loop and still raises NotImplementedError.

The contract

A chain of strict projections: the final SELECT over a CTE or derived table, projecting over another, down to __THIS__ — flattened into one serving projection. The window surface is wide: any aggregate (including FILTER, DISTINCT, ordered arguments, and order-sensitive ones), running windows and RANGE/GROUPS frames (the order values join the key set), the rank family, first_value/last_value/nth_value, expression partition keys, named windows — plus scalar and EXISTS subqueries over __THIS__, which are provably uncorrelated (there is no syntax to reach the outer row) and run verbatim at fit time.

Declaring a schema unlocks more: SQLProjection(sql, this_model=Row) (the pydantic model is the authoritative __THIS__ schema) makes unknown columns refuse at construction, expands COLUMNS('regex') (matched by DuckDB's own regex engine) and * EXCLUDE/REPLACE/RENAME at any level, and resolves lateral aliases by DuckDB's column-wins rule. Schema-free construction keeps working, with the ambiguous cases refused.

The rule deciding it all: a window is marginalizable iff its value is a function of row-visible values. What depends on physical row position — row_number, ntile, lag/lead, bounded ROWS frames, EXCLUDE — is refused, as is everything non-projection (WHERE, GROUP BY, joins, set operations, IN-subqueries), each with a named error. Serve-or-refuse, no third mode.

Parsing is DuckDB's own (json_serialize_sql / json_deserialize_sql): the oracle's grammar and the oracle's printer, no second SQL dialect anywhere.

The gate and the metric

For every accepted query, the training-set round-trip invariant must hold bit-exactly: fit + transform on the training set equals the original query with __THIS__ pointing at the training set — both executed by DuckDB (single-threaded: DuckDB's parallel float window aggregation is not bit-deterministic even against itself). NULL partition keys are one partition and join back via IS NOT DISTINCT FROM; join multiplicity is provable, not tested.

Progression is measured by corpus replay (_corpus_test.py): queries mined from DuckDB's own window test suite plus a curated family list, each either MARGINALIZED (gate passes), REFUSED (named error), or FAILED (always a bug; asserted empty). The pinned scoreboard — currently 11/22 mined marginalized and 39 curated families supported against 17 named refusal families — is the progression record; widening a loop edits it upward in a reviewable diff.

Design specs, in order: loop 1 — marginalization, loop 2 — window widening, loop 3 — chains and the fit plan.