django-data-shape

A realistically shaped test database from Django models.

Declare the shape of your data -- cardinality, value skew, foreign-key fan-out as a distribution with a long tail, and where related rows physically sit -- then load it by COPY and ANALYZE it, so the query planner makes the same choices it will make in production.

It exists because a plan over ten rows is a lie, and because the loop it replaces is not merely smaller: uniform fan-out makes the planner always right, and generating children parent-by-parent clusters them perfectly, which flatters every index scan. A test database can be wrong in the flattering direction, and usually is.

Install

pip install django-data-shape[postgres]

Use

import datetime

from django_data_shape import Sequential, Shape, Skew, Table, Uniform, build

shape = Shape(
    Table(
        Order,
        rows=1_000_000,
        status=Skew({"complete": 0.98, "pending": 0.015, "cancelled": 0.005}),
        total=Uniform(0, 500, places=2),
        created_at=Sequential(
            datetime.datetime(2020, 1, 1, tzinfo=datetime.timezone.utc),
            datetime.timedelta(seconds=3),
        ),
    ),
    seed=1234,
)

build(shape)

build() generates the rows, loads them with COPY, moves the identity sequence past the keys it assigned, and runs ANALYZE so the planner can see the shape. It raises on any backend that is not PostgreSQL rather than degrading quietly.

Relations

Table(
    Order,
    rows=2_000_000,
    # A distribution, not a number: giving every parent ten children is the one
    # shape in which the planner is never wrong, because its n_distinct average
    # is then the truth.
    company=FanOut(Zipf(1.2), childless=0.35),
    ...
)

The parents can be rows this package built or rows your own code did -- their real keys are read, not assumed, so the ORM can own the small tables while this owns the large ones.

Status

Early. Single tables and the model graph. Derived fields, collections copied along a join, per-group invariants and template-database reuse come next.

Full documentation: https://artui.github.io/django-data-shape/

License

MIT

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