DataFusion Query Plans for Governed APIs

A data API that hides its query plan is asking you to trust a box.

The practical problem

Embedded query engines are becoming part of the data product surface. Instead of every consumer connecting directly to a warehouse, a product team may expose a governed API backed by files, tables, or an in-process execution engine.

Apache DataFusion is relevant because it is an extensible query engine with logical planning, optimization, and physical execution built on Apache Arrow. That makes the plan inspectable in a way that ordinary API code often is not.

Plans are not just performance artifacts

DataFusion EXPLAIN output can show the logical or physical execution plan for a statement. In a governed API, that plan can become evidence. It can show which table source was touched, which projection was applied, which filter was pushed down, and which execution boundary mattered.

That does not make a query plan a policy engine. It makes it a useful companion to policy. The policy decision says whether an action is allowed. The plan helps prove what action the system was about to perform.

The API contract should include behavior, not only schema

Most data API contracts stop at inputs and outputs. That is not enough for Open Data Infrastructure. The contract should also describe which sources are eligible, which filters are mandatory, which policy checks run, and which plan shapes are unacceptable.

This matters for DataFusion data product APIs and for agent tool design. If an agent can call a query tool, the tool needs explainable boundaries. Otherwise a simple question can become an accidental broad scan, policy bypass, or hidden data movement event.

What breaks first

• The API validates parameters but not the resulting query behavior.
• Policy checks run before planning, then optimization changes which data is touched.
• Plans are available during debugging but never logged with the request.
• The API exposes a clean schema while hiding expensive or risky execution paths.

Questions to ask

• Can the API explain which datasets and columns a request touched?
• Can policy decisions be tied to the planned operation?
• Which plan patterns should fail closed?
• Can an agent receive a safe, useful error when the plan is rejected?

For adjacent design, read Apache DataFusion and Composable Query Engines, DataFusion as an Embedded Query Engine for Agents, and Agentic Data Contracts for Tool Calls.

Sources to start with

These primary sources anchor the technical claims in this guide.

• Apache DataFusion documentation
• Apache DataFusion EXPLAIN documentation
• Apache DataFusion query optimizer
• OpenLineage documentation

A governed data API should be able to explain its own path through the data.