Indexes & Query-Driven Design Design access paths from real application queries
Phase 10 - Relational Data Modeling Volume 03 - Database Engineering
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Phase 10 decides what the durable data should mean before Phase 11 writes application behavior against it.
Do not index every column Indexes trade storage and write cost for faster access paths. Index design should start from important query patterns, not from a desire to "optimize" every field.
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Indexes are consequences of access patterns.
Expected TaskFlow queries Query Likely columns
Candidate Prisma indexes
model Task {
// ...
@@index([projectId, status])
@@index([assigneeId, status])
@@index([createdAt])
}model Project { // ... @@index([workspaceId]) }
Composite index order matters An index on (projectId, status) is naturally useful for queries beginning with projectId. It is not equivalent to (status, projectId) for every workload. The order should reflect leading filter/selectivity and sort patterns.
Foreign keys are not automatically all the indexes you need A foreign-key constraint protects referential integrity. Query performance is a separate concern. PostgreSQL does not automatically create an index on the referencing foreign-key column simply because the foreign key exists. Performance design rule: model correctness first, then choose indexes from observed high-value queries, and validate with EXPLAIN/production-like data later.
Authentication answers who the request represents. Treating decoded or client-supplied identity as trusted is a security boundary failure.
Restate the goal of “Indexes and Query Driven Design” as one observable behavior. Write the success signal before touching code.
Trace the boundary map: credentials/session → token → verification → trusted identity → protected operation. Mark the one boundary this lesson changes.
Inspect the current repository state and record the exact files, route, table, or runtime involved. Avoid creating an abstraction before you can name the existing owner.
Implement the smallest vertical slice. Add one coherent change, run the narrowest verification command, and read the output before continuing.
Connect the slice to TaskFlow’s phase artifact: A complete Prisma/ER model for TaskFlow core entities with keys, relation tables, uniqueness, and important indexes.. Verify behavior from the outside as a user or API client would.
Create one controlled failure that is safe to reproduce. Predict the symptom first, capture evidence, locate the earliest broken boundary, then fix only that cause.
Close the reference and rebuild or explain the core change from memory. Record what you would monitor or test before trusting the same change in production.
const token = readBearerToken(request);
const claims = await verifier.verify(token);
request.user = {
id: claims.sub,
email: claims.email,
};Do not copy this as a finished answer. Predict what each boundary does, type the smallest useful slice, run it, then compare the result with your prediction.
Inspect whether a token exists, whether it is expired, which issuer/audience signed it, and what the API verifier expects.
Trust only verified server claims; never repair identity by accepting a client-sent user id.
Without copying the final source, create a second small scenario that applies “Indexes and Query Driven Design” to a different TaskFlow resource or input. Write the acceptance criteria first, implement it, and provide the evidence that each criterion is satisfied.
Seed a small realistic graph and query it from several directions to validate the model.
Use the smallest change that demonstrates this lesson. Stop after it works and capture evidence.Ambiguous cascade-delete behavior
Predict the symptom before causing the failure. Then locate the earliest boundary where reality diverges from expectation.Why does this responsibility live here, what input does it trust, what output proves success, and what would go wrong if another layer owned it?
If your explanation is vague, repeat the boundary trace before moving on.Close the reference. Reproduce the core idea from memory or explain the exact request/data flow aloud. Then check your diff or notes for accidental shortcuts that bypass validation, authorization, error handling, or verification earned in earlier phases.