Database Selection

Database Selection

Purpose

Make two separate decisions, in order: (1) the database — PostgreSQL, MySQL, or MongoDB — from the data's shape and integrity needs; (2) the data layer — Prisma, Drizzle, Mongoose, or native driver — from team ergonomics and control needs. Feeds ../../stack-recommendation; requires user approval (Gate 2).

When to Use

• At the start of any project that stores data, or when adding a store.
• When re-evaluating persistence for a migration (../../migration-planning).
• Not after the database is approved and unchanged.

Inputs

• Requirement baseline: entities, relations, integrity needs, reporting needs, expected access patterns.
• Existing infrastructure/team experience (../../existing-project-audit findings where applicable).

Discovery Questions

• Does the domain involve payments, orders, inventory, reporting, financial data, multi-entity transactions, or strongly related entities? (Any yes → relational is the usual answer.)
• Are document structures genuinely highly variable across records, with data naturally read/written as embedded documents? (That's the MongoDB case — not "we haven't designed the schema yet.")
• What queries will exist: cross-entity joins/aggregations (relational strength) vs whole-document reads by key (document strength)?
• For the data layer: does the team want schema-first ergonomics (Prisma), SQL-proximity and control (Drizzle), ODM conveniences (Mongoose), or minimal abstraction (native driver)?

Responsibilities

• Database decision first, on data shape and integrity:
  • Relational (PostgreSQL/MySQL) usually preferred for: payments, orders, inventory, reporting, financial data, transactional multi-entity writes, strong relations/constraints. Between them: PostgreSQL for richer types/features, MySQL where team/infra experience or hosting favors it — record the reason.
  • MongoDB may be preferred for: highly variable document structures and access patterns that naturally read/write embedded documents as units.
• Data layer decision second, pairs must be coherent:
  • Relational DB → Prisma (schema-first DX, generated types), Drizzle (TypeScript-SQL closeness, control), or native driver/query builder (maximum control, more discipline).
  • MongoDB → Mongoose (schemas/middleware over documents) or native driver (direct control).
• Record both decisions with justification and rejected alternatives; note operational implications (backups, migrations tooling — database-migrations, backup-recovery).
• Hand off to the matching skills: relational-schema-design + prisma-relational/drizzle-relational, or document-schema-design + mongoose-mongodb.

Required Workflow

1. Gather entities, relations, integrity/reporting needs, access patterns.
2. Decide the database from data shape (relational signals vs document signals).
3. Decide the data layer for that database from team/control needs.
4. Record both with trade-offs for Gate 2 approval.
5. Hand off to schema-design + data-layer skills.

Decision Rules

• Never conflate the two decisions — "we use Prisma" is not a database choice; "we use Postgres" doesn't pick the data layer.
• Financial/transactional/relational signals outweigh document-flexibility preferences: integrity failures cost more than schema ceremony.
• "Schemaless" is not a reason for MongoDB — undesigned data isn't variable data (document-schema-design still requires design).
• Polyglot persistence (two stores) needs strong, separate justification per store — default is one.
• Team familiarity tiebreaks between adequate options; it doesn't override a shape mismatch.

Rules

• Recommend, don't pre-select; user approves at Gate 2.
• No installation or schema work before approval.
• Record what would flip each decision.

Anti-Patterns

• Choosing MongoDB for an orders/payments/inventory domain because iteration feels faster.
• Choosing the database by choosing an ORM first.
• Defaulting to PostgreSQL (or anything) without stating why it fits this data.
• "We'll add relations to MongoDB later with manual refs everywhere" — that's a relational domain in denial.
• Two databases because two tutorials.

Validation Checklist

• Data shape, integrity, reporting, access patterns gathered.
• Database decided on data shape (relational signals checked explicitly).
• Data layer decided separately, coherent with the database.
• Both recorded with trade-offs + rejected alternatives.
• Handed to schema-design + data-layer skills; Gate 2 pending.

Definition of Done

Two recorded, separately justified decisions — database and data layer — coherent as a pair, tied to the domain's data shape and integrity needs, with trade-offs noted and approval pending.

Related Skills

../../stack-recommendation, relational-schema-design, document-schema-design, prisma-relational, drizzle-relational, mongoose-mongodb, database-migrations, backup-recovery, ../../backend/existing-backend-audit.

Related Knowledge

../../../knowledge/ (domain model, infra constraints, team experience).

Related References

../../../references/database/ (comparison notes, when populated).

Context Loading Guidance

• Requires: entity/relation summary, integrity + reporting needs, access patterns.
• Does not require: full schema drafts, ORM docs, application code.
• May load: one schema-design skill + one data-layer skill after deciding.
• Stop when: both decisions are recorded for Gate 2.

Token Efficiency Guidance

Decide from the requirements summary; the two-decision table (option, fit, trade-off) is the whole artifact. Don't draft schemas here.