# Research: PostgreSQL Database Replacement

> For the complete documentation index, see [llms.txt](https://deploy-preview-472--lucky-concha-f3599f.netlify.app/llms.txt).

## Objective

Define the transition from state `004` to `005` by replacing H2 with PostgreSQL while preserving behavior.

## Inputs Reviewed

*   `spec.md`
*   `plan.md`
*   `tasks.md`
*   `requirements/functional-delta.md`
*   `requirements/nonfunctional-delta.md`
*   `system/architecture.md`
*   `system/runtime-topology.md`

## Key Decisions

1.  Use PostgreSQL as the default SQL persistence engine for this branch.
2.  Preserve application-level behaviors and API compatibility from predecessor state.
3.  Keep migration reversible and clearly isolated as a state-specific delta.