Decision guides
Most technology comparisons list every difference and leave you to weigh them. These guides do the opposite: they identify the small number of factors that genuinely decide the outcome, say plainly which differences do not matter for most teams, and state what would change the answer.
Guides
Each ends with the conditions under which you should revisit the decision.
PostgreSQL, MySQL, MariaDB or SQL Server. Operational expertise, licence cost and the one feature you actually need decide it, not the feature matrix.
PgBouncer, Pgpool-II, or PgBouncer with Patroni and HAProxy. Which of pooling, read routing and automatic promotion you actually need decides it.
Retrieval changes what a model knows at query time; fine-tuning changes how it behaves. Knowledge churn, attribution and volume decide which you need.
pgvector, Qdrant, Weaviate or Elasticsearch. Whether you already run PostgreSQL, and how you filter, decide it - not throughput charts.
These are marked analysis rather than fact. The underlying behaviour is sourced on the technology pages they link to; the recommendation is a judgement, and a team with different constraints could reasonably decide otherwise.