Backend performance checklist

I walk every check below against the code in hand and state a verdict on each one. Applied, clean, or not applicable and why. A check I did not name is a check I did not run.

Checks marked [pg] are PostgreSQL-specific. I skip them on other stores rather than translating them.

• Parallel async. Independent async calls run together in Promise.all, never awaited one by one.
• N+1 queries. No query inside a sequential for or for await loop. Batch with IN clauses or a join instead. Promise.all(items.map(q => oneQuery(q))) over a few index-friendly queries runs in parallel and is not a sequential N+1. Cap the fan-out: an unbounded items list gets an explicit bound or concurrency limit, since N parallel queries still open N connections. Keep the parallel shape unless EXPLAIN shows the batched join runs faster, because batching often defeats the per-row index.
• Pagination counts. Count and rows come from one query: a count(*) OVER () window or a single CTE, not a separate COUNT(*) next to the data query.
• Index coverage [pg]. Every production WHERE clause needs a matching index. similarity(col, ...), @@ to_tsvector(...), or LIKE '%pattern%' needs the matching gin (col gin_trgm_ops) or gin (to_tsvector('lang', col)) index in the same PR, or a one-line comment citing the ADR or issue that accepts the sequential scan. Prefer the indexable % operator over a raw similarity() > threshold comparison, but only with a matching threshold: % tests against the session pg_trgm.similarity_threshold (default 0.3), so set it to the query's threshold and confirm result-set equivalence, and EXPLAIN-verify that the planner picks the index.
• STABLE function reuse [pg]. pg_trgm.similarity and ts_rank are STABLE, not IMMUTABLE, so PostgreSQL does not reliably memoize duplicate calls in one SELECT. A value needed twice, for example once in the projection and once in GREATEST(...), is computed once in a WITH ... AS MATERIALIZED CTE. A plain subquery alias does not guarantee this. The planner can flatten it and evaluate per row.
• Select only needed columns. List endpoints name their columns instead of selecting the whole row.
• Reuse fetched rows. A row fetched for an ownership check is the row the handler returns. No second fetch by id.
• EXPLAIN before rewriting [pg]. Any query restructure, batching, joining, indexing, or replacement of parallel-N with batched-1 ships EXPLAIN (ANALYZE, BUFFERS) numbers for current versus proposed shape on representative data. Keep the faster shape. A rewrite that looks faster in the diff can run 10-100× slower on real data.
• Race conditions. When 2+ writers can touch one row, use a transaction. Async completion order is not source order. Order explicitly with await or Promise.all instead of relying on accidental serialization. A check plus a write that must stay atomic uses SELECT ... FOR UPDATE on [pg] or a unique constraint, not application-level locking.