Cloudflare D1 query tuning

Use this skill when

• A Cloudflare Workers repo has a slow D1 query and the task is to tune query shape or repository logic without touching schema or migration files.
• A D1-backed adapter or repository is over-fetching, doing N+1 lookups, or filtering/paginating in application code when SQL should do the work.
• You need D1-specific inspection such as wrangler d1 execute, EXPLAIN QUERY PLAN, or environment-aware repro steps before rewriting the query path.

Do not use this skill when

• The task changes schema, indexes, constraints, or migration files; use cloudflare-d1-migrations.
• The repository does not use Cloudflare D1 and the main work is a generic adapter/query change; use repository-adapters.
• The problem is D1 binding setup, preview_database_id, or Wrangler environment drift rather than runtime query behavior; use configuration-env.
• The main request is caching, HTTP contract changes, or higher-level service behavior rather than the D1 query path itself.

Inputs to gather

Required before editing

• The Wrangler config file and exact d1_databases binding in scope.
• The slow query path: repository/adapter method, raw SQL or query builder call, representative parameters, and expected result shape.
• Current evidence of slowness, such as logs, a reproducible request path, or direct wrangler d1 execute timing / EXPLAIN QUERY PLAN output.
• The repo's existing wrappers for D1 execution, tests, and local-vs-preview-vs-remote workflows.

Helpful if present

• Whether the problem reproduces in --local, --preview, --remote, or all three.
• The surrounding access pattern, such as per-row follow-up queries, repeated existence checks, or offset-heavy pagination.
• Whether the repo already uses prepared statements, batches, or transactions for nearby D1 work.

Only investigate if encountered

• Whether D1/SQLite feature limits like FTS, JSON functions, or virtual tables constrain the rewrite.
• Whether the only viable fix is actually an index or schema change; if so, stop and redirect rather than widening scope.
• Whether the query is generated and needs a code-generation step after edits.

First move

First confirm the problem is runtime query shape or D1 access behavior, not a missing index, migration, or binding issue. Then inspect the exact D1 binding and reproduce the query with representative parameters in the right environment before proposing a rewrite.

Workflow

1. Confirm the D1 runtime contract.

   • Read wrangler.toml, wrangler.json, or wrangler.jsonc and note the exact d1_databases entry in scope.
   • Record the binding name, database_name, and any preview-vs-remote distinction before measuring anything.
   • Prefer repo wrappers over raw Wrangler once you understand how they map to D1 execution.

2. Capture the real slow path before rewriting.

   • Find the exact repository method, adapter call, or query source used by the slow request.
   • Reproduce it with representative parameters and inspect EXPLAIN QUERY PLAN in the same environment when feasible.
   • Check whether the issue is over-fetching, post-filtering in application code, N+1 follow-up queries, redundant existence checks, or offset-heavy pagination.

3. Tune the query path without changing schema.

   • Push filtering, ordering, limiting, aggregation, and existence checks into SQL when that preserves the contract.
   • Prefer narrower projections, EXISTS, set-based fetches, and query consolidation over SELECT *, row-by-row follow-ups, or post-processing large result sets.
   • Reuse the repo's prepared statement, batch, or transaction patterns when they reduce repeated round trips without changing semantics.
   • If the only meaningful improvement would require a new index or schema change, stop and hand off to cloudflare-d1-migrations instead of smuggling schema advice into this workflow.

4. Validate behavior and performance deltas together.

   • Re-run the same representative query path after the rewrite.
   • Compare the work done, such as plan shape, rows touched, or round trips, not just the final latency text.
   • Run the repo's normal tests or affected checks for the changed adapter/query path.

Guardrails

• Must inspect the exact D1 binding and environment before measuring or rewriting the query.
• Must preserve the query contract and result semantics while tuning.
• Must not recommend schema, index, or migration-file changes inside this workflow.
• Must not treat local D1 behavior as proof that preview or remote has the same runtime characteristics.
• Should keep rewrites at the query / adapter boundary instead of leaking D1-specific behavior upward.
• Should call out plainly when the no-schema constraint blocks the real fix.
• May pair with repository-adapters when the repo uses a strong domain-interface boundary and the adapter contract also needs care.

Validation

• The same D1 query path was inspected before and after with comparable parameters.
• The tuned version reduces unnecessary work, such as wider scans, over-fetching, redundant round trips, or post-filtering outside SQL.
• The repo's relevant tests or validation commands still pass for the touched query or adapter path.

Examples

• "Tune a slow D1 query without changing schema or migration files."
• "This Worker does N+1 D1 lookups in a loop. Collapse it into a better query path without touching migrations."
• "Rewrite this D1 pagination query so it stops over-fetching, but keep the same result contract."

Reference files

• Read references/inspection-and-rewrite.md when you need D1-specific repro commands, EXPLAIN QUERY PLAN usage, or a checklist of no-schema rewrite wins.