chore(trials): aggregate_trials_summary as single CTE-based query

[SoundMindsAI]

Summary

Wave 2 PR A of the pre-MVP2 backlog sweep: SQL refactor per chore_trial_summary_single_query.

backend/app/db/repo/trial.py:aggregate_trials_summary was making two round-trips: one SELECT with COUNT(*) FILTER (...) aggregations + MAX(primary_metric) for counts/best, then a second SELECT to find the trial id matching the best metric. The Phase 2 impl plan committed to this as a single SQL statement.

Rewritten as a CTE (summary) that does the aggregation, joined with a correlated scalar subquery that selects the winner id. One round-trip. Same result.

The 2-query form was functionally correct + well within spec §13's <100ms p99 target on small studies, but documented as single-query in the contract. Closing the drift now while context is loaded.

Test plan

• pytest backend/tests/unit/ — 696/696 pass
• mypy backend/app/db/repo/trial.py — clean
• Existing integration tests (TestTrialsSummary::test_summary_with_mixed_statuses + ::test_summary_empty_study) cover the contract per the original idea spec — no test changes needed
• CI service container Postgres runs the integration tests against the new query

Archives

chore_trial_summary_single_query/idea.md → docs/00_overview/implemented_features/2026_05_13_chore_trial_summary_single_query/idea.md. Dashboard regenerated.

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[gemini-code-assist]

Code Review

This pull request refactors the aggregate_trials_summary function in backend/app/db/repo/trial.py to use a single CTE-based SQL query, improving performance and adhering to the technical specification. Corresponding updates were made to the project dashboard documentation to reflect the completion of this task. The reviewer suggested using the more idiomatic .filter() syntax for conditional aggregations instead of func.count(case(...)) to improve readability and align with the SQL FILTER clause.

[gemini-code-assist]

The use of func.count(case(...)) for conditional aggregation is functional, but SQLAlchemy and modern SQL (like PostgreSQL) offer a more readable and explicit syntax using the .filter() modifier on aggregate functions. This corresponds to the SQL FILTER (WHERE ...) clause.

Using .filter() would make the query's intent clearer and aligns with both the function's docstring and the SQL example in idea.md, which both mention using FILTER.

I suggest updating the conditional aggregations to use this more idiomatic pattern.

Suggested change

	func.count(case((Trial.status == "complete", 1))).label("complete"),
	func.count(case((Trial.status == "failed", 1))).label("failed"),
	func.count(case((Trial.status == "pruned", 1))).label("pruned"),
	func.max(case((Trial.status == "complete", Trial.primary_metric))).label("best"),
	func.count(Trial.id).filter(Trial.status == "complete").label("complete"),
	func.count(Trial.id).filter(Trial.status == "failed").label("failed"),
	func.count(Trial.id).filter(Trial.status == "pruned").label("pruned"),
	func.max(Trial.primary_metric).filter(Trial.status == "complete").label("best"),

[SoundMindsAI]

Gemini Code Assist review — adjudication

#	Finding	Severity	Verdict	Notes
F1	Use func.X(val).filter(cond) idiom instead of func.X(case((cond, val)))	Medium	Accept (applied in 36b7935)	Produces SQL COUNT(*) FILTER (WHERE ...) which matches the docstring + the original idea spec's SQL example. More idiomatic in PostgreSQL. Per the new rubric: same file, ~4 LOC, no test changes → inline. IDE diagnostic caught case import becoming unused; dropped from the sqlalchemy import. Verified: 696/696 unit tests pass, mypy clean.

Net: 1 finding → 1 accept (applied).

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