elixir-performance

name	elixir-performance
description	Find and fix performance problems in portal/ — N+1 queries, missing preloads, unbounded lists/assigns, missing DB indexes, slow context reads. Use when a page or list is slow, the DB is hot, LiveView sockets are heavy, or before shipping a list/heavy read path.
effort	medium
argument-hint	[context, LiveView, or query to profile]
allowed-tools	Read, Grep, Glob, Bash

Performance pass

Measure, then fix the cause. Don't micro-optimize what isn't hot. The big wins in a Phoenix/Ecto app are almost always N+1 queries, unbounded result sets, and missing indexes — in that order.

The usual suspects (emisar-specific)

1. N+1 queries. An association loaded per-row in a loop. Symptom: the same SELECT repeated in the log.
   • Fix: declare the association in the Query module's preloads/0 and pass :preload to Repo.fetch/list (IL-10) — never Repo.preload inside an Enum.map. Detect: rg -n 'Repo\.preload|\.\w+\b' ... for context calls or preloads inside Enum./comprehensions.
2. Unbounded lists. Loading every row, or assigning a big list to the socket.
   • Lists that can grow (runs, audit events, runners): page with Repo.list/3 (keyset via cursor_fields), and render with stream/3 in LiveView, never assign(socket, :items, all) (IL-18).
3. Missing indexes. Every Query.by_* filter, every FK, and the not_deleted soft-delete predicate should be backed by an index. Cross-check the Query helpers against the migration; a partial index where deleted_at is null matches not_deleted/1.
4. Slow query / wrong plan. Confirm with Repo.explain(:all, query) in iex -S mix (verify the call exists/shape first) — look for Seq Scan on a large table where an index should apply.
5. Over-preloading. The opposite problem: loading associations a screen doesn't use. Preload only what the caller renders.

How to run it

cd portal
# repeated identical SELECTs in dev = N+1 (queries log by default in dev)
# profile a specific read in iex:
echo 'Emisar.Repo.explain(:all, Emisar.Runs.ActionRun.Query.not_deleted())' | iex -S mix

Read the LiveView's mount/handle_* for list assigns; read the context read path for preload shape; read the migration for indexes.

Output

Findings ordered by impact: issue · where · cost · fix. Fix the unambiguous ones (add a preload to preloads/0, switch an assign to a stream, add a missing index in a NEW migration — IL-11: a committed migration is frozen, never edit it). Leave a judgment call (is this list big enough to page?) as a flagged question, not a silent change. Re-measure after fixing.