posthog/diagnosing-endpoint-performance

Diagnosing endpoint performance

This skill walks through a specific endpoint that is slow, expensive, or unreliable, and produces a concrete recommendation. It is the deep-dive counterpart to auditing-endpoints (which finds candidates).

When to use this skill

• "This endpoint is slow / timing out"
• "Why is my endpoint hitting the cost cap?"
• "Should I materialise X?"
• An endpoint surfaced from auditing-endpoints as a failing materialisation or expensive caller
• The user has a specific endpoint in mind and wants advice

If the question is project-wide ("what should I clean up?"), use auditing-endpoints first.

Available tools

| Tool | Purpose | | ----------------------------------- | ---------------------------------------------------------------------------------------------- | | endpoint-get | Full endpoint config: query, current version, data_freshness_seconds, materialisation status | | endpoint-versions | History of every version (query + materialisation state); which version is current | | endpoint-materialization-status | Whether materialisation is eligible, current state, last run, last error | | endpoints-materialization-preview | What the materialised query would look like, plus the rejection reason if ineligible | | endpoints-last-execution-times | When was it last called (endpoint-level sanity-check that it is in active use) | | execute-sql | Query query_log for endpoint-level call frequency and per-call duration/bytes |

The decision tree

When deciding what to recommend, walk these in order — the first one that applies is the cheapest fix.

Step 1 — Is it cached at all?

Fetch the endpoint and look at data_freshness_seconds (it sets both the cache TTL and, when materialised, the refresh cadence). If the user's traffic calls the same parameters repeatedly within that window, every call after the first is a cache hit and effectively free.

• TTL is at the default (24h / 86400s) and the data really doesn't need fresher than that → done, no change needed.
• TTL is at the 900s floor (15 min) and the user is hitting the endpoint many times per minute → bump the TTL. This is almost always the cheapest first move. (data_freshness_seconds is an enum: 900, 1800, 3600, 21600, 43200, 86400, 604800 — there is no sub-15-minute value.)
• TTL is at the floor because the data must be fresh (e.g. real-time dashboard) → cache won't help, skip to step 2.

The shape of the variables matters here: if every call passes different user_id or date_from values, the cache has many distinct keys and a higher TTL helps less. If almost every call uses the same handful of parameter combinations, the cache helps a lot.

Step 2 — Should it be materialised?

Materialisation pre-computes the query into a saved view that's refreshed on a schedule. Reads become near-instant — at the cost of staleness equal to the refresh interval, plus storage and compute for the materialisation itself.

Call endpoints-materialization-preview. The response tells you:

• Eligible + clean transform → strong candidate. Recommend enabling, especially for endpoints with predictable filter shapes (variables, breakdowns).
• Not eligible, with a rejection reason → cannot materialise. The reason often hints at the next step (see step 3 — rewrite).
• Eligible but the transform is gnarly (lots of range pairs, complex aggregation re-derivation) → materialisation will work but may not save much. Worth flagging before flipping the switch.

When materialisation is enabled, callers must pass all materialised variables — calls without them are rejected (security: prevents returning unfiltered data). Pair the recommendation with a note about which variables become required.

Step 3 — Does the query need rewriting?

If the endpoint isn't eligible for materialisation, the rejection reason from endpoints-materialization-preview is usually the lead:

• Cohort breakdown / compare mode rejection → regular property breakdowns materialise fine; only cohort breakdowns and compare mode are blocked. Swap a cohort breakdown for a property breakdown, or drop compare mode (expose the comparison window as a variable instead).
• JOINs combined with variables → a top-level JOIN plus a variable filter is rejected for materialisation, because applying the variable changes the joined row cardinality and silently produces wrong results (e.g. LEFT JOIN non-matches lose the variable column). Restructure so the variable filters a single table — push the filter into a subquery/CTE that's then joined, rather than filtering across the join. This is the most common "looks fine but won't materialise" trap.
• "Missing variables" / unbounded scan → the query reads too much data without a filter. Encourage adding a required time-window variable (e.g. date_from, lookback_days).
• HogQL with * / non-deterministic functions → narrow the columns selected, replace now() / today() with a variable when possible.

Check endpoint-versions to see whether the query was recently changed. Often the regression came from a specific commit and reverting that version is faster than rewriting.

Step 4 — Is the slow version even the one being called?

Only the latest version runs by default; older versions run only when a caller pins ?version=N. So the version to tune is almost always the current one — unless a pinned older version is the culprit. Call endpoint-versions and read each version's last_executed_at to see which versions have been hit recently; a materialised version with a null or long-stale last_executed_at is a candidate to unmaterialise or delete rather than tune (confirm first — that signal only counts API-key runs and can be sparse).

For endpoint-level call frequency and per-call cost, query query_log with execute-sql — it carries query_duration_ms, read_rows, and read_bytes, handy for confirming how heavy the endpoint's calls actually are:

SELECT count() AS calls, max(query_start_time) AS last_called, avg(query_duration_ms) AS avg_ms
FROM query_log
WHERE name = '<endpoint_name>' AND endpoint LIKE '%/endpoints/%' AND is_personal_api_key_request

Workflow

1. Identify the endpoint by name. If the user gave a URL, parse the name from /api/projects/{team_id}/endpoints/{name}/run.
2. endpoint-get to fetch full config. Note: data_freshness_seconds, is_materialized, current version, query kind.
3. endpoint-materialization-status to see materialisation health (failed? eligible?).
4. endpoint-versions to see the query history and which version is current (the one callers hit unless they pin ?version=N).
5. If the user gave specific symptoms ("times out", "memory limit", "very slow"), walk the decision tree above. Otherwise, present the four checkpoints and ask which matches.
6. End with a concrete recommendation — bump TTL, materialise, rewrite, or rollback — and confirm before applying it.

Example interaction

User: "monthly_active_users is timing out for some callers"

Agent steps:
- endpoint-get monthly_active_users → HogQL endpoint, data_freshness_seconds=900,
  is_materialized=false, current version v4
- endpoint-materialization-status monthly_active_users → not materialised; eligible: true
- endpoint-versions monthly_active_users
  → v4 is current; v1–v3 are prior query revisions (callers hit v4 unless pinned)
- endpoints-materialization-preview → eligible, clean transform with one range pair

- "Three things stand out:
   1. Cache TTL is 900 seconds (the 15-min floor), but the data is
      aggregating a month — that aggressive a refresh probably isn't
      necessary. Bumping to 1 hour (3600s) would dramatically reduce
      ClickHouse load.
   2. It's eligible for materialisation with a clean transform. If
      callers can tolerate 5-15 minute staleness, materialisation
      would make reads near-instant.
   3. v4 is the current version, so callers hit it by default — the
      tuning target is clear.

   Quickest win: bump TTL to 3600s. Bigger win: materialise.

   Which trade-off matches your needs? If you need real-time freshness,
   neither helps and we'd need to rewrite the query — likely narrowing
   the aggregation window."

Important notes

• Cache is almost always the first fix. It's free, instantly reversible, and doesn't change data semantics. Resist jumping to materialisation if a higher TTL would do.
• Materialisation has hidden costs. Storage of the materialised view, refresh compute, and the requirement that callers pass all variables.
• Don't rewrite the query without the user. A query change creates a new version and may break callers!!! Surface the suggested change, get sign-off, then apply.
• Three usage signals. endpoint-get's last_executed_at is endpoint-level recency; endpoint-versions gives each version's own last_executed_at; query_log (via execute-sql) gives endpoint-level call frequency and per-call cost. All count only personal-API-key calls, and per-version recency can be sparse — confirm with the user before calling a version dead.
• The "right" fix depends on the SLA, not the query. Always ask the user about acceptable staleness before recommending materialisation. A 15-minute-stale materialised view is wrong for a real-time dashboard, regardless of how cheap it'd be.
• Tell PostHog what's missing. If the diagnosis runs into a product limitation (an eligibility rule, the TTL enum, required variables), nudge the team via agent-feedback.