.tip

Jun 26, 2026 · one of the small things

p99, not the average

An average of 80 milliseconds can hide the one request in a hundred that takes six seconds, and those are the ones people complain about.

Graph p50, p95 and p99 on the same panel, and alert on p99. The average is useful for capacity maths and nothing else. A histogram in your metrics library gives you all three for free, while a gauge of the mean gives you none of them.

observabilityperformance

~/tips/today — 2026-10-19

~cat tips/today

Never build an index on a live table without CONCURRENTLY

postgresqlmigrations

A plain CREATE INDEX takes a lock that blocks every write to the table for the whole build. On a big table that means minutes of outage that look like a hung app.

CREATE INDEX CONCURRENTLY orders_customer_idx ON orders (customer_id);

It's slower and can't run inside a transaction, so you'll have to tell your migration tool. If it fails halfway, it leaves an invalid index behind, so check pg_index.indisvalid and drop it before you retry.

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