pg_hdr_histogram is a PostgreSQL extension for exact-latency percentiles.
Most monitoring tools approximate percentiles from fixed buckets. pg_hdr_histogram
stores a full HDR histogram as a first-class type, so p50 / p99 / p99.9 are
measurements (~0.1% relative precision over 6 orders of magnitude), not estimates —
and histograms merge losslessly across time windows and hosts.
hdr_add and hdr_merge, parallel-safe, trusted = true.