> ## Documentation Index
> Fetch the complete documentation index at: https://docs.vortexiq.ai/llms.txt
> Use this file to discover all available pages before exploring further.

# Query Latency p50 (ms), ClickHouse

> Query Latency p50 (ms) for ClickHouse instances. Tracked live in Vortex IQ Nerve Centre. How to read it, why it matters, and how to act on it.

**Card class:** [Non-Hero](/nerve-centre/overview#card-classes-explained)  •  **Category:** [Performance](/nerve-centre/connectors#connectors-by-type)

## At a glance

> Query Latency p50 (ms) is the median query duration in milliseconds: half of all queries finish faster, half slower. It tells a platform team how a typical query feels on the ClickHouse instance right now. Read it alongside the tail percentiles ([p95](/nerve-centre/kpi-cards/clickhouse/query-latency-p95-ms) and [p99](/nerve-centre/kpi-cards/clickhouse/query-latency-p99-ms)): a flat p50 with a rising p95 means a slow minority, while a rising p50 means the whole instance has slowed down.

## What it tracks

Query Latency p50 (ms) for the selected period is the 50th-percentile of `query_duration_ms` over completed queries in a rolling window, drawn from `system.query_log`. It is server-side wall-clock time per query, not network or client render time, and it covers all completed query types (SELECT, INSERT, DDL). Because it is the median, it is robust to a handful of slow outliers: one runaway query barely moves p50, which is exactly why it is the right metric for "how does a normal query feel?" rather than "how bad is the worst case?". The card has no alert threshold of its own; it is a context metric that you read against the tail-latency cards and [Queries per Second (live)](/nerve-centre/kpi-cards/clickhouse/queries-per-second-live) to tell genuine load from a slow-query problem. A rising p50 is the signal that the instance as a whole is under pressure (load, contention, or a stall), not just a slow subset of queries.

## Reconciling against the source

Reproduce the headline directly in ClickHouse with `SELECT quantile(0.5)(query_duration_ms) FROM system.query_log WHERE type = 'QueryFinish' AND event_time >= now() - 300` (use `quantileExact(0.5)` for an exact, slower figure). On ClickHouse Cloud the Metrics tab plots latency percentiles per service. Small differences come from approximate-vs-exact quantiles, log sampling (`log_queries_probability` below 1), and the sliding-window boundary.

***

### Tracked live in Vortex IQ Nerve Centre

*Query Latency p50 (ms)* is one of hundreds of KPI pulses Vortex IQ tracks across ClickHouse and 70+ other ecommerce connectors. Nerve Centre runs the detection layer; Vortex Mind investigates the cause when something moves; Ask Viq lets you interrogate any number in plain English.

[Start for free](https://app.vortexiq.ai/login) or [book a demo](https://www.vortexiq.ai/contact-us) to see this metric running on your own data.