This configuration parameter dictates the maximum number of simultaneously executing queries permitted within a ClickHouse server instance. It serves as a crucial control mechanism, preventing resource exhaustion and ensuring system stability. If the number of incoming queries exceeds this limit, subsequent queries are queued until existing ones complete, thereby preventing overload.
Properly configuring this limit is paramount for maintaining responsiveness and preventing performance degradation. Setting it too high can lead to excessive resource consumption, causing memory pressure, CPU contention, and potentially instability. Conversely, setting it too low can artificially restrict throughput, leaving system resources underutilized and increasing query latency. Historically, the need for such a mechanism arose from the inherent challenges of managing concurrent workloads in database systems, especially under conditions of high demand.
