Polling & Performance

Work In Progress

The tiered polling model described below is planned but not yet fully implemented.

Reference for every polling interval in the app - what each page queries, which system tables are hit, and the load impact.

Polling Tiers

Tier	Interval	Tables	Cost
Tier 1 (live state)	2–5s	asynchronous_metrics, metrics, processes, merges, mutations	Near-zero (all virtual)
Tier 2 (recent completed)	30s	query_log, part_log, events (deltas)	MergeTree reads, small time windows
Tier 3 (structural)	60s	parts, disks, replicas, dictionaries	Virtual or light aggregation

Total Overhead

~10 lightweight read-only queries per 30-second cycle. All are SELECT statements against system.* tables.

System Table Types

Type	Examples	Cost
Virtual	metrics, processes, merges, parts	Computed from in-memory state - free
MergeTree	query_log, part_log, metric_log	Persisted, grow over time, need TTL management

Per-Page Breakdown

Cluster Overview

• system.asynchronous_metrics (2s) - CPU, memory gauges
• system.metrics (2s) - active connections, running queries
• system.events (5s) - cumulative counters delta
• system.clusters (60s) - topology

Database Explorer

• system.databases (60s) - database list
• system.tables (60s) - table metadata
• system.parts (30s) - part details for selected table
• system.columns (60s) - column statistics

Merge Tracker

• system.merges (2s) - active merges
• system.part_log (30s) - completed merge history
• system.mutations (5s) - active mutations

Query Monitor

• system.processes (2s) - running queries
• system.query_log (30s) - historical queries

Engine Internals

• system.asynchronous_metrics (5s) - thread pools, memory
• system.dictionaries (60s) - dictionary status
• system.trace_log (30s) - CPU profiling data

Scaling Considerations

• For clusters with 100+ tables, structural queries (Tier 3) may take longer
• system.parts can be expensive on tables with thousands of parts - the app uses WHERE active = 1 filtering
• system.query_log queries use time-bounded windows to avoid scanning the full log