How to Check Memory Usage in ClickHouse

How do I check and control memory usage in ClickHouse?

The memory usage command monitors RAM consumed by ClickHouse queries or the whole server, helping prevent OOM errors and optimize performance.

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Description

Example H2

Example H3

What is the fastest way to see current memory usage?

Query the system.metrics and system.asynchronous_metrics tables. They expose real-time counters for total and per-query memory.

SELECT metric, value FROM system.metrics WHERE metric ILIKE '%Memory%';

How do I check memory used by a running query?

The system.processes table holds one row per live query, including the memory_usage column.

SELECT query_id, user, formatReadableSize(memory_usage) AS mem FROM system.processes ORDER BY memory_usage DESC;

Can I audit historical peaks?

Enable the internal system.query_log. It records read_rows, read_bytes, and memory_usage after each query finishes.

SELECT query, formatReadableSize(memory_usage) AS mem FROM system.query_log WHERE event_time > now() - INTERVAL 1 HOUR ORDER BY memory_usage DESC;

How to limit memory per session or query?

Use the max_memory_usage setting when connecting or in each statement. ClickHouse aborts the query if the limit is exceeded.

SET max_memory_usage = 2e9; -- 2 GB SELECT ...;

Session example in the Galaxy SQL editor

In Galaxy, open a new tab, paste the SET statement, run it, then execute your Orders analysis. The editor shows a warning if the limit triggers.

Which system metrics matter most?

MemoryTracking (allocated), RealMemoryUsage (RSS), and UncompressedCacheBytes (cache) give a complete picture. Monitor them with Grafana or CLI scripts.

Best practices for low-memory environments

Use SAMPLE for large tables, avoid ORDER BY RAND(), prefer LIMIT early, and partition Orders data by order_date to prune scans.

Why How to Check Memory Usage in ClickHouse is important

How to Check Memory Usage in ClickHouse Example Usage


-- Find top 5 memory-hungry queries touching Orders in the last day
SELECT query,
       formatReadableSize(memory_usage) AS mem
FROM system.query_log
WHERE event_time >= yesterday()
  AND query ILIKE '%Orders%'
ORDER BY memory_usage DESC
LIMIT 5;

How to Check Memory Usage in ClickHouse Syntax


-- View server-wide memory counters
SELECT metric, value
FROM system.metrics
WHERE metric ILIKE '%Memory%';

-- Monitor memory of live ecommerce queries
SELECT query_id,
       user,
       formatReadableSize(memory_usage) AS mem,
       query
FROM system.processes;

-- Enforce a 1 GB cap before running a big Orders join
SET max_memory_usage = 1e9;  -- bytes
SELECT c.name, SUM(oi.quantity * p.price) AS lifetime_value
FROM Customers c
JOIN Orders o  ON o.customer_id = c.id
JOIN OrderItems oi ON oi.order_id = o.id
JOIN Products p ON p.id = oi.product_id
GROUP BY c.name;

Common Mistakes

Relying only on OS tools like top or htop. They miss per-query stats. Fix: query system.metrics and system.processes for ClickHouse-native data.
Setting max_memory_usage globally without testing. It can kill legitimate large reports. Fix: tune per-session or per-user, then raise slowly.