ClickHouse Query

Use one command:

bun run --cwd packages/ch-schema chcli -- -F json -q "<SQL>"

The wrapper reads CLICKHOUSE_URL, CLICKHOUSE_USERNAME, and CLICKHOUSE_PASSWORD from the current process. The URL must not contain credentials. See references/connection.md.

Guardrails

• Run only read-only queries. Never run DDL, inserts, mutations, deletes, or administrative commands.
• Local and staging use their configured identities. Production requires a separate least-privilege, read-only identity; stop if it is unavailable.
• Never print or persist credentials or production-derived result data.
• Use -F json by default; use -F jsonl only for bounded streaming output.

Per ClickHouse's agent-query-safety rule, every application-data query must have:

• a filter on a discovered sorting or partition key;
• a finite LIMIT;
• max_execution_time;
• max_rows_to_read or max_bytes_to_read.

LIMIT does not cap scanned rows. Metadata queries must also have a finite LIMIT, a time limit, and a read limit.

Workflow

Per ClickHouse's agent-discovery-schema rule, run these steps in order. Execute every SQL block with the wrapper command above and structured output.

1. Databases

SELECT name
FROM system.databases
WHERE name NOT IN ('system', 'information_schema', 'INFORMATION_SCHEMA')
ORDER BY name
LIMIT 100
SETTINGS max_execution_time = 10, max_rows_to_read = 100000

2. Tables and active-part sizes

Replace analytics with the selected database.

SELECT
  t.name,
  t.engine,
  coalesce(sumIf(p.rows, p.active), 0) AS rows,
  formatReadableSize(coalesce(sumIf(p.bytes_on_disk, p.active), 0)) AS size
FROM system.tables AS t
LEFT JOIN system.parts AS p
  ON p.database = t.database AND p.table = t.name
WHERE t.database = 'analytics'
GROUP BY t.name, t.engine
ORDER BY sumIf(p.bytes_on_disk, p.active) DESC
LIMIT 200
SETTINGS max_execution_time = 15, max_rows_to_read = 1000000

3. Columns and comments

SELECT position, name, type, default_expression, comment
FROM system.columns
WHERE database = 'analytics' AND table = 'events'
ORDER BY position
LIMIT 500
SETTINGS max_execution_time = 10, max_rows_to_read = 100000

4. Sorting and partition keys

SELECT sorting_key, primary_key, partition_key
FROM system.tables
WHERE database = 'analytics' AND name = 'events'
LIMIT 1
SETTINGS max_execution_time = 10, max_rows_to_read = 100000

5. Skipping indexes

SELECT name, type_full, expr, granularity
FROM system.data_skipping_indices
WHERE database = 'analytics' AND table = 'events'
ORDER BY name
LIMIT 100
SETTINGS max_execution_time = 10, max_rows_to_read = 100000

6. Bounded sample

Use fields discovered in steps 3–4. This example assumes event_date is a key:

SELECT event_date, user_id, event_type
FROM analytics.events
WHERE event_date >= today() - 1
LIMIT 10
SETTINGS max_execution_time = 30,
         max_rows_to_read = 10000000,
         timeout_before_checking_execution_speed = 0

7. Explain, then execute

Explain the exact bounded query:

EXPLAIN indexes = 1
SELECT event_type, count() AS events
FROM analytics.events
WHERE event_date >= today() - 1
GROUP BY event_type
ORDER BY events DESC
LIMIT 100
SETTINGS max_execution_time = 30,
         max_rows_to_read = 10000000,
         timeout_before_checking_execution_speed = 0

If the plan shows useful part or granule pruning, run the same query without EXPLAIN. Otherwise narrow the key filter. On timeout, read-limit, or memory errors, narrow the query rather than increasing limits.