How to Get an Execution Plan in ClickHouse

How do I view the execution plan of a query in ClickHouse?

EXPLAIN shows how ClickHouse will run a query, detailing stages, indexes, and resource costs.

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Description

Why inspect a ClickHouse execution plan?

Knowing the plan lets you spot full-table scans, missing indexes, or expensive JOINs before they hurt production performance.

How do I request the plan?

Run EXPLAIN before your SELECT. Choose a mode—AST, SYNTAX, PLAN, or PIPELINE—depending on the depth of detail you need.

Which EXPLAIN mode answers which question?

AST shows the parsed tree, useful for debugging rewritten queries. PLAN reveals the costed steps ClickHouse will execute. PIPELINE breaks down thread-level processing.SYNTAX returns the rewritten SQL.

What is the basic workflow?

1) Write the SELECT. 2) Prefix with EXPLAIN PLAN. 3) Examine output and adjust query, data types, or settings. 4) Re-EXPLAIN until the plan is optimal.

Practical example: Optimizing order totals

Suppose you need daily revenue. If EXPLAIN shows a full scan on OrderItems, add a partition key or materialized view to pre-aggregate totals.

Best practices for EXPLAIN

Set allow_experimental_analyzer = 1 for richer plans.Collect statistics on columns used in WHERE and JOIN. Test changes in staging first.

What common mistakes should I avoid?

Do not ignore parts_to_read; a high value signals poor partitioning. Avoid relying solely on SYNTAX mode—PLAN mode gives cost info.

Next steps after reading the plan

Create skip indexes, reorder JOINs, or use pre-aggregated tables. Re-run EXPLAIN to verify improvements.

Why How to Get an Execution Plan in ClickHouse is important

How to Get an Execution Plan in ClickHouse Example Usage


EXPLAIN PLAN
SELECT
    Orders.order_date::Date AS day,
    sum(OrderItems.quantity * Products.price) AS daily_revenue
FROM Orders
JOIN OrderItems ON Orders.id = OrderItems.order_id
JOIN Products ON Products.id = OrderItems.product_id
WHERE Orders.order_date >= '2024-09-01'
  AND Orders.order_date <  '2024-10-01'
GROUP BY day
ORDER BY day;

How to Get an Execution Plan in ClickHouse Syntax


EXPLAIN [AST | SYNTAX | PLAN | PIPELINE]
SETTINGS allow_experimental_analyzer = 1
SELECT column_list
FROM Orders
JOIN OrderItems ON Orders.id = OrderItems.order_id
JOIN Products ON Products.id = OrderItems.product_id
WHERE order_date BETWEEN '2024-09-01' AND '2024-09-30'
  AND Products.stock > 0;

Common Mistakes

Ignoring pipeline details: Users look only at the high-level PLAN and miss that threads are imbalanced in PIPELINE mode. Always check PIPELINE to ensure even workload distribution.
Running EXPLAIN without turning on allow_experimental_analyzer, which hides modern cost estimates. Fix by adding SETTINGS allow_experimental_analyzer = 1.