ClickHouse/docs/en/sql-reference/statements/explain.md

toc_priority	toc_title
39	EXPLAIN

EXPLAIN Statement

Shows the execution plan of a statement.

Syntax:

EXPLAIN [AST | SYNTAX | PLAN | PIPELINE] [setting = value, ...] SELECT ... [FORMAT ...]

Example:

EXPLAIN SELECT sum(number) FROM numbers(10) UNION ALL SELECT sum(number) FROM numbers(10) ORDER BY sum(number) ASC FORMAT TSV;

Union
  Expression (Projection)
    Expression (Before ORDER BY and SELECT)
      Aggregating
        Expression (Before GROUP BY)
          SettingQuotaAndLimits (Set limits and quota after reading from storage)
            ReadFromStorage (SystemNumbers)
  Expression (Projection)
    MergingSorted (Merge sorted streams for ORDER BY)
      MergeSorting (Merge sorted blocks for ORDER BY)
        PartialSorting (Sort each block for ORDER BY)
          Expression (Before ORDER BY and SELECT)
            Aggregating
              Expression (Before GROUP BY)
                SettingQuotaAndLimits (Set limits and quota after reading from storage)
                  ReadFromStorage (SystemNumbers)

EXPLAIN Types

• AST — Abstract syntax tree.
• SYNTAX — Query text after AST-level optimizations.
• PLAN — Query execution plan.
• PIPELINE — Query execution pipeline.

EXPLAIN AST

Dump query AST.

Example:

EXPLAIN AST SELECT 1;

SelectWithUnionQuery (children 1)
 ExpressionList (children 1)
  SelectQuery (children 1)
   ExpressionList (children 1)
    Literal UInt64_1

EXPLAIN SYNTAX

Returns query after syntax optimizations.

Example:

EXPLAIN SYNTAX SELECT * FROM system.numbers AS a, system.numbers AS b, system.numbers AS c;

SELECT
    `--a.number` AS `a.number`,
    `--b.number` AS `b.number`,
    number AS `c.number`
FROM
(
    SELECT
        number AS `--a.number`,
        b.number AS `--b.number`
    FROM system.numbers AS a
    CROSS JOIN system.numbers AS b
) AS `--.s`
CROSS JOIN system.numbers AS c

EXPLAIN PLAN

Dump query plan steps.

Settings:

• header — Prints output header for step. Default: 0.
• description — Prints step description. Default: 1.
• actions — Prints detailed information about step actions. Default: 0.

Example:

EXPLAIN SELECT sum(number) FROM numbers(10) GROUP BY number % 4;

Union
  Expression (Projection)
  Expression (Before ORDER BY and SELECT)
    Aggregating
      Expression (Before GROUP BY)
        SettingQuotaAndLimits (Set limits and quota after reading from storage)
          ReadFromStorage (SystemNumbers)

!!! note "Note" Step and query cost estimation is not supported.

EXPLAIN PIPELINE

Settings:

• header — Prints header for each output port. Default: 0.
• graph — Prints a graph described in the DOT graph description language. Default: 0.
• compact — Prints graph in compact mode if graph setting is enabled. Default: 1.
• indexes — Shows used indexes, the number of filtered parts, and granules for every index applied. Default: 0. Supported for MergeTree tables.

Example:

EXPLAIN PIPELINE SELECT sum(number) FROM numbers_mt(100000) GROUP BY number % 4;

(Union)
(Expression)
ExpressionTransform
  (Expression)
  ExpressionTransform
    (Aggregating)
    Resize 2 → 1
      AggregatingTransform × 2
        (Expression)
        ExpressionTransform × 2
          (SettingQuotaAndLimits)
            (ReadFromStorage)
            NumbersMt × 2 0 → 1

Оriginal article