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* Add EXPLAIN query description * Check examples * update * Update docs/en/sql-reference/statements/explain.md Co-authored-by: BayoNet <da-daos@yandex.ru> * Update docs/en/sql-reference/statements/explain.md Co-authored-by: Nikolai Kochetov <KochetovNicolai@users.noreply.github.com> Co-authored-by: BayoNet <da-daos@yandex.ru> Co-authored-by: Nikolai Kochetov <KochetovNicolai@users.noreply.github.com>
151 lines
3.4 KiB
Markdown
151 lines
3.4 KiB
Markdown
---
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toc_priority: 39
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toc_title: EXPLAIN
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---
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# EXPLAIN Statement {#explain}
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Show the execution plan of a statement.
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Syntax:
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```sql
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EXPLAIN [AST | SYNTAX | PLAN | PIPELINE] [setting = value, ...] SELECT ... [FORMAT ...]
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```
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Example:
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```sql
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EXPLAIN SELECT sum(number) FROM numbers(10) UNION ALL SELECT sum(number) FROM numbers(10) ORDER BY sum(number) ASC FORMAT TSV;
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```
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```sql
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Union
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Expression (Projection)
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Expression (Before ORDER BY and SELECT)
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Aggregating
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Expression (Before GROUP BY)
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SettingQuotaAndLimits (Set limits and quota after reading from storage)
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ReadFromStorage (SystemNumbers)
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Expression (Projection)
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MergingSorted (Merge sorted streams for ORDER BY)
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MergeSorting (Merge sorted blocks for ORDER BY)
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PartialSorting (Sort each block for ORDER BY)
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Expression (Before ORDER BY and SELECT)
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Aggregating
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Expression (Before GROUP BY)
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SettingQuotaAndLimits (Set limits and quota after reading from storage)
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ReadFromStorage (SystemNumbers)
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```
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## EXPLAIN Types {#explain-types}
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- `AST` — Abstract syntax tree.
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- `SYNTAX` — Query text after AST-level optimizations.
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- `PLAN` — Query execution plan.
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- `PIPELINE` — Query execution pipeline.
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### EXPLAIN AST {#explain-ast}
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Dump query AST.
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Example:
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```sql
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EXPLAIN AST SELECT 1;
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```
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```sql
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SelectWithUnionQuery (children 1)
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ExpressionList (children 1)
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SelectQuery (children 1)
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ExpressionList (children 1)
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Literal UInt64_1
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```
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### EXPLAIN SYNTAX {#explain-syntax}
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Return query after syntax optimizations.
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Example:
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```sql
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EXPLAIN SYNTAX SELECT * FROM system.numbers AS a, system.numbers AS b, system.numbers AS c;
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```
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```sql
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SELECT
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`--a.number` AS `a.number`,
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`--b.number` AS `b.number`,
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number AS `c.number`
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FROM
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(
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SELECT
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number AS `--a.number`,
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b.number AS `--b.number`
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FROM system.numbers AS a
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CROSS JOIN system.numbers AS b
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) AS `--.s`
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CROSS JOIN system.numbers AS c
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```
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### EXPLAIN PLAN {#explain-plan}
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Dump query plan steps.
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Settings:
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- `header` — Print output header for step. Default: 0.
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- `description` — Print step description. Default: 1.
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- `actions` — Print detailed information about step actions. Default: 0.
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Example:
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```sql
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EXPLAIN SELECT sum(number) FROM numbers(10) GROUP BY number % 4;
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```
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```sql
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Union
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Expression (Projection)
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Expression (Before ORDER BY and SELECT)
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Aggregating
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Expression (Before GROUP BY)
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SettingQuotaAndLimits (Set limits and quota after reading from storage)
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ReadFromStorage (SystemNumbers)
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```
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!!! note "Note"
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Step and query cost estimation is not supported.
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### EXPLAIN PIPELINE {#explain-pipeline}
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Settings:
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- `header` — Print header for each output port. Default: 0.
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- `graph` — Use DOT graph description language. Default: 0.
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- `compact` — Print graph in compact mode if graph is enabled. Default: 1.
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Example:
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```sql
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EXPLAIN PIPELINE SELECT sum(number) FROM numbers_mt(100000) GROUP BY number % 4;
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```
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```sql
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(Union)
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(Expression)
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ExpressionTransform
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(Expression)
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ExpressionTransform
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(Aggregating)
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Resize 2 → 1
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AggregatingTransform × 2
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(Expression)
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ExpressionTransform × 2
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(SettingQuotaAndLimits)
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(ReadFromStorage)
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NumbersMt × 2 0 → 1
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```
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[Оriginal article](https://clickhouse.tech/docs/en/sql-reference/statements/explain/) <!--hide-->
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