mirror of
https://github.com/ClickHouse/ClickHouse.git
synced 2024-11-13 11:04:10 +00:00
416 lines
10 KiB
Markdown
416 lines
10 KiB
Markdown
---
|
||
toc_priority: 39
|
||
toc_title: EXPLAIN
|
||
---
|
||
|
||
# EXPLAIN Statement {#explain}
|
||
|
||
Shows the execution plan of a statement.
|
||
|
||
Syntax:
|
||
|
||
```sql
|
||
EXPLAIN [AST | SYNTAX | PLAN | PIPELINE] [setting = value, ...] SELECT ... [FORMAT ...]
|
||
```
|
||
|
||
Example:
|
||
|
||
```sql
|
||
EXPLAIN SELECT sum(number) FROM numbers(10) UNION ALL SELECT sum(number) FROM numbers(10) ORDER BY sum(number) ASC FORMAT TSV;
|
||
```
|
||
|
||
```sql
|
||
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 {#explain-types}
|
||
|
||
- `AST` — Abstract syntax tree.
|
||
- `SYNTAX` — Query text after AST-level optimizations.
|
||
- `PLAN` — Query execution plan.
|
||
- `PIPELINE` — Query execution pipeline.
|
||
|
||
### EXPLAIN AST {#explain-ast}
|
||
|
||
Dump query AST. Supports all types of queries, not only `SELECT`.
|
||
|
||
Examples:
|
||
|
||
```sql
|
||
EXPLAIN AST SELECT 1;
|
||
```
|
||
|
||
```sql
|
||
SelectWithUnionQuery (children 1)
|
||
ExpressionList (children 1)
|
||
SelectQuery (children 1)
|
||
ExpressionList (children 1)
|
||
Literal UInt64_1
|
||
```
|
||
|
||
```sql
|
||
EXPLAIN AST ALTER TABLE t1 DELETE WHERE date = today();
|
||
```
|
||
|
||
```sql
|
||
explain
|
||
AlterQuery t1 (children 1)
|
||
ExpressionList (children 1)
|
||
AlterCommand 27 (children 1)
|
||
Function equals (children 1)
|
||
ExpressionList (children 2)
|
||
Identifier date
|
||
Function today (children 1)
|
||
ExpressionList
|
||
```
|
||
|
||
### EXPLAIN SYNTAX {#explain-syntax}
|
||
|
||
Returns query after syntax optimizations.
|
||
|
||
Example:
|
||
|
||
```sql
|
||
EXPLAIN SYNTAX SELECT * FROM system.numbers AS a, system.numbers AS b, system.numbers AS c;
|
||
```
|
||
|
||
```sql
|
||
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 {#explain-plan}
|
||
|
||
Dump query plan steps.
|
||
|
||
Settings:
|
||
|
||
- `header` — Prints output header for step. Default: 0.
|
||
- `description` — Prints step description. Default: 1.
|
||
- `indexes` — Shows used indexes, the number of filtered parts and the number of filtered granules for every index applied. Default: 0. Supported for [MergeTree](../../engines/table-engines/mergetree-family/mergetree.md) tables.
|
||
- `actions` — Prints detailed information about step actions. Default: 0.
|
||
- `json` — Prints query plan steps as a row in [JSON](../../interfaces/formats.md#json) format. Default: 0. It is recommended to use [TSVRaw](../../interfaces/formats.md#tabseparatedraw) format to avoid unnecessary escaping.
|
||
|
||
Example:
|
||
|
||
```sql
|
||
EXPLAIN SELECT sum(number) FROM numbers(10) GROUP BY number % 4;
|
||
```
|
||
|
||
```sql
|
||
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.
|
||
|
||
When `json = 1`, the query plan is represented in JSON format. Every node is a dictionary that always has the keys `Node Type` and `Plans`. `Node Type` is a string with a step name. `Plans` is an array with child step descriptions. Other optional keys may be added depending on node type and settings.
|
||
|
||
Example:
|
||
|
||
```sql
|
||
EXPLAIN json = 1, description = 0 SELECT 1 UNION ALL SELECT 2 FORMAT TSVRaw;
|
||
```
|
||
|
||
```json
|
||
[
|
||
{
|
||
"Plan": {
|
||
"Node Type": "Union",
|
||
"Plans": [
|
||
{
|
||
"Node Type": "Expression",
|
||
"Plans": [
|
||
{
|
||
"Node Type": "SettingQuotaAndLimits",
|
||
"Plans": [
|
||
{
|
||
"Node Type": "ReadFromStorage"
|
||
}
|
||
]
|
||
}
|
||
]
|
||
},
|
||
{
|
||
"Node Type": "Expression",
|
||
"Plans": [
|
||
{
|
||
"Node Type": "SettingQuotaAndLimits",
|
||
"Plans": [
|
||
{
|
||
"Node Type": "ReadFromStorage"
|
||
}
|
||
]
|
||
}
|
||
]
|
||
}
|
||
]
|
||
}
|
||
}
|
||
]
|
||
```
|
||
|
||
With `description` = 1, the `Description` key is added to the step:
|
||
|
||
```json
|
||
{
|
||
"Node Type": "ReadFromStorage",
|
||
"Description": "SystemOne"
|
||
}
|
||
```
|
||
|
||
With `header` = 1, the `Header` key is added to the step as an array of columns.
|
||
|
||
Example:
|
||
|
||
```sql
|
||
EXPLAIN json = 1, description = 0, header = 1 SELECT 1, 2 + dummy;
|
||
```
|
||
|
||
```json
|
||
[
|
||
{
|
||
"Plan": {
|
||
"Node Type": "Expression",
|
||
"Header": [
|
||
{
|
||
"Name": "1",
|
||
"Type": "UInt8"
|
||
},
|
||
{
|
||
"Name": "plus(2, dummy)",
|
||
"Type": "UInt16"
|
||
}
|
||
],
|
||
"Plans": [
|
||
{
|
||
"Node Type": "SettingQuotaAndLimits",
|
||
"Header": [
|
||
{
|
||
"Name": "dummy",
|
||
"Type": "UInt8"
|
||
}
|
||
],
|
||
"Plans": [
|
||
{
|
||
"Node Type": "ReadFromStorage",
|
||
"Header": [
|
||
{
|
||
"Name": "dummy",
|
||
"Type": "UInt8"
|
||
}
|
||
]
|
||
}
|
||
]
|
||
}
|
||
]
|
||
}
|
||
}
|
||
]
|
||
```
|
||
|
||
With `indexes` = 1, the `Indexes` key is added. It contains an array of used indexes. Each index is described as JSON with `Type` key (a string `MinMax`, `Partition`, `PrimaryKey` or `Skip`) and optional keys:
|
||
|
||
- `Name` — An index name (for now, is used only for `Skip` index).
|
||
- `Keys` — An array of columns used by the index.
|
||
- `Condition` — A string with condition used.
|
||
- `Description` — An index (for now, is used only for `Skip` index).
|
||
- `Initial Parts` — A number of parts before the index is applied.
|
||
- `Selected Parts` — A number of parts after the index is applied.
|
||
- `Initial Granules` — A number of granules before the index is applied.
|
||
- `Selected Granulesis` — A number of granules after the index is applied.
|
||
|
||
Example:
|
||
|
||
```json
|
||
"Node Type": "ReadFromMergeTree",
|
||
"Indexes": [
|
||
{
|
||
"Type": "MinMax",
|
||
"Keys": ["y"],
|
||
"Condition": "(y in [1, +inf))",
|
||
"Initial Parts": 5,
|
||
"Selected Parts": 4,
|
||
"Initial Granules": 12,
|
||
"Selected Granules": 11
|
||
},
|
||
{
|
||
"Type": "Partition",
|
||
"Keys": ["y", "bitAnd(z, 3)"],
|
||
"Condition": "and((bitAnd(z, 3) not in [1, 1]), and((y in [1, +inf)), (bitAnd(z, 3) not in [1, 1])))",
|
||
"Initial Parts": 4,
|
||
"Selected Parts": 3,
|
||
"Initial Granules": 11,
|
||
"Selected Granules": 10
|
||
},
|
||
{
|
||
"Type": "PrimaryKey",
|
||
"Keys": ["x", "y"],
|
||
"Condition": "and((x in [11, +inf)), (y in [1, +inf)))",
|
||
"Initial Parts": 3,
|
||
"Selected Parts": 2,
|
||
"Initial Granules": 10,
|
||
"Selected Granules": 6
|
||
},
|
||
{
|
||
"Type": "Skip",
|
||
"Name": "t_minmax",
|
||
"Description": "minmax GRANULARITY 2",
|
||
"Initial Parts": 2,
|
||
"Selected Parts": 1,
|
||
"Initial Granules": 6,
|
||
"Selected Granules": 2
|
||
},
|
||
{
|
||
"Type": "Skip",
|
||
"Name": "t_set",
|
||
"Description": "set GRANULARITY 2",
|
||
"Initial Parts": 1,
|
||
"Selected Parts": 1,
|
||
"Initial Granules": 2,
|
||
"Selected Granules": 1
|
||
}
|
||
]
|
||
```
|
||
|
||
With `actions` = 1, added keys depend on step type.
|
||
|
||
Example:
|
||
|
||
```sql
|
||
EXPLAIN json = 1, actions = 1, description = 0 SELECT 1 FORMAT TSVRaw;
|
||
```
|
||
|
||
```json
|
||
[
|
||
{
|
||
"Plan": {
|
||
"Node Type": "Expression",
|
||
"Expression": {
|
||
"Inputs": [],
|
||
"Actions": [
|
||
{
|
||
"Node Type": "Column",
|
||
"Result Type": "UInt8",
|
||
"Result Type": "Column",
|
||
"Column": "Const(UInt8)",
|
||
"Arguments": [],
|
||
"Removed Arguments": [],
|
||
"Result": 0
|
||
}
|
||
],
|
||
"Outputs": [
|
||
{
|
||
"Name": "1",
|
||
"Type": "UInt8"
|
||
}
|
||
],
|
||
"Positions": [0],
|
||
"Project Input": true
|
||
},
|
||
"Plans": [
|
||
{
|
||
"Node Type": "SettingQuotaAndLimits",
|
||
"Plans": [
|
||
{
|
||
"Node Type": "ReadFromStorage"
|
||
}
|
||
]
|
||
}
|
||
]
|
||
}
|
||
}
|
||
]
|
||
```
|
||
|
||
### EXPLAIN PIPELINE {#explain-pipeline}
|
||
|
||
Settings:
|
||
|
||
- `header` — Prints header for each output port. Default: 0.
|
||
- `graph` — Prints a graph described in the [DOT](https://en.wikipedia.org/wiki/DOT_(graph_description_language)) graph description language. Default: 0.
|
||
- `compact` — Prints graph in compact mode if `graph` setting is enabled. Default: 1.
|
||
|
||
Example:
|
||
|
||
```sql
|
||
EXPLAIN PIPELINE SELECT sum(number) FROM numbers_mt(100000) GROUP BY number % 4;
|
||
```
|
||
|
||
```sql
|
||
(Union)
|
||
(Expression)
|
||
ExpressionTransform
|
||
(Expression)
|
||
ExpressionTransform
|
||
(Aggregating)
|
||
Resize 2 → 1
|
||
AggregatingTransform × 2
|
||
(Expression)
|
||
ExpressionTransform × 2
|
||
(SettingQuotaAndLimits)
|
||
(ReadFromStorage)
|
||
NumbersMt × 2 0 → 1
|
||
```
|
||
### EXPLAIN ESTIMATE {#explain-estimate}
|
||
|
||
Shows the estimated number of rows, marks and parts to be read from the tables while processing the query. Works with tables in the [MergeTree](../../engines/table-engines/mergetree-family/mergetree.md#table_engines-mergetree) family.
|
||
|
||
**Example**
|
||
|
||
Creating a table:
|
||
|
||
```sql
|
||
CREATE TABLE ttt (i Int64) ENGINE = MergeTree() ORDER BY i SETTINGS index_granularity = 16, write_final_mark = 0;
|
||
INSERT INTO ttt SELECT number FROM numbers(128);
|
||
OPTIMIZE TABLE ttt;
|
||
```
|
||
|
||
Query:
|
||
|
||
```sql
|
||
EXPLAIN ESTIMATE SELECT * FROM ttt;
|
||
```
|
||
|
||
Result:
|
||
|
||
```text
|
||
┌─database─┬─table─┬─parts─┬─rows─┬─marks─┐
|
||
│ default │ ttt │ 1 │ 128 │ 8 │
|
||
└──────────┴───────┴───────┴──────┴───────┘
|
||
```
|
||
|
||
[Оriginal article](https://clickhouse.tech/docs/en/sql-reference/statements/explain/) <!--hide-->
|