thevar1able/ClickHouse
1
0
Fork 0
mirror of https://github.com/ClickHouse/ClickHouse.git synced 2024-11-23 08:02:02 +00:00
Code Issues Packages Projects Releases Wiki Activity

Merge pull request #63187 from kitaisreal/recursive-cte-documentation

Browse Source 
Added recursive CTE documentation
This commit is contained in:
Alexey Milovidov 2024-05-07 19:06:17 +00:00 committed by GitHub
commit 35b5a80e0e
No known key found for this signature in database
GPG Key ID: B5690EEEBB952194
1 changed files with 229 additions and 6 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

235
docs/en/sql-reference/statements/select/with.md
View File

@ -5,21 +5,21 @@ sidebar_label: WITH
# WITH Clause # WITH Clause
ClickHouse supports Common Table Expressions ([CTE](https://en.wikipedia.org/wiki/Hierarchical_and_recursive_queries_in_SQL)) and substitutes the code defined in the `WITH` clause in all places of use for the rest of `SELECT` query. Named subqueries can be included to the current and child query context in places where table objects are allowed. Recursion is prevented by hiding the current level CTEs from the WITH expression. ClickHouse supports Common Table Expressions ([CTE](https://en.wikipedia.org/wiki/Hierarchical_and_recursive_queries_in_SQL)) and substitutes the code defined in the `WITH` clause in all places of use for the rest of `SELECT` query. Named subqueries can be included to the current and child query context in places where table objects are allowed. Recursion is prevented by hiding the current level CTEs from the WITH expression.
Please note that CTEs do not guarantee the same results in all places they are called because the query will be re-executed for each use case. Please note that CTEs do not guarantee the same results in all places they are called because the query will be re-executed for each use case.
An example of such behavior is below An example of such behavior is below
``` sql ``` sql
with cte_numbers as with cte_numbers as
( (
select select
num num
from generateRandom('num UInt64', NULL) from generateRandom('num UInt64', NULL)
limit 1000000 limit 1000000
) )
select select
count() count()
from cte_numbers from cte_numbers
where num in (select num from cte_numbers) where num in (select num from cte_numbers)
``` ```
@ -87,3 +87,226 @@ LIMIT 10;
WITH test1 AS (SELECT i + 1, j + 1 FROM test1) WITH test1 AS (SELECT i + 1, j + 1 FROM test1)
SELECT * FROM test1; SELECT * FROM test1;
``` ```
# Recursive Queries
The optional RECURSIVE modifier allows for a WITH query to refer to its own output. Example:
**Example:** Sum integers from 1 through 100
```sql
WITH RECURSIVE test_table AS (
SELECT 1 AS number
UNION ALL
SELECT number + 1 FROM test_table WHERE number < 100
)
SELECT sum(number) FROM test_table;
```
``` text
┌─sum(number)─┐
│ 5050 │
└─────────────┘
```
The general form of a recursive `WITH` query is always a non-recursive term, then `UNION ALL`, then a recursive term, where only the recursive term can contain a reference to the query's own output. Recursive CTE query is executed as follows:
1. Evaluate the non-recursive term. Place result of non-recursive term query in a temporary working table.
2. As long as the working table is not empty, repeat these steps:
1. Evaluate the recursive term, substituting the current contents of the working table for the recursive self-reference. Place result of recursive term query in a temporary intermediate table.
2. Replace the contents of the working table with the contents of the intermediate table, then empty the intermediate table.
Recursive queries are typically used to work with hierarchical or tree-structured data. For example, we can write a query that performs tree traversal:
**Example:** Tree traversal
First let's create tree table:
```sql
DROP TABLE IF EXISTS tree;
CREATE TABLE tree
(
id UInt64,
parent_id Nullable(UInt64),
data String
) ENGINE = MergeTree ORDER BY id;
INSERT INTO tree VALUES (0, NULL, 'ROOT'), (1, 0, 'Child_1'), (2, 0, 'Child_2'), (3, 1, 'Child_1_1');
```
We can traverse those tree with such query:
**Example:** Tree traversal
```sql
WITH RECURSIVE search_tree AS (
SELECT id, parent_id, data
FROM tree t
WHERE t.id = 0
UNION ALL
SELECT t.id, t.parent_id, t.data
FROM tree t, search_tree st
WHERE t.parent_id = st.id
)
SELECT * FROM search_tree;
```
```text
┌─id─┬─parent_id─┬─data──────┐
│ 0 │ ᴺᵁᴸᴸ │ ROOT │
│ 1 │ 0 │ Child_1 │
│ 2 │ 0 │ Child_2 │
│ 3 │ 1 │ Child_1_1 │
└────┴───────────┴───────────┘
```
## Search order
To create a depth-first order, we compute for each result row an array of rows that we have already visited:
**Example:** Tree traversal depth-first order
```sql
WITH RECURSIVE search_tree AS (
SELECT id, parent_id, data, [t.id] AS path
FROM tree t
WHERE t.id = 0
UNION ALL
SELECT t.id, t.parent_id, t.data, arrayConcat(path, [t.id])
FROM tree t, search_tree st
WHERE t.parent_id = st.id
)
SELECT * FROM search_tree ORDER BY path;
```
```text
┌─id─┬─parent_id─┬─data──────┬─path────┐
│ 0 │ ᴺᵁᴸᴸ │ ROOT │ [0] │
│ 1 │ 0 │ Child_1 │ [0,1] │
│ 3 │ 1 │ Child_1_1 │ [0,1,3] │
│ 2 │ 0 │ Child_2 │ [0,2] │
└────┴───────────┴───────────┴─────────┘
```
To create a breadth-first order, standard approach is to add column that tracks the depth of the search:
**Example:** Tree traversal breadth-first order
```sql
WITH RECURSIVE search_tree AS (
SELECT id, parent_id, data, [t.id] AS path, toUInt64(0) AS depth
FROM tree t
WHERE t.id = 0
UNION ALL
SELECT t.id, t.parent_id, t.data, arrayConcat(path, [t.id]), depth + 1
FROM tree t, search_tree st
WHERE t.parent_id = st.id
)
SELECT * FROM search_tree ORDER BY depth;
```
```text
┌─id─┬─link─┬─data──────┬─path────┬─depth─┐
│ 0 │ ᴺᵁᴸᴸ │ ROOT │ [0] │ 0 │
│ 1 │ 0 │ Child_1 │ [0,1] │ 1 │
│ 2 │ 0 │ Child_2 │ [0,2] │ 1 │
│ 3 │ 1 │ Child_1_1 │ [0,1,3] │ 2 │
└────┴──────┴───────────┴─────────┴───────┘
```
## Cycle detection
First let's create graph table:
```sql
DROP TABLE IF EXISTS graph;
CREATE TABLE graph
(
from UInt64,
to UInt64,
label String
) ENGINE = MergeTree ORDER BY (from, to);
INSERT INTO graph VALUES (1, 2, '1 -> 2'), (1, 3, '1 -> 3'), (2, 3, '2 -> 3'), (1, 4, '1 -> 4'), (4, 5, '4 -> 5');
```
We can traverse that graph with such query:
**Example:** Graph traversal without cycle detection
```sql
WITH RECURSIVE search_graph AS (
SELECT from, to, label FROM graph g
UNION ALL
SELECT g.from, g.to, g.label
FROM graph g, search_graph sg
WHERE g.from = sg.to
)
SELECT DISTINCT * FROM search_graph ORDER BY from;
```
```text
┌─from─┬─to─┬─label──┐
│ 1 │ 4 │ 1 -> 4 │
│ 1 │ 2 │ 1 -> 2 │
│ 1 │ 3 │ 1 -> 3 │
│ 2 │ 3 │ 2 -> 3 │
│ 4 │ 5 │ 4 -> 5 │
└──────┴────┴────────┘
```
But if we add cycle in that graph, previous query will fail with `Maximum recursive CTE evaluation depth` error:
```sql
INSERT INTO graph VALUES (5, 1, '5 -> 1');
WITH RECURSIVE search_graph AS (
SELECT from, to, label FROM graph g
UNION ALL
SELECT g.from, g.to, g.label
FROM graph g, search_graph sg
WHERE g.from = sg.to
)
SELECT DISTINCT * FROM search_graph ORDER BY from;
```
```text
Code: 306. DB::Exception: Received from localhost:9000. DB::Exception: Maximum recursive CTE evaluation depth (1000) exceeded, during evaluation of search_graph AS (SELECT from, to, label FROM graph AS g UNION ALL SELECT g.from, g.to, g.label FROM graph AS g, search_graph AS sg WHERE g.from = sg.to). Consider raising max_recursive_cte_evaluation_depth setting.: While executing RecursiveCTESource. (TOO_DEEP_RECURSION)
```
The standard method for handling cycles is to compute an array of the already visited nodes:
**Example:** Graph traversal with cycle detection
```sql
WITH RECURSIVE search_graph AS (
SELECT from, to, label, false AS is_cycle, [tuple(g.from, g.to)] AS path FROM graph g
UNION ALL
SELECT g.from, g.to, g.label, has(path, tuple(g.from, g.to)), arrayConcat(sg.path, [tuple(g.from, g.to)])
FROM graph g, search_graph sg
WHERE g.from = sg.to AND NOT is_cycle
)
SELECT * FROM search_graph WHERE is_cycle ORDER BY from;
```
```text
┌─from─┬─to─┬─label──┬─is_cycle─┬─path──────────────────────┐
│ 1 │ 4 │ 1 -> 4 │ true │ [(1,4),(4,5),(5,1),(1,4)] │
│ 4 │ 5 │ 4 -> 5 │ true │ [(4,5),(5,1),(1,4),(4,5)] │
│ 5 │ 1 │ 5 -> 1 │ true │ [(5,1),(1,4),(4,5),(5,1)] │
└──────┴────┴────────┴──────────┴───────────────────────────┘
```
## Infinite queries
It is also possible to use infinite recursive CTE queries if `LIMIT` is used in outer query:
**Example:** Infinite recursive CTE query
```sql
WITH RECURSIVE test_table AS (
SELECT 1 AS number
UNION ALL
SELECT number + 1 FROM test_table
)
SELECT sum(number) FROM (SELECT number FROM test_table LIMIT 100);
```
```text
┌─sum(number)─┐
│ 5050 │
└─────────────┘
```