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61 lines
2.8 KiB
Markdown
61 lines
2.8 KiB
Markdown
---
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toc_priority: 62
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toc_title: Window Functions
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---
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# [experimental] Window Functions
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!!! warning "Warning"
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This is an experimental feature that is currently in development and is not ready for general use. It will change in unpredictable backwards-incompatible ways in the future releases. Set `allow_experimental_window_functions = 1` to enable it.
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ClickHouse supports the standard grammar for defining windows and window functions. The following features are currently supported:
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| Feature | Support or workaround |
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| --------| ----------|
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| ad hoc window specification (`count(*) over (partition by id order by time desc)`) | supported |
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| expressions involving window functions, e.g. `(count(*) over ()) / 2)` | not supported, wrap in a subquery ([feature request](https://github.com/ClickHouse/ClickHouse/issues/19857)) |
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| `WINDOW` clause (`select ... from table window w as (partiton by id)`) | supported |
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| `ROWS` frame | supported |
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| `RANGE` frame | supported, the default |
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| `INTERVAL` syntax for `DateTime` `RANGE OFFSET` frame | not supported, specify the number of seconds instead |
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| `GROUPS` frame | not supported |
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| Calculating aggregate functions over a frame (`sum(value) over (order by time)`) | all aggregate functions are supported |
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| `rank()`, `dense_rank()`, `row_number()` | supported |
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| `lag/lead(value, offset)` | Not supported. Workarounds: |
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| | 1) replace with `any(value) over (.... rows between <offset> preceding and <offset> preceding)`, or `following` for `lead`|
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| | 2) use `lagInFrame/leadInFrame`, which are analogous, but respect the window frame. To get behavior identical to `lag/lead`, use `rows between unbounded preceding and unbounded following` |
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## References
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### GitHub Issues
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The roadmap for the initial support of window functions is [in this issue](https://github.com/ClickHouse/ClickHouse/issues/18097).
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All GitHub issues related to window funtions have the [comp-window-functions](https://github.com/ClickHouse/ClickHouse/labels/comp-window-functions) tag.
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### Tests
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These tests contain the examples of the currently supported grammar:
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https://github.com/ClickHouse/ClickHouse/blob/master/tests/performance/window_functions.xml
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https://github.com/ClickHouse/ClickHouse/blob/master/tests/queries/0_stateless/01591_window_functions.sql
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### Postgres Docs
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https://www.postgresql.org/docs/current/sql-select.html#SQL-WINDOW
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https://www.postgresql.org/docs/devel/sql-expressions.html#SYNTAX-WINDOW-FUNCTIONS
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https://www.postgresql.org/docs/devel/functions-window.html
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https://www.postgresql.org/docs/devel/tutorial-window.html
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### MySQL Docs
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https://dev.mysql.com/doc/refman/8.0/en/window-function-descriptions.html
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https://dev.mysql.com/doc/refman/8.0/en/window-functions-usage.html
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https://dev.mysql.com/doc/refman/8.0/en/window-functions-frames.html
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