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docs: Add documentation for supported window expressions #2696

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1dd2ba8
clarify supported window expressions
andygrove Nov 5, 2025
fb72c92
Merge remote-tracking branch 'apache/main' into window-expr-docs
andygrove Nov 5, 2025
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13 changes: 13 additions & 0 deletions docs/source/user-guide/latest/expressions.md
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Expand Up @@ -201,6 +201,19 @@ incompatible expressions.
| VariancePop | | Yes | |
| VarianceSamp | | Yes | |

## Window Functions

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@comphead comphead Nov 6, 2025

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@andygrove I'm preparing set of tests today to get real status for windows, let me get back to this PR later today

Comet supports using aggregate functions within window contexts with PARTITION BY and ORDER BY clauses. Window frames (ROWS and RANGE) are supported with various bounds (UNBOUNDED PRECEDING, UNBOUNDED FOLLOWING, CURRENT ROW, and numeric offsets).

| Expression | Spark-Compatible? | Compatibility Notes |
| ---------- | ----------------- | -------------------------------------------- |
| Count | Yes | Aggregate function used in window context |
| Max | Yes | Aggregate function used in window context |
| Min | Yes | Aggregate function used in window context |
| Sum | Yes | Aggregate function used in window context |

**Note:** Dedicated window functions such as `rank`, `dense_rank`, `row_number`, `lag`, `lead`, `ntile`, `cume_dist`, `percent_rank`, and `nth_value` are not currently supported and will fall back to Spark.

## Array Expressions

| Expression | Spark-Compatible? | Compatibility Notes |
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