docs: clarify FFT limits

Author: DaZuiZui

src/UserGuide/Master/Table/SQL-Manual/Featured-Functions_apache.md

@@ -693,8 +693,6 @@ IoTDB> SELECT window_start, window_end, stock_id, avg(price) as avg FROM CUMULAT
 |2021-01-01T09:00:00.000+08:00|2021-01-01T09:10:00.000+08:00|    AAPL|101.66666666666667|
 +-----------------------------+-----------------------------+--------+------------------+
 ```
-+-----------------------------+-----------------------------+--------+------------------+
-```
 
 ## 4. `FFT` Function
 
@@ -730,11 +728,12 @@ FFT(
 
 ### 4.4 Input Requirements and Limitations
 
-* Supported FFT input types are `INT32`, `INT64`, `FLOAT`, and `DOUBLE`. Other non-partition columns are ignored.
+* Supported FFT input types are `INT32`, `INT64`, `FLOAT`, and `DOUBLE`. Other non-partition, non-time columns are ignored.
 * Every numeric input value must be non-`NULL`.
 * Timestamps must be strictly ascending within each partition.
 * If `SAMPLE_INTERVAL` is omitted, each partition must contain at least two rows. For regularly sampled data, explicitly specifying the interval is recommended.
 * FFT assumes equally spaced samples. For irregular timestamps, the implementation uses the average interval (or the supplied interval), so the frequency axis is an approximation.
+* The total number of spectrum values (`2 × N × number of numeric columns`) is limited to 16,777,216 in addition to the `N <= 65,536` limit.
 
 ### 4.5 Returned Results
 

src/UserGuide/latest-Table/SQL-Manual/Featured-Functions_apache.md

@@ -693,8 +693,6 @@ IoTDB> SELECT window_start, window_end, stock_id, avg(price) as avg FROM CUMULAT
 |2021-01-01T09:00:00.000+08:00|2021-01-01T09:10:00.000+08:00|    AAPL|101.66666666666667|
 +-----------------------------+-----------------------------+--------+------------------+
 ```
-+-----------------------------+-----------------------------+--------+------------------+
-```
 
 ## 4. `FFT` Function
 
@@ -730,11 +728,12 @@ FFT(
 
 ### 4.4 Input Requirements and Limitations
 
-* Supported FFT input types are `INT32`, `INT64`, `FLOAT`, and `DOUBLE`. Other non-partition columns are ignored.
+* Supported FFT input types are `INT32`, `INT64`, `FLOAT`, and `DOUBLE`. Other non-partition, non-time columns are ignored.
 * Every numeric input value must be non-`NULL`.
 * Timestamps must be strictly ascending within each partition.
 * If `SAMPLE_INTERVAL` is omitted, each partition must contain at least two rows. For regularly sampled data, explicitly specifying the interval is recommended.
 * FFT assumes equally spaced samples. For irregular timestamps, the implementation uses the average interval (or the supplied interval), so the frequency axis is an approximation.
+* The total number of spectrum values (`2 × N × number of numeric columns`) is limited to 16,777,216 in addition to the `N <= 65,536` limit.
 
 ### 4.5 Returned Results
 

src/zh/UserGuide/latest-Table/SQL-Manual/Featured-Functions_apache.md

@@ -694,8 +694,6 @@ IoTDB> SELECT window_start, window_end, stock_id, avg(price) as avg FROM CUMULAT
 |2021-01-01T09:00:00.000+08:00|2021-01-01T09:10:00.000+08:00|    AAPL|101.66666666666667|
 +-----------------------------+-----------------------------+--------+------------------+
 ```
-+-----------------------------+-----------------------------+--------+------------------+
-```
 
 ## 4. `FFT` 函数
 
@@ -731,11 +729,12 @@ FFT(
 
 ### 4.4 输入要求与限制
 
-* 支持的 FFT 输入类型为 `INT32`、`INT64`、`FLOAT` 和 `DOUBLE`，其他非分区列会被忽略。
+* 支持的 FFT 输入类型为 `INT32`、`INT64`、`FLOAT` 和 `DOUBLE`，其他非分区、非时间列会被忽略。
 * 所有数值输入都不能为 `NULL`。
 * 每个分区内的时间戳必须严格递增。
 * 省略 `SAMPLE_INTERVAL` 时，每个分区至少需要两行数据。对于等间隔采样数据，建议显式指定采样间隔。
 * FFT 假设样本等间隔。对于时间戳不规则的数据，函数使用平均间隔（或用户提供的间隔），因此频率轴结果是近似值。
+* 除了 `N <= 65,536` 的限制外，频谱值总数（`2 × N × 数值列数`）不能超过 16,777,216。
 
 ### 4.5 返回结果