featzhang opened a new pull request, #27351:
URL: https://github.com/apache/flink/pull/27351
**Background**
Apache Flink SQL currently provides `URL_DECODE(str)` to decode strings in
`application/x-www-form-urlencoded` format (introduced in [FLINK-34108]). In
practice, log data, tracking data, or external system inputs may contain
multi-level URL-encoded values, e.g., strings encoded multiple times by
upstream systems or due to redirects/proxies. The existing function only
performs a single decoding pass, requiring manual repeated calls for full
decoding.
**Problem**
* Single-pass decoding is insufficient for multi-level encoded values.
* Users must repeatedly apply `URL_DECODE`, reducing SQL readability and
increasing error risk.
**Solution**
* Extend `URL_DECODE` with an optional boolean parameter `recursive`.
* Signature:
```sql
URL_DECODE(str) -- existing behavior
URL_DECODE(str, recursive) -- new recursive option
```
* Behavior:
* `recursive = false` (default): single-pass decoding.
* `recursive = true`: repeatedly decodes until the result no longer
changes.
**Examples**
```sql
-- Single-pass decoding
SELECT URL_DECODE('%252Fpath%252Fto%252Fresource');
-- Result: '%2Fpath%2Fto%2Fresource'
-- Recursive decoding
SELECT URL_DECODE('%252Fpath%252Fto%252Fresource', true);
-- Result: '/path/to/resource'
```
**Compatibility**
* Fully backward-compatible; existing queries using `URL_DECODE(str)` remain
unchanged.
* The new parameter is optional and extends functionality without breaking
existing pipelines.
**Notes**
* Recursive decoding stops when the decoded result matches the previous
value.
* Useful for data cleansing, normalization, and multi-level encoded input
scenarios.
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