LucaCappelletti94 commented on code in PR #2352:
URL: 
https://github.com/apache/datafusion-sqlparser-rs/pull/2352#discussion_r3310483572


##########
src/parser/mod.rs:
##########
@@ -1717,6 +1731,23 @@ impl<'a> Parser<'a> {
             return prefix;
         }
 
+        // Memoize parse_prefix failures to break 2^N speculation when both
+        // prefix arms fail at every level (e.g. `IF(current_time(...x`).
+        // The per-arm cache in `parse_prefix_inner` complements this for
+        // chains where the reserved arm fails but the unreserved fallback
+        // succeeds (e.g. `case-case-...c`).
+        let start_index = self.index;
+        if let Some(cached) = self.failed_prefix_positions.get(&start_index) {
+            return Err(cached.clone());
+        }
+        let result = self.parse_prefix_inner();
+        if let Err(ref e) = result {
+            self.failed_prefix_positions.insert(start_index, e.clone());

Review Comment:
   I have a ~2GB limit on the fuzzer processes and after many billions of 
iterations it did not manage to OOM - it hit before the ASAN thread number 
limit than an OOM, so at the very least I believe it is not exponential. I can 
try and get a scaling though.



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