Parser: fix exponential parse time on table-factor paren chains

`parse_table_factor` speculatively parses `(...)` as a derived table; on
failure it rewinds and falls through to `parse_table_and_joins`, which
recurses back into `parse_table_factor` and re-attempts the same
speculative parse at every deeper paren. Both arms walk the remaining
chain, so on inputs like `SELECT 1 FROM ((((...` work doubles per level.
Caching the position at which the speculative arm failed short-circuits
the second descent.

Measured on `GenericDialect` with `with_recursion_limit(256)`, release
build:

| N  | Before  | After  |
|----|---------|--------|
| 10 |  20 ms  | 57 us  |
| 20 | 820 ms  | 119 us |
| 25 |  2.8 s  | 281 us |
| 30 |  7.9 s  | 345 us |

Author: moshap-firebolt

sqlparser_bench/benches/sqlparser_bench.rs

@@ -245,6 +245,34 @@ fn parse_prefix_case_chain(c: &mut Criterion) {
     group.finish();
 }
 
+/// Benchmark parsing pathological paren chains that previously caused 2^N
+/// work in `parse_table_factor`. The input `SELECT 1 FROM ((((...` rejects
+/// at EOF, which used to force exponential backtracking through the chain.
+fn parse_table_factor_paren_chain(c: &mut Criterion) {
+    let mut group = c.benchmark_group("parse_table_factor_paren_chain");
+    let dialect = GenericDialect {};
+
+    for &n in &[10usize, 20, 30] {
+        let mut sql = String::from("SELECT 1 ");
+        for _ in 0..5 {
+            sql.push_str("FROM ");
+            sql.push_str(&"(".repeat(n));
+            sql.push(' ');
+        }
+
+        group.bench_function(format!("chain_{n}"), |b| {
+            b.iter(|| {
+                let _ = Parser::new(&dialect)
+                    .with_recursion_limit(256)
+                    .try_with_sql(std::hint::black_box(&sql))
+                    .and_then(|mut p| p.parse_statements());
+            });
+        });
+    }
+
+    group.finish();
+}
+
 criterion_group!(
     benches,
     basic_queries,
@@ -253,6 +281,7 @@ criterion_group!(
     parse_compound_chain,
     parse_compound_keyword_chain,
     parse_prefix_keyword_call_chain,
-    parse_prefix_case_chain
+    parse_prefix_case_chain,
+    parse_table_factor_paren_chain
 );
 criterion_main!(benches);

src/parser/mod.rs

@@ -15,7 +15,7 @@
 #[cfg(not(feature = "std"))]
 use alloc::{
     boxed::Box,
-    collections::BTreeMap,
+    collections::{BTreeMap, BTreeSet},
     format,
     string::{String, ToString},
     vec,
@@ -26,7 +26,7 @@ use core::{
     str::FromStr,
 };
 #[cfg(feature = "std")]
-use std::collections::BTreeMap;
+use std::collections::{BTreeMap, BTreeSet};
 
 use helpers::attached_token::AttachedToken;
 
@@ -369,6 +369,10 @@ pub struct Parser<'a> {
     /// Cached failures from the speculative reserved-word prefix arm. See
     /// [`Parser::parse_prefix`] for the 2^N patterns this guards.
     failed_reserved_word_prefix_positions: BTreeMap<usize, ExprPrefixError>,
+    /// Cached failures from the speculative derived-table arm of
+    /// `parse_table_factor`. See [`Parser::parse_table_factor`] for the 2^N
+    /// pattern this guards.
+    failed_derived_table_factor_positions: BTreeSet<usize>,
 }
 
 /// Copy marker for a [`ParserError`] cached by the `parse_prefix` failure
@@ -414,6 +418,7 @@ impl<'a> Parser<'a> {
             options: ParserOptions::new().with_trailing_commas(dialect.supports_trailing_commas()),
             failed_prefix_positions: BTreeMap::new(),
             failed_reserved_word_prefix_positions: BTreeMap::new(),
+            failed_derived_table_factor_positions: BTreeSet::new(),
         }
     }
 
@@ -477,6 +482,7 @@ impl<'a> Parser<'a> {
         self.index = 0;
         self.failed_prefix_positions.clear();
         self.failed_reserved_word_prefix_positions.clear();
+        self.failed_derived_table_factor_positions.clear();
         self
     }
 
@@ -16172,9 +16178,26 @@ impl<'a> Parser<'a> {
             // `parse_derived_table_factor` below will return success after parsing the
             // subquery, followed by the closing ')', and the alias of the derived table.
             // In the example above this is case (3).
-            if let Some(mut table) =
-                self.maybe_parse(|parser| parser.parse_derived_table_factor(NotLateral))?
+            //
+            // Memoize failures to break the 2^N work on inputs like
+            // `FROM ((((...`, where the nested-join fallback recurses back into
+            // `parse_table_factor` and re-attempts the same speculative parse.
+            let derived_pos = self.index;
+            let derived = if self
+                .failed_derived_table_factor_positions
+                .contains(&derived_pos)
             {
+                None
+            } else {
+                match self.maybe_parse(|parser| parser.parse_derived_table_factor(NotLateral))? {
+                    Some(t) => Some(t),
+                    None => {
+                        self.failed_derived_table_factor_positions.insert(derived_pos);
+                        None
+                    }
+                }
+            };
+            if let Some(mut table) = derived {
                 while let Some(kw) = self.parse_one_of_keywords(&[Keyword::PIVOT, Keyword::UNPIVOT])
                 {
                     table = match kw {

tests/sqlparser_common.rs

@@ -19111,3 +19111,34 @@ fn parse_prefix_case_chain_no_exponential_blowup() {
     rx.recv_timeout(Duration::from_secs(5))
         .expect("parser should reject this quickly, not loop exponentially");
 }
+
+/// Regression test for the 2^N parse-time blowup in `parse_table_factor` on
+/// inputs like `SELECT 1 FROM ((((...`. The speculative derived-table arm
+/// and the nested-join fallback both recurse through the remaining paren
+/// chain, doubling work per level. Post-fix the per-position failure cache
+/// short-circuits the second descent.
+#[test]
+fn parse_table_factor_paren_chain_no_exponential_blowup() {
+    use std::sync::mpsc;
+    use std::thread;
+    use std::time::Duration;
+
+    let mut sql = String::from("SELECT 1 ");
+    for _ in 0..5 {
+        sql.push_str("FROM ");
+        sql.push_str(&"(".repeat(30));
+        sql.push(' ');
+    }
+
+    let (tx, rx) = mpsc::channel();
+    thread::spawn(move || {
+        let _ = Parser::new(&GenericDialect {})
+            .with_recursion_limit(256)
+            .try_with_sql(&sql)
+            .and_then(|mut p| p.parse_statements());
+        let _ = tx.send(());
+    });
+
+    rx.recv_timeout(Duration::from_secs(5))
+        .expect("parser should reject this quickly, not loop exponentially");
+}