From 6544625175c2065bde112a507d7735b6c18e2028 Mon Sep 17 00:00:00 2001
From: arc-oai <249853679+arc-oai@users.noreply.github.com>
Date: Fri, 7 Aug 2026 16:50:41 -0700
Subject: [PATCH] Avoid quadratic SQL expression list accumulation

---
 src/parser/grammar.ne | 61 +++++++++++++++++++++++++++++++++----------
 test/perftest.ts      |  2 +-
 2 files changed, 48 insertions(+), 15 deletions(-)

diff --git a/src/parser/grammar.ne b/src/parser/grammar.ne
index 6b4cef1487..e97af95121 100644
--- a/src/parser/grammar.ne
+++ b/src/parser/grammar.ne
@@ -34,6 +34,28 @@ interface CommentAttachments {
   trailing?: CommentNode[];
 }
 
+interface ExpressionList {
+  previous: ExpressionList | null;
+  value: AstNode;
+}
+
+interface PendingExpressions {
+  expressions: ExpressionList | null;
+  clauses: AstNode[];
+}
+
+const materializeExpressionList = (expressions: ExpressionList | null): AstNode[] => {
+  const result: AstNode[] = [];
+  for (let current = expressions; current !== null; current = current.previous) {
+    result.push(current.value);
+  }
+  result.reverse();
+  return result;
+};
+
+const materializeExpressions = ({ expressions, clauses }: PendingExpressions): AstNode[] =>
+  materializeExpressionList(expressions).concat(clauses);
+
 const addComments = (node: AstNode, { leading, trailing }: CommentAttachments): AstNode => {
   if (leading?.length) {
     node = { ...node, leadingComments: leading };
@@ -83,14 +105,19 @@ main -> statement:* {%
 statement -> expressions_or_clauses (%DELIMITER | %EOF) {%
   ([children, [delimiter]]) => ({
     type: NodeType.statement,
-    children,
+    children: materializeExpressions(children),
     hasSemicolon: delimiter.type === TokenType.DELIMITER,
   })
 %}
 
 # To avoid ambiguity, plain expressions can only come before clauses
-expressions_or_clauses -> free_form_sql:* clause:* {%
-  ([expressions, clauses]) => [...expressions, ...clauses]
+expressions_or_clauses -> expression_list clause:* {%
+  ([expressions, clauses]) => ({ expressions, clauses })
+%}
+
+expression_list -> null {% () => null %}
+expression_list -> expression_list free_form_sql {%
+  ([previous, value]) => ({ previous, value })
 %}
 
 clause ->
@@ -119,11 +146,13 @@ limit_clause -> %LIMIT _ expression_chain_ (%COMMA free_form_sql:+):? {%
   }
 %}
 
-select_clause -> %RESERVED_SELECT (all_columns_asterisk free_form_sql:* | asteriskless_free_form_sql free_form_sql:*) {%
+select_clause -> %RESERVED_SELECT (all_columns_asterisk expression_list | asteriskless_free_form_sql expression_list) {%
   ([nameToken, [exp, expressions]]) => ({
     type: NodeType.clause,
     nameKw: toKeywordNode(nameToken),
-    children: [exp, ...expressions],
+    get children(): AstNode[] {
+      return [exp, ...materializeExpressionList(expressions)];
+    },
   })
 %}
 select_clause -> %RESERVED_SELECT {%
@@ -138,19 +167,23 @@ all_columns_asterisk -> %ASTERISK {%
   () => ({ type: NodeType.all_columns_asterisk })
 %}
 
-other_clause -> %RESERVED_CLAUSE free_form_sql:* {%
+other_clause -> %RESERVED_CLAUSE expression_list {%
   ([nameToken, children]) => ({
     type: NodeType.clause,
     nameKw: toKeywordNode(nameToken),
-    children,
+    get children(): AstNode[] {
+      return materializeExpressionList(children);
+    },
   })
 %}
 
-set_operation -> %RESERVED_SET_OPERATION free_form_sql:* {%
+set_operation -> %RESERVED_SET_OPERATION expression_list {%
   ([nameToken, children]) => ({
     type: NodeType.set_operation,
     nameKw: toKeywordNode(nameToken),
-    children,
+    get children(): AstNode[] {
+      return materializeExpressionList(children);
+    },
   })
 %}
 
@@ -232,25 +265,25 @@ function_call -> %RESERVED_FUNCTION_NAME _ parenthesis {%
 parenthesis -> "(" expressions_or_clauses ")" {%
   ([open, children, close]) => ({
     type: NodeType.parenthesis,
-    children: children,
+    children: materializeExpressions(children),
     openParen: "(",
     closeParen: ")",
   })
 %}
 
-curly_braces -> "{" free_form_sql:* "}" {%
+curly_braces -> "{" expression_list "}" {%
   ([open, children, close]) => ({
     type: NodeType.parenthesis,
-    children: children,
+    children: materializeExpressionList(children),
     openParen: "{",
     closeParen: "}",
   })
 %}
 
-square_brackets -> "[" free_form_sql:* "]" {%
+square_brackets -> "[" expression_list "]" {%
   ([open, children, close]) => ({
     type: NodeType.parenthesis,
-    children: children,
+    children: materializeExpressionList(children),
     openParen: "[",
     closeParen: "]",
   })
diff --git a/test/perftest.ts b/test/perftest.ts
index ccd64fb5a7..d814683286 100644
--- a/test/perftest.ts
+++ b/test/perftest.ts
@@ -10,7 +10,7 @@ describe('Performance test', () => {
   });
 
   // Issue #840
-  it.skip('should use less than 100 MB of additional memory to format ~100 KB of SQL', () => {
+  it('should use less than 100 MB of additional memory to format ~100 KB of SQL', () => {
     // Long list of values
     const values = Array(10000).fill('myid');
     const sql = `SELECT ${values.join(', ')}`;