perf(sql): clamp oversized LIMIT / FETCH (#416 onto dev)

docs/user-guide.md

@@ -46,6 +46,11 @@ High-level feature depth varies by database type. PostgreSQL, MySQL, and SQLite
 
 ### Result row caps (SQL workspaces)
 
-Preferences → **Max rows in results** caps how many rows the grid loads. For **PostgreSQL** and **SQLite**, ad-hoc `SELECT` / `WITH` / `VALUES` without an author `LIMIT` get a `LIMIT` injected before execution so the engine does not materialize an unbounded result. Queries that already include `LIMIT`, and non-SELECT statements (`INSERT`, `PRAGMA`, …), are left unchanged; the UI may still truncate the displayed grid as a fallback.
+Preferences → **Max rows in results** caps how many rows the grid loads. For **PostgreSQL** and **SQLite**, ad-hoc `SELECT` / `WITH` / `VALUES` are bounded **before** execution:
+
+- No author `LIMIT` → a `LIMIT` equal to the preference is injected.
+- Author `LIMIT` / `LIMIT ALL` / `FETCH FIRST n ROWS ONLY` larger than the preference → clamped down to the preference (OFFSET kept when present).
+
+Queries that already use a smaller `LIMIT`, and non-SELECT statements (`INSERT`, `PRAGMA`, …), are left unchanged. The UI may still truncate the displayed grid as a fallback. **MySQL** SQL workspace streams rows and stops at the cap client-side.
 
 For troubleshooting build/run issues, see the main [README.md](../README.md).

lib/core/database/sql_limit.dart

@@ -15,13 +15,25 @@ String stripLeadingWhitespaceAndLineComments(String sql) {
   }
 }
 
-/// Injects a `LIMIT` clause into a read-only query (`SELECT`, `WITH`, `VALUES`)
-/// when it does not already contain `LIMIT`.
+final _limitAll = RegExp(r'\bLIMIT\s+ALL\b', caseSensitive: false);
+final _limitCount = RegExp(
+  r'\bLIMIT\s+(\d+)(\s+OFFSET\s+\d+)?',
+  caseSensitive: false,
+);
+final _fetchFirst = RegExp(
+  r'\bFETCH\s+(?:FIRST|NEXT)\s+(\d+)\s+ROWS?\s+ONLY\b',
+  caseSensitive: false,
+);
+
+/// Injects or clamps a `LIMIT` on read-only queries (`SELECT`, `WITH`, `VALUES`).
 ///
-/// Existing `LIMIT` is left unchanged (caller may still apply a client-side
-/// display cap). Non-select statements are returned as-is.
+/// - No `LIMIT` / `FETCH … ONLY` → appends `LIMIT [limit]`.
+/// - `LIMIT ALL` → replaced with `LIMIT [limit]`.
+/// - `LIMIT n [OFFSET m]` where `n > limit` → clamped to [limit].
+/// - `FETCH FIRST/NEXT n ROWS ONLY` where `n > limit` → clamped.
+/// - Non-select statements are returned unchanged.
 ///
-/// Trailing semicolons are preserved after the injected clause.
+/// Trailing semicolons are preserved after an injected clause.
 String injectSqlLimit(String sql, int limit) {
   if (limit <= 0) return sql;
 
@@ -36,9 +48,38 @@ String injectSqlLimit(String sql, int limit) {
     return sql;
   }
 
-  // Already bounded by the author (may still exceed UI cap — see clamp issue).
-  final hasLimit = RegExp(r'\bLIMIT\b', caseSensitive: false).hasMatch(sql);
-  if (hasLimit) {
+  if (_limitAll.hasMatch(sql)) {
+    return sql.replaceFirst(_limitAll, 'LIMIT $limit');
+  }
+
+  final limitMatch = _limitCount.firstMatch(sql);
+  if (limitMatch != null) {
+    final existing = int.tryParse(limitMatch.group(1)!);
+    if (existing == null || existing <= limit) {
+      return sql;
+    }
+    final offsetPart = limitMatch.group(2) ?? '';
+    return sql.replaceFirst(
+      limitMatch.group(0)!,
+      'LIMIT $limit$offsetPart',
+    );
+  }
+
+  final fetchMatch = _fetchFirst.firstMatch(sql);
+  if (fetchMatch != null) {
+    final existing = int.tryParse(fetchMatch.group(1)!);
+    if (existing == null || existing <= limit) {
+      return sql;
+    }
+    return sql.replaceFirst(
+      fetchMatch.group(0)!,
+      'FETCH FIRST $limit ROWS ONLY',
+    );
+  }
+
+  if (RegExp(r'\bLIMIT\b', caseSensitive: false).hasMatch(sql) ||
+      RegExp(r'\bFETCH\b', caseSensitive: false).hasMatch(sql)) {
+    // Unrecognized LIMIT/FETCH shape — leave unchanged.
     return sql;
   }
 

test/core/database/postgres_sql_test.dart

@@ -95,13 +95,18 @@ void main() {
           'SELECT * FROM users\nLIMIT 5000;;');
     });
 
-    test('does not append LIMIT if LIMIT already exists', () {
+    test('does not append LIMIT if LIMIT already within cap', () {
       expect(injectSqlLimit('SELECT * FROM users LIMIT 10', 5000),
           'SELECT * FROM users LIMIT 10');
       expect(injectSqlLimit('SELECT * FROM users limit 10;', 5000),
           'SELECT * FROM users limit 10;');
     });
 
+    test('clamps oversized LIMIT', () {
+      expect(injectSqlLimit('SELECT * FROM users LIMIT 999999', 5000),
+          'SELECT * FROM users LIMIT 5000');
+    });
+
     test('does not modify non-select/non-read queries', () {
       expect(injectSqlLimit('INSERT INTO users VALUES (1)', 5000),
           'INSERT INTO users VALUES (1)');

test/core/database/sql_limit_test.dart

@@ -25,7 +25,7 @@ void main() {
       );
     });
 
-    test('does not append LIMIT if LIMIT already exists', () {
+    test('does not append LIMIT if LIMIT already exists and within cap', () {
       expect(
         injectSqlLimit('SELECT * FROM users LIMIT 10', 5000),
         'SELECT * FROM users LIMIT 10',
@@ -36,6 +36,41 @@ void main() {
       );
     });
 
+    test('clamps LIMIT larger than cap', () {
+      expect(
+        injectSqlLimit('SELECT * FROM users LIMIT 999999', 5000),
+        'SELECT * FROM users LIMIT 5000',
+      );
+      expect(
+        injectSqlLimit('SELECT * FROM users LIMIT 100000 OFFSET 20;', 1000),
+        'SELECT * FROM users LIMIT 1000 OFFSET 20;',
+      );
+    });
+
+    test('replaces LIMIT ALL with cap', () {
+      expect(
+        injectSqlLimit('SELECT * FROM users LIMIT ALL', 5000),
+        'SELECT * FROM users LIMIT 5000',
+      );
+    });
+
+    test('clamps FETCH FIRST n ROWS ONLY', () {
+      expect(
+        injectSqlLimit(
+          'SELECT * FROM users FETCH FIRST 100000 ROWS ONLY',
+          5000,
+        ),
+        'SELECT * FROM users FETCH FIRST 5000 ROWS ONLY',
+      );
+      expect(
+        injectSqlLimit(
+          'SELECT * FROM users FETCH FIRST 10 ROWS ONLY',
+          5000,
+        ),
+        'SELECT * FROM users FETCH FIRST 10 ROWS ONLY',
+      );
+    });
+
     test('does not modify non-select/non-read queries', () {
       expect(
         injectSqlLimit('INSERT INTO users VALUES (1)', 5000),