Address code review feedback - fix spacing and add performance notes

Co-authored-by: hotlong <50353452+hotlong@users.noreply.github.com>

Author: Copilot

packages/spec/src/data/query.test.ts

@@ -1441,7 +1441,7 @@ describe('QuerySchema - Window Functions', () => {
   // Real-World Window Function Examples
   // ============================================================================
 
-  it('should accept query for top N per group (SQL: ROW_NUMBER() OVER (PARTITION BY ...))  ', () => {
+  it('should accept query for top N per group (SQL: ROW_NUMBER() OVER (PARTITION BY ...))', () => {
     const query: QueryAST = {
       object: 'product',
       fields: ['category_id', 'name', 'price'],

packages/spec/src/data/query.zod.ts

@@ -59,6 +59,12 @@ export const SortNodeSchema = z.object({
  * - **array_agg**: Aggregate values into array (SQL: ARRAY_AGG(field))
  * - **string_agg**: Concatenate values (SQL: STRING_AGG(field, delimiter))
  * 
+ * Performance Considerations:
+ * - COUNT(*) is typically faster than COUNT(field) as it doesn't check for nulls
+ * - COUNT DISTINCT may require additional memory for tracking unique values
+ * - Window aggregates (with OVER clause) can be more efficient than subqueries
+ * - Large GROUP BY operations benefit from proper indexing on grouped fields
+ * 
  * @example
  * // SQL: SELECT region, SUM(amount) FROM sales GROUP BY region
  * {