fix(db): compile filter expression once instead of on every invocation (#954)

* fix(db): compile filter expression once instead of on every invocation

Previously, `createFilterFunctionFromExpression` was calling
`compileSingleRowExpression(expression)` inside the returned filter
function, causing the expression to be recompiled for every item
being filtered. This resulted in severe performance degradation when
filtering large collections, as `normalizeValue` and other compilation
functions were called thousands of times unnecessarily.

Move the compilation outside the returned function so it happens once
when the filter function is created.

* chore: add changeset for filter expression compile fix

---------

Co-authored-by: Claude <[email protected]>

Author: KyleAMathews

.changeset/fix-filter-expression-compile.md

@@ -0,0 +1,7 @@
+---
+"@tanstack/db": patch
+---
+
+fix(db): compile filter expression once in createFilterFunctionFromExpression
+
+Fixed a performance issue in `createFilterFunctionFromExpression` where the expression was being recompiled on every filter call. This only affected realtime change event filtering for pushed-down predicates at the collection level when using orderBy + limit. The core query engine was not affected as it already compiled predicates once.

packages/db/src/collection/change-events.ts

@@ -221,9 +221,11 @@ export function createFilterFunction<T extends object>(
 export function createFilterFunctionFromExpression<T extends object>(
   expression: BasicExpression<boolean>
 ): (item: T) => boolean {
+  // Compile expression once when filter function is created, not on every invocation
+  const evaluator = compileSingleRowExpression(expression)
+
   return (item: T): boolean => {
     try {
-      const evaluator = compileSingleRowExpression(expression)
       const result = evaluator(item as Record<string, unknown>)
       return toBooleanPredicate(result)
     } catch {