# doctrine-orm-0003 — SqlWalker::walkObjectExpression: O(fields²) partial field set check per query

**Status:** OPEN
**Severity:** MEDIUM
**Component:** `doctrine/orm` — `src/Doctrine/ORM/Query/SqlWalker.php`
**Affects:** DQL queries using partial object selects (`SELECT PARTIAL p.{id,name}`)

---

## Root Cause

`SqlWalker.php:1405` and `1445` — `walkObjectExpression()`:

```php
// DEFECTIVE — O(partialFieldSet) per field, inside foreach fieldMappings
foreach ($class->fieldMappings as $fieldName => $mapping) {
    if ($partialFieldSet && ! in_array($fieldName, $partialFieldSet, true)) {
        continue;
    }
    // ... build SQL column list
}

// And for subclasses (line 1445):
foreach ($subClass->fieldMappings as $fieldName => $mapping) {
    if (isset($mapping->inherited) || ($partialFieldSet && ! in_array($fieldName, $partialFieldSet, true))) {
        continue;
    }
```

`$partialFieldSet` is a plain PHP array of field name strings. For each field in
`$class->fieldMappings` (F fields), `in_array` scans all P entries in `$partialFieldSet`.
Total: O(F × P) per class, O(F × P × subclasses) for STI/CTI hierarchies.

Called once per query compilation (not per result row), but DQL parsing without a query
result cache repeats this on every request.

---

## Complexity Analysis

| Metric | Defective | Fixed |
|--------|-----------|-------|
| Single field check | O(partialFieldSet size P) | O(1) |
| Full entity (F fields) | O(F × P) | O(F) |
| With S subclasses (STI) | O(F × P × S) | O(F × S) |

---

## Fix

Convert `$partialFieldSet` array to a hash set before entering the loop:

```php
// FIXED — build O(1)-lookup set once, before the loops
$partialFieldIndex = $partialFieldSet ? array_flip($partialFieldSet) : null;

foreach ($class->fieldMappings as $fieldName => $mapping) {
    if ($partialFieldIndex !== null && ! isset($partialFieldIndex[$fieldName])) {
        continue;
    }
    // ...
}
```

Same fix applies at line 1445 for the subclass loop.

---

## Call Sites

| File | Line | Context |
|------|------|---------|
| `src/Query/SqlWalker.php` | 1405 | `walkObjectExpression()` — primary class fields |
| `src/Query/SqlWalker.php` | 1445 | `walkObjectExpression()` — STI/CTI subclass fields |

---

## Speedup Estimate

Queries with large partial field sets (P > 10) or large entities (F > 20) with deep STI
hierarchies (S > 10): O(P)× improvement in query compilation time on the partial-select
field filter path.