java-topology/docs/tickets/doctrine-orm-0001-hydrator-discriminator-in-array.md

doctrine-orm-0001 — AbstractHydrator: O(rows × subclasses) discriminatorValues linear scan per result row

Status: OPEN Severity: HIGH Component: doctrine/orm — src/Doctrine/ORM/Internal/Hydration/AbstractHydrator.php Affects: All queries over inheritance hierarchies (CTI/STI) that return multiple rows

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Root Cause

AbstractHydrator.php:328 — inside gatherRowData(), called once per DB result row:

// DEFECTIVE — O(subclasses) per row, per column with inheritance field collision
if (
    isset($cacheKeyInfo['discriminatorColumn'], $data[$cacheKeyInfo['discriminatorColumn']])
    && ! in_array((string) $data[$cacheKeyInfo['discriminatorColumn']], $cacheKeyInfo['discriminatorValues'], true)
) {
    break;
}

$cacheKeyInfo['discriminatorValues'] is a PHP array of discriminator string values, built in getDiscriminatorValues() (line 541–551) from $classMetadata->subClasses[]. in_array performs a full linear scan of this array.

The outer loop (foreach ($data as $key => $value)) runs once per column per result row. gatherRowData() is called per row in the while loop at AbstractHydrator.php:114:

while (true) {
    $row = $this->statement()->fetchAssociative();
    ...
    $this->hydrateRowData($row, $result);  // calls gatherRowData()

ObjectHydrator.php:327 confirms this is the hot path:

$rowData = $this->gatherRowData($row, $id, $nonemptyComponents);

The same pattern appears in SimpleObjectHydrator.php:128:

if (isset($cacheKeyInfo['discriminatorValues']) && ! in_array((string) $discrColumnValue, $cacheKeyInfo['discriminatorValues'], true)) {
    continue;
}

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Complexity Analysis

Metric	Defective	Fixed
Discriminator check per row	O(subclasses)	O(1)
Full query (N rows, C affected columns)	O(N × C × subclasses)	O(N × C)
Growth pattern	Quadratic in subclasses	Linear

For a CTI hierarchy with 50 subclasses, a 10-column SELECT returning 10,000 rows performs 50 × 10 × 10,000 = 5,000,000 comparisons for discriminator membership alone.

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Fix

getDiscriminatorValues() (line 541) builds a flat array. Change it to build a Set-equivalent (array keyed by discriminator value for O(1) isset lookup):

// FIXED — O(1) per row
private function getDiscriminatorValues(ClassMetadata $classMetadata): array
{
    $values = [];
    foreach ($classMetadata->subClasses as $subClass) {
        $val = (string) $this->getClassMetadata($subClass)->discriminatorValue;
        $values[$val] = true;
    }
    $values[(string) $classMetadata->discriminatorValue] = true;
    return $values;
}

Then change both call sites from in_array($val, $discriminatorValues, true) to isset($discriminatorValues[$val]).

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Call Sites

File	Line	Path
src/Internal/Hydration/AbstractHydrator.php	328	gatherRowData() — ObjectHydrator hot path
src/Internal/Hydration/SimpleObjectHydrator.php	128	hydrateRowData() — SimpleObjectHydrator hot path

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Speedup Estimate

For queries over CTI/STI hierarchies with many subclasses: linear in subclass count. A hierarchy with S subclasses sees S× speedup on the discriminator-check portion of hydration. Real-world inheritance hierarchies of 20–100 subclasses → 20–100× reduction in comparisons on the discriminator check path.