Magento 2 DB Performance Patterns

Query Reduction Summary

Operation	Before	After
DELETE in loop	N queries	1 query
UPDATE same value in loop	N queries	1 per group
CSV multiselect UPDATE	N SELECTs + N UPDATEs	1 UPDATE per duplicate ID

---

Pattern 1 — Bulk DELETE (IN clause)

Anti-pattern: $connection->delete() called inside a foreach loop.

// BAD — N queries
foreach ($rows as $row) {
    $connection->delete($table, ['id = ?' => $row['id']]);
}

Fix: Collect IDs first, then delete once after the loop.

// GOOD — 1 query
$ids = [];
foreach ($rows as $row) {
    // ... process ...
    $ids[] = $row['id'];
}
if (!empty($ids)) {
    $connection->delete($table, ['id IN (?)' => $ids]);
}

When to apply: Any loop that deletes rows by a scalar ID or key value.

---

Pattern 2 — Bulk UPDATE same value (IN clause)

Anti-pattern: Calling $connection->update() per row when all rows get the same new value (e.g. merging duplicates to a canonical value).

// BAD — N queries
foreach ($duplicates as $dupId) {
    $connection->update($table, ['value' => $canonical], ['value = ?' => $dupId]);
}

Fix: Collect all duplicate values and update in one shot.

// GOOD — 1 query per group
$connection->update(
    $table,
    ['value' => $canonical],
    ['value IN (?)' => $duplicates]
);

When to apply: Multiple rows need to be set to the same new value (e.g. merging EAV option IDs, normalising foreign keys after deduplication).

---

Pattern 3 — SQL-side CSV replacement (FIND_IN_SET + REPLACE)

Magento stores multiselect EAV values as comma-separated IDs in catalog_product_entity_varchar (e.g. "12,45,78"). Replacing one ID requires touching every row that contains it.

Anti-pattern: PHP fetch-loop-update.

// BAD — N SELECTs + N UPDATEs
$rows = $connection->fetchAll("SELECT value_id, value FROM $table WHERE FIND_IN_SET($dupId, value)");
foreach ($rows as $row) {
    $parts = array_map('intval', explode(',', $row['value']));
    $parts = array_map(fn($v) => $v === $dupId ? $canonicalId : $v, $parts);
    $parts = array_unique($parts);
    $connection->update($table, ['value' => implode(',', $parts)], ['value_id = ?' => $row['value_id']]);
}

Fix: Single UPDATE with FIND_IN_SET condition and Zend_Db_Expr REPLACE/TRIM expression.

// GOOD — 1 query
$affected = $connection->update(
    $table,
    [
        'value' => new \Zend_Db_Expr(
            "TRIM(',' FROM REPLACE(
                REPLACE(
                    CONCAT(',', value, ','),
                    ',$duplicateId,',
                    ',$canonicalId,'
                ),
                ',$canonicalId,$canonicalId,',
                ',$canonicalId,'
            ))"
        )
    ],
    ["FIND_IN_SET($duplicateId, value) > 0"]
);

How it works:

1. CONCAT(',', value, ',') — wrap value in commas so every element (including first and last) has a delimiter on both sides.
2. First REPLACE — swap ,{duplicate}, → ,{canonical},.
3. Second REPLACE — collapse accidental double canonical ,$canonical,$canonical, → ,$canonical, (handles the case where canonical was already present in the row).
4. TRIM(',') — strip the leading/trailing commas added in step 1.

When to apply: Multiselect EAV values stored as comma-separated IDs in catalog_product_entity_varchar (or any CSV-in-varchar column). Also applicable to catalog_product_entity_text if the same storage pattern is used.

Edge case: If $canonicalId was already in the value before the replacement, step 3 prevents storing the canonical twice.

---

General Rules

• Always collect IDs/values into an array before any DB call; never query inside a tight loop.
• Use $connection->delete($table, ['col IN (?)' => $array]) — the ? placeholder handles array binding automatically in Magento's Zend_Db_Adapter.
• Use new \Zend_Db_Expr(...) for raw SQL expressions in update $bind arrays.
• Use FIND_IN_SET(needle, column) in WHERE conditions — it is index-unfriendly but acceptable for batch migration scripts run once.
• Always check !empty($ids) before issuing a bulk DELETE or UPDATE to avoid accidental full-table operations.
• Log both to $output->writeln() (CLI) and $this->logger->info() (file) for auditability.