Magento + MySQL: the 3 queries I always check first

Critical queries:
1) `catalog_product_entity` joins
2) `inventory_source_item` (MSI)
3) `sales_order_grid`

In Magento, the usual culprit is an outdated `sales_order_grid`.

Quick fix:
```
bin/magento indexer:reindex sales_order_grid
```

If it doesn’t scale, consider moving grids to a separate DB or tuning indexes.

With MySQL slow query log, check:
- `SHOW PROCESSLIST` during peaks.
- Queries without indexes using `LIKE %`.
- Locks on `sales_order` from batch processes.

Magento doesn’t forgive large tables without well‑designed indexes.