Limitations of Zend_Db_Table::save on missing primary key

Goal

Determine how Zend Framework 2’s Zend_Db_Table::save reacts when a caller supplies a primary‑key value that does not match any existing row, and whether retrying the same operation can create duplicate records.

Constraints & Uncertainty

• Framework 2 documentation specifies that save performs an INSERT if the primary‑key is null or not found, otherwise an UPDATE.
• The behavior for a supplied key that is missing is not explicitly covered, and it varies across adapters (MySQL, PostgreSQL).
• Retries execute the same SQL again; without a transaction that checks existence first, an auto‑increment key may generate a duplicate row.
• Zend_Db_Table offers transaction APIs but no built‑in idempotent retry or optimistic locking.

Specific Questions

• When a primary‑key value is supplied but the corresponding row does not exist, does save perform an INSERT or throw an exception?
• During a retry, how does save decide whether to re‑execute an INSERT or an UPDATE, and can this lead to duplicate rows?
• What documented best practices or patterns can be adopted to prevent duplicate writes in retry scenarios within Zend Framework 2?