Negative stock and lost updates in a Postgres transaction

<p>Stock goes negative even though the code is wrapped in a transaction. Or a stocktake finds less physical stock than the system shows. Nothing appears in the error log. Tests that process one order at a time in sequence never show the symptom.</p> <p>I measured two patterns that cause this symptom under PostgreSQL 16's default isolation level (Read Committed).<sup id="fnref1">1</sup> The symptom

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