Dimitry Sibiryakov,
вот ты упертый. Ясное дело, что в стандарте снапшота нет. Но про снапшот уже давно говорят в отношении СУБД, и он именно то, что в ИБ и ФБ concurrency = SNAPSHOT.
https://en.wikipedia.org/wiki/Snapshot_isolation
Snapshot isolation has been adopted by several major database management systems, such as SQL Anywhere, InterBase ,
Firebird , Oracle, PostgreSQL, MongoDB[1] and Microsoft SQL Server (2005 and later). The main reason for its adoption is that it allows better performance than serializability, yet still avoids most of the concurrency anomalies that serializability avoids (but not always all). In practice snapshot isolation is implemented within multiversion concurrency control (MVCC), where generational values of each data item (versions) are maintained: MVCC is a common way to increase concurrency and performance by generating a new version of a database object each time the object is written, and allowing transactions' read operations of several last relevant versions (of each object). Snapshot isolation has been used[2] to criticize the ANSI SQL-92 standard's definition of isolation levels, as it exhibits none of the "anomalies" that the SQL standard prohibited, yet is not serializable (the anomaly-free isolation level defined by ANSI).
