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1

I assume that X refers to the same table row everywhere. In that case, the two transactions in your picture are not serializable, and indeed a lost update would occur. Any serial execution of the transactions would produce a different result from the nested execution you are showing. To clarify the term "serializable", here is how the SQL standard defines ...


7

To long for a comment, so I'll just add this as a complement to David's solution. Even though the question was of theoretical nature, it may be of interest to see how that can be implemented in real life. In some situations, the anti-reflexive property, and the symmetric property is too strong, but we still want the "uniqueness" property to hold. One common ...


9

Are there official terms for this type of scenario? Yes. This is a Symmetric Relation. And "relation" here has the same meaning as in "Relational Database". An RDBMS is a database management system designed around storing relations. However RDBMSs don't have a native way to store symmetric relations. You either have to store both tuples, eg (a,b) and (...


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