Decisions based on an outdated premise
When we previously discussed write skew in snapshot isolation (see “Write Skew and Phantoms” on page 246), we observed a recurring pattern: a transaction reads some data from the database, examines the result of the query, and decides to take some action (write to the database) based on the result that it saw. However, under snapshot isolation, the result from the original query may no longer be up-to-date by the time the transaction commits, because the data may have been modified in the meantime.
Put another way, the transaction is taking an action based on a premise (a fact that was true at the beginning of the transaction, e.g., “There are currently two doctors on call”). Later, when the transaction wants to commit, the original data may have changed—the premise may no longer be true.
When the application makes a query (e.g., “How many doctors are currently on call?”), the database doesn’t know how the application logic uses the result of that query. To be safe, the database needs to assume that any change in the query result (the premise) means that writes in that transaction may be invalid. In other words, there may be a causal dependency between the queries and the writes in the transaction. In order to provide serializable isolation, the database must detect situations in which a transaction may have acted on an outdated premise and abort the transaction in that case.
How does the database know if a query result might have changed? There are two cases to consider:
- • Detecting reads of a stale MVCC object version (uncommitted write occurred before the read)
- • Detecting writes that affect prior reads (the write occurs after the read)