It is reading "in progress" data, which may not be complete, and may never actually be committed. So, while the Non-Repeatable Read applies to a single row, the Phantom Read is about a range of records which satisfy a given query filtering criteria.
It happens when the DELETE statement unfortunately, removes the very same row which your current transaction was querying.
But this is a rare case, and far more unlikely to occur in a database which have millions of rows in each table. Tables containing transaction data usually have high data volume in any production environment. Actually we need to analyze the use case and decide an isolation level so that we optimize the transaction time and also prevent most anomalies.
Admins and architects may have an affinity towards choosing this setting as default, to exhibit better performance of the platform. There is a difference in the implementation between these two kinds isolation levels. For "non-repeatable read", row-locking is needed. We can implement these two levels by using two-phase-locking protocol. In a system with non-repeatable reads, the result of Transaction A's second query will reflect the update in Transaction B - it will see the new amount.
The accepted answer indicates most of all that the so-called distinction between the two is actually not significant at all. For example, suppose transaction 1 updates a row. Transaction 2 reads the updated row before transaction 1 commits the update. If transaction 1 rolls back the change, transaction 2 will have read data that is considered never to have existed.
Nonrepeatable Reads A nonrepeatable read occurs when a transaction reads the same row twice but gets different data each time.
For example, suppose transaction 1 reads a row. Transaction 2 updates or deletes that row and commits the update or delete. If transaction 1 rereads the row, it retrieves different row values or discovers that the row has been deleted. Phantoms A phantom is a row that matches the search criteria but is not initially seen. For example, suppose transaction 1 reads a set of rows that satisfy some search criteria.
Transaction 2 generates a new row through either an update or an insert that matches the search criteria for transaction 1. If transaction 1 reexecutes the statement that reads the rows, it gets a different set of rows.
The four transaction isolation levels as defined by SQL are defined in terms of these phenomena. Learn how your comment data is processed. The opinions expressed here represent my own and not those of my employer. My employer do not endorse any tools, applications, books, or concepts mentioned on the blog.
I have documented my personal experience on this blog. Email Address:. Like this: Like Loading Comments 11 Trackbacks 3 Leave a comment Trackback. Sileet Shafique. Eric Vanderburg evanderburg. You might need to update this with newer isolation methods such as snapshot isolation. Easy to Understand.. Explain useFocusEffect hook in React-native. What are the Class component lifecycle methods in React? Mention the differences between redux persist and redux offline packages if you have used it.
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