CODEWITHSUNDEEP
sqloracle-database
user3444106
user3444106

Reputation: 29

Differences between TRANSACTION's levels: READ WRITE and ISOLATION LEVEL SERIALIZABLE

What's the differences between these two transaction's levels: READ WRITE and ISOLATION LEVEL SERIALIZABLE ? As I understand, READ WRITE allows dirty reads, while ISOLATION LEVEL SERIALIZABLE prevents data from changing by other users(think that I'm mistaken here) or just read that data that is available at the beginning of the transaction(don't see the data, that has been changed by other users during my transaction).

Upvotes: 1

Views: 1083

Answers (1)

Ulrich Thomas Gabor
Ulrich Thomas Gabor

Reputation: 6654

You can find detailed information about this topic on the oracle site.

Basically READ COMMITTED allows "nonrepeatable reads" and "phantom reads", while both are prohibited in SERIALIZABLE.

If non-repeatable reads are permitted, the same SELECT query inside of the same transaction, might return different results based on when the query is issued. Other parallel transactions may change the data and this changes might become visible inside your transaction.

If phantom reads are permitted, it can happen that when you issue the same SELECT query twice inside of one transaction, and another transactions inserts rows into the table in parallel, these rows might become visible inside of your transaction, but only in the resultset of the second select. So the same select statement will return for example 5 rows the first time and 10 rows the second time it was executed.

Both properties are similar, but the first only says something about data which may change, while the scond property says something about additional rows which might be returned.

Upvotes: 1

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