Definition relationship between Read Consistency in Oracle and Consistency in database ACID

Question

Definition for Consistency:

In database systems, a consistent transaction is one that starts with a database in a consistent state and ends with the database in a consistent state. Any data written to the database must be valid according to all defined rules, including but not limited to constraints, cascades, triggers, and any combination thereof.

Definition for Read Consistency:

Oracle always enforces statement-level read consistency. This guarantees that all the data returned by a single query comes from a single point in time—the time that the query began. Therefore, a query never sees dirty data or any of the changes made by transactions that commit during query execution.

I've got puzzled, it seems that Read Consistency is kind of Isolation but not Consistency. Is that true?

Answer 1

That's right, but the concepts are related and the verbage maybe mixed up a bit.

"Consistency" (as in ACID) means that when you update the database, you cannot put it into an inconsistent state (the database will enforce that all constraints are met).

"Read Consistency" is one of the "Transaction Isolation" levels that describe how good concurrent transactions are isolated from each-other (i.e. in how far they can treat the database as if only they work on it).

Okay, let me rephrase that. The four transaction isolation levels describe what happens when you run multiple queries in the same transaction. So that is a potentially much stronger consistency.

The "Read Consistency" you mentioned is about a single query returning results that are "internally consistent": They represent data that existed at one point in time (when the query started). You won't get weird results for long-running queries.

I think this is a bit stronger of a guarantee than READ COMMITTED (you don't see anything that is not yet committed -- no dirty reads -- but you also don't see anything that has been committed after your query started).

But it does not imply that you get the same results when you run the same query in the same transaction five minutes later: you might see data that has been updated (and committed!) in the mean-time. If you don't want that, you need REPEATABLE READ.

As far as I know, READ UNCOMMITTED is not available in Oracle. That is probably what they mean by "Oracle always enforces statement-level read consistency".