Which isolation level is used by default

Restrictions on Subqueries. Transactional and Locking Statements. Statements That Cause an Implicit Commit. Restrictions on XA Transactions. Replication Statements. Variables in Stored Programs. Local Variable Scope and Resolution. Flow Control Statements. Restrictions on Server-Side Cursors. Restrictions on Condition Handling. Database Administration Statements. Account Management Statements. Table Maintenance Statements.

Plugin and Loadable Function Statements. Other Administrative Statements. A transaction re-reads data it has previously read and finds that data has been modified by another transaction that committed since the initial read. A transaction re-executes a query returning a set of rows that satisfy a search condition and finds that the set of rows satisfying the condition has changed due to another recently-committed transaction.

The result of successfully committing a group of transactions is inconsistent with all possible orderings of running those transactions one at a time. In PostgreSQL , you can request any of the four standard transaction isolation levels, but internally only three distinct isolation levels are implemented, i.

This is because it is the only sensible way to map the standard isolation levels to PostgreSQL's multiversion concurrency control architecture. Stricter behavior is permitted by the SQL standard: the four isolation levels only define which phenomena must not happen, not which phenomena must happen.

The behavior of the available isolation levels is detailed in the following subsections. Some PostgreSQL data types and functions have special rules regarding transactional behavior. In particular, changes made to a sequence and therefore the counter of a column declared using serial are immediately visible to all other transactions and are not rolled back if the transaction that made the changes aborts.

See Section 9. However, SELECT does see the effects of previous updates executed within its own transaction, even though they are not yet committed. However, such a target row might have already been updated or deleted or locked by another concurrent transaction by the time it is found. In this case, the would-be updater will wait for the first updating transaction to commit or roll back if it is still in progress.

If the first updater rolls back, then its effects are negated and the second updater can proceed with updating the originally found row. If the first updater commits, the second updater will ignore the row if the first updater deleted it, otherwise it will attempt to apply its operation to the updated version of the row.

The search condition of the command the WHERE clause is re-evaluated to see if the updated version of the row still matches the search condition. If so, the second updater proceeds with its operation using the updated version of the row. In Read Committed mode, each row proposed for insertion will either insert or update. Unless there are unrelated errors, one of those two outcomes is guaranteed. If a conflict originates in another transaction whose effects are not yet visible to the INSERT , the UPDATE clause will affect that row, even though possibly no version of that row is conventionally visible to the command.

Again, this is only the case in Read Committed mode. Because of the above rules, it is possible for an updating command to see an inconsistent snapshot: it can see the effects of concurrent updating commands on the same rows it is trying to update, but it does not see effects of those commands on other rows in the database.

This behavior makes Read Committed mode unsuitable for commands that involve complex search conditions; however, it is just right for simpler cases.

For example, consider updating bank balances with transactions like:. If two such transactions concurrently try to change the balance of account , we clearly want the second transaction to start with the updated version of the account's row.

Because each command is affecting only a predetermined row, letting it see the updated version of the row does not create any troublesome inconsistency. More complex usage can produce undesirable results in Read Committed mode. For example, consider a DELETE command operating on data that is being both added and removed from its restriction criteria by another command, e.

Because Read Committed mode starts each command with a new snapshot that includes all transactions committed up to that instant, subsequent commands in the same transaction will see the effects of the committed concurrent transaction in any case. The point at issue above is whether or not a single command sees an absolutely consistent view of the database. The partial transaction isolation provided by Read Committed mode is adequate for many applications, and this mode is fast and simple to use; however, it is not sufficient for all cases.

Applications that do complex queries and updates might require a more rigorously consistent view of the database than Read Committed mode provides.

The Repeatable Read isolation level only sees data committed before the transaction began; it never sees either uncommitted data or changes committed during transaction execution by concurrent transactions. However, the query does see the effects of previous updates executed within its own transaction, even though they are not yet committed. This is a stronger guarantee than is required by the SQL standard for this isolation level, and prevents all of the phenomena described in Table As mentioned above, this is specifically allowed by the standard, which only describes the minimum protections each isolation level must provide.

This level is different from Read Committed in that a query in a repeatable read transaction sees a snapshot as of the start of the first non-transaction-control statement in the transaction , not as of the start of the current statement within the transaction. Applications using this level must be prepared to retry transactions due to serialization failures. Improve this question. Jonathan Allen Jonathan Allen If you downvote, please explain why — Remus Rusanu.

RemusRusanu: Not reading the docs first I guess. I did not downvote — Neolisk. Neolisk: Things are often more complex than they seem Just curious about the fragment of your question which specifically mentions with ADO. I believe irrespective of the provider w. Net, Python etc the default is driven by the database engine and NOT the provider. Kindly correct me if I'm wrong.

For other settings, there are differences between the defaults for ADO. So that's not really a safe assumption. Add a comment. Active Oldest Votes. BeginTransaction also states Read committed MSDN article. Improve this answer.

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