The BEGIN statement initiates a transaction, which either successfully executes all of the statements it contains or none at all.
Synopsis
Required Privileges
No privileges are required to initiate a transaction. However, privileges are required for each statement within a transaction.
Aliases
In CockroachDB, the following are aliases for the BEGIN statement:
BEGIN TRANSACTIONSTART TRANSACTION
The following aliases also exist for isolation levels:
READ UNCOMMITTED,READ COMMITTED, andREPEATABLE READare aliases forSERIALIZABLE
For more information on isolation level aliases, see Comparison to ANSI SQL Isolation Levels.
Parameters
| Parameter | Description |
|---|---|
ISOLATION LEVEL |
By default, transactions in CockroachDB implement the strongest ANSI isolation level: SERIALIZABLE. At this isolation level, transactions will never result in anomalies. The SNAPSHOT isolation level is still supported as well for backwards compatibility, but you should avoid using it. It provides little benefit in terms of performance and can result in inconsistent state under certain complex workloads. For more information, see Transactions: Isolation Levels.Default: SERIALIZABLE |
PRIORITY |
If you do not want the transaction to run with NORMAL priority, you can set it to LOW or HIGH.Transactions with higher priority are less likely to need to be retried. For more information, see Transactions: Priorities. Default: NORMAL |
Examples
Begin a Transaction
Use Default Settings
Without modifying the BEGIN statement, the transaction uses SERIALIZABLE isolation and NORMAL priority.
> BEGIN;
> SAVEPOINT cockroach_restart;
> UPDATE products SET inventory = 0 WHERE sku = '8675309';
> INSERT INTO orders (customer, sku, status) VALUES (1001, '8675309', 'new');
> RELEASE SAVEPOINT cockroach_restart;
> COMMIT;
Change Isolation Level & Priority
You can set a transaction's isolation level to SNAPSHOT, as well as its priority to LOW or HIGH.
> BEGIN ISOLATION LEVEL SNAPSHOT, PRIORITY HIGH;
> SAVEPOINT cockroach_restart;
> UPDATE products SET inventory = 0 WHERE sku = '8675309';
> INSERT INTO orders (customer, sku, status) VALUES (1001, '8675309', 'new');
> RELEASE SAVEPOINT cockroach_restart;
> COMMIT;
You can also set a transaction's isolation level and priority with SET TRANSACTION.
Begin a Transaction with Automatic Retries
CockroachDB will automatically retry all transactions that contain both BEGIN and COMMIT in the same batch. Batching is controlled by your driver or client's behavior, but means that CockroachDB receives all of the statements as a single unit, instead of a number of requests.
From the perspective of CockroachDB, a transaction sent as a batch looks like this:
> BEGIN; DELETE FROM customers WHERE id = 1; DELETE orders WHERE customer = 1; COMMIT;
However, in your application's code, batched transactions are often just multiple statements sent at once. For example, in Go, this transaction would sent as a single batch (and automatically retried):
db.Exec(
"BEGIN;
DELETE FROM customers WHERE id = 1;
DELETE orders WHERE customer = 1;
COMMIT;"
)
Issuing statements this way signals to CockroachDB that you do not need to change any of the statement's values if the transaction doesn't immediately succeed, so it can continually retry the transaction until it's accepted.
See Also
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