The BEGIN statement initiates a transaction, which either successfully executes all of the statements it contains or none at all.
When running under the default SERIALIZABLE isolation level, your application should use a retry loop to handle transaction retry errors that can occur under contention.
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
Parameters
| Parameter | Description |
|---|---|
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 |
READ |
Set the transaction access mode to READ ONLY or READ WRITE. The current transaction access mode is also exposed as the session variable transaction_read_only.Default: READ WRITE |
AS OF SYSTEM TIME |
Execute the transaction using the database contents "as of" a specified time in the past. The AS OF SYSTEM TIME clause can be used only when the transaction is read-only. If the transaction contains any writes, or if the READ WRITE mode is specified, an error will be returned.For more information, see AS OF SYSTEM TIME. |
NOT DEFERRABLEDEFERRABLE |
This clause is supported for compatibility with PostgreSQL. NOT DEFERRABLE is a no-op and the default behavior for CockroachDB. DEFERRABLE returns an unimplemented error. |
CockroachDB now only supports SERIALIZABLE isolation, so transactions can no longer be meaningfully set to any other ISOLATION LEVEL. In previous versions of CockroachDB, you could set transactions to SNAPSHOT isolation, but that feature has been removed.
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 priority
You can set a transaction's priority to LOW or HIGH.
> BEGIN 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 priority with SET TRANSACTION.
This example assumes you're using client-side retry handling.
Use the AS OF SYSTEM TIME option
You can execute the transaction using the database contents "as of" a specified time in the past.
> BEGIN AS OF SYSTEM TIME '2019-04-09 18:02:52.0+00:00';
> SELECT * FROM orders;
> SELECT * FROM products;
> COMMIT;
You can also use the SET TRANSACTION statement inside the transaction to achieve the same results. This syntax is easier to use from drivers and ORMs.
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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