> ## Documentation Index
> Fetch the complete documentation index at: https://cockroachlabs.mintlify.site/llms.txt
> Use this file to discover all available pages before exploring further.
# SET TRANSACTION
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The `SET TRANSACTION` statement sets the transaction priority, access mode, "as of" timestamp, and isolation level. These are applied after you `BEGIN` the transaction and before executing the first statement that manipulates a database.
Cockroach Labs recommends leaving the transaction priority at the default setting in almost all cases. Changing the transaction priority to `HIGH` in particular can lead to difficult-to-debug interactions with other transactions executing on the system.
If you are setting a transaction priority to avoid contention or hot spots, or to get better query performance, it is usually a sign that you need to update your schema design and/or review the data access patterns of your workload.
## Synopsis
## Required privileges
No privileges are required to set the transaction priority. However, privileges are required for each statement within a 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. The current priority is also exposed as the read-only session variable`transaction\_priority`. **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 DEFERRABLE` `DEFERRABLE` | 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. |
| `ISOLATION LEVEL` | Set the transaction isolation level. Transactions use `SERIALIZABLE` isolation by default. They can be configured to run at `READ COMMITTED` isolation. This clause only takes effect if specified at the beginning of the transaction. |
## Examples
### Set isolation level
You can set the transaction isolation level to `SERIALIZABLE` or `READ COMMITTED`.
If not specified, transactions use the value of the current session's `default_transaction_isolation` variable.
```sql theme={"theme":{"light":"catppuccin-mocha","dark":"catppuccin-mocha"}}
BEGIN;
```
```sql theme={"theme":{"light":"catppuccin-mocha","dark":"catppuccin-mocha"}}
SET TRANSACTION ISOLATION LEVEL READ COMMITTED;
```
### Set priority
This example assumes you're using client-side retry handling.
```sql theme={"theme":{"light":"catppuccin-mocha","dark":"catppuccin-mocha"}}
> BEGIN;
```
```sql theme={"theme":{"light":"catppuccin-mocha","dark":"catppuccin-mocha"}}
> SET TRANSACTION PRIORITY HIGH;
```
```sql theme={"theme":{"light":"catppuccin-mocha","dark":"catppuccin-mocha"}}
> SAVEPOINT cockroach_restart;
```
```sql theme={"theme":{"light":"catppuccin-mocha","dark":"catppuccin-mocha"}}
> UPDATE products SET inventory = 0 WHERE sku = '8675309';
```
```sql theme={"theme":{"light":"catppuccin-mocha","dark":"catppuccin-mocha"}}
> INSERT INTO orders (customer, sku, status) VALUES (1001, '8675309', 'new');
```
```sql theme={"theme":{"light":"catppuccin-mocha","dark":"catppuccin-mocha"}}
> RELEASE SAVEPOINT cockroach_restart;
```
```sql theme={"theme":{"light":"catppuccin-mocha","dark":"catppuccin-mocha"}}
> COMMIT;
```
### Use the `AS OF SYSTEM TIME` option
You can execute the transaction using the database contents "as of" a specified time in the past.
```sql theme={"theme":{"light":"catppuccin-mocha","dark":"catppuccin-mocha"}}
> BEGIN;
```
```sql theme={"theme":{"light":"catppuccin-mocha","dark":"catppuccin-mocha"}}
> SET TRANSACTION AS OF SYSTEM TIME '2019-04-09 18:02:52.0+00:00';
```
```sql theme={"theme":{"light":"catppuccin-mocha","dark":"catppuccin-mocha"}}
> SELECT * FROM orders;
```
```sql theme={"theme":{"light":"catppuccin-mocha","dark":"catppuccin-mocha"}}
> SELECT * FROM products;
```
```sql theme={"theme":{"light":"catppuccin-mocha","dark":"catppuccin-mocha"}}
> COMMIT;
```
### Set the default transaction priority for a session
To set the default transaction priority for all transactions in a session, use the `default_transaction_priority` session variable. For example:
```sql theme={"theme":{"light":"catppuccin-mocha","dark":"catppuccin-mocha"}}
SET default_transaction_priority = 'high';
```
```sql theme={"theme":{"light":"catppuccin-mocha","dark":"catppuccin-mocha"}}
SHOW transaction_priority;
```
```text theme={"theme":{"light":"catppuccin-mocha","dark":"catppuccin-mocha"}}
transaction_priority
------------------------
high
```
Note that `transaction_priority` is a read-only session variable that cannot be set directly.
## See also
* `SET`
* Transactions: Priority levels
* `BEGIN`
* `COMMIT`
* `SAVEPOINT`
* `RELEASE SAVEPOINT`
* `ROLLBACK`