> For the complete documentation index, see [llms.txt](https://docs.postgresql.tw/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.postgresql.tw/reference/sql-commands/sql-begin.md).

# BEGIN

### BEGIN

BEGIN — start a transaction block

### Synopsis

```

BEGIN [ WORK | TRANSACTION ] [ transaction_mode [, ...] ]

where transaction_mode is one of:

    ISOLATION LEVEL { SERIALIZABLE | REPEATABLE READ | READ COMMITTED | READ UNCOMMITTED }
    READ WRITE | READ ONLY
    [ NOT ] DEFERRABLE
```

### Description

`BEGIN` initiates a transaction block, that is, all statements after a `BEGIN` command will be executed in a single transaction until an explicit [`COMMIT`](/reference/sql-commands/sql-commit.md) or [`ROLLBACK`](/reference/sql-commands/sql-rollback.md) is given. By default (without `BEGIN`), PostgreSQL executes transactions in “autocommit” mode, that is, each statement is executed in its own transaction and a commit is implicitly performed at the end of the statement (if execution was successful, otherwise a rollback is done).

Statements are executed more quickly in a transaction block, because transaction start/commit requires significant CPU and disk activity. Execution of multiple statements inside a transaction is also useful to ensure consistency when making several related changes: other sessions will be unable to see the intermediate states wherein not all the related updates have been done.

If the isolation level, read/write mode, or deferrable mode is specified, the new transaction has those characteristics, as if [`SET TRANSACTION`](/reference/sql-commands/sql-set-transaction.md) was executed.

### Parameters

`WORK`\
`TRANSACTION` : Optional key words. They have no effect.

Refer to [SET TRANSACTION](/reference/sql-commands/sql-set-transaction.md) for information on the meaning of the other parameters to this statement.

### Notes

[`START TRANSACTION`](/reference/sql-commands/sql-start-transaction.md) has the same functionality as `BEGIN`.

Use [`COMMIT`](/reference/sql-commands/sql-commit.md) or [`ROLLBACK`](/reference/sql-commands/sql-rollback.md) to terminate a transaction block.

Issuing `BEGIN` when already inside a transaction block will provoke a warning message. The state of the transaction is not affected. To nest transactions within a transaction block, use savepoints (see [SAVEPOINT](/reference/sql-commands/sql-savepoint.md)).

For reasons of backwards compatibility, the commas between successive *`transaction_modes`* can be omitted.

### Examples

To begin a transaction block:

```

BEGIN;
```

### Compatibility

`BEGIN` is a PostgreSQL language extension. It is equivalent to the SQL-standard command [`START TRANSACTION`](/reference/sql-commands/sql-start-transaction.md), whose reference page contains additional compatibility information.

The `DEFERRABLE` *`transaction_mode`* is a PostgreSQL language extension.

Incidentally, the `BEGIN` key word is used for a different purpose in embedded SQL. You are advised to be careful about the transaction semantics when porting database applications.

### See Also

[COMMIT](/reference/sql-commands/sql-commit.md), [ROLLBACK](/reference/sql-commands/sql-rollback.md), [START TRANSACTION](/reference/sql-commands/sql-start-transaction.md), [SAVEPOINT](/reference/sql-commands/sql-savepoint.md)

***

原文：[PostgreSQL 18.6 Documentation](https://www.postgresql.org/docs/18/sql-begin.html)（英文原文，待翻譯）


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