---
title: "BEGIN"
id: sql-begin
pg_version: "20devel"
---

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# 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
```

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## 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`](sql-commit.md) or [`ROLLBACK`](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`](sql-set-transaction.md) was executed. 

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## Parameters

**`WORK`**, **`TRANSACTION`**

 Optional key words. They have no effect. Refer to [SET TRANSACTION](sql-set-transaction.md) for information on the meaning of the other parameters to this statement. 

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## Notes

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

Use [`COMMIT`](sql-commit.md) or [`ROLLBACK`](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](sql-savepoint.md)). 

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

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## Examples

To begin a transaction block: 
BEGIN;


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## Compatibility

`BEGIN` is a PostgreSQL language extension. It is equivalent to the SQL-standard command [`START TRANSACTION`](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. 

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## See Also

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