SET TRANSACTION v17

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Name

SET TRANSACTION — Set the characteristics of the current transaction.

Synopsis

SET TRANSACTION transaction_mode

Where transaction_mode is one of:

ISOLATION LEVEL { SERIALIZABLE | READ COMMITTED }
READ WRITE | READ ONLY

Description

The SET TRANSACTION command sets the characteristics of the current transaction. It has no effect on any later transactions. The available transaction characteristics are the transaction isolation level and the transaction access mode (read/write or read-only). The isolation level of a transaction determines the data the transaction can see when other transactions are running concurrently.

READ COMMITTED

A statement can see only rows committed before it began. This is the default.

SERIALIZABLE

All statements of the current transaction can see only rows committed before the first query or data-modification statement was executed in this transaction.

You can't change the transaction isolation level after the first query or data-modification statement (SELECT, INSERT, DELETE, UPDATE, or FETCH) of a transaction is executed. The transaction access mode determines whether the transaction is read/write or read-only. Read/write is the default.

When a transaction is read-only, the following SQL commands aren't allowed:

  • INSERT, UPDATE, and DELETE if the table they write to isn't a temporary table
  • All CREATE, ALTER, and DROP commands
  • COMMENT, GRANT, REVOKE, TRUNCATE
  • EXECUTE if the command it executes is among those listed.

This is a high-level notion of read-only that doesn't prevent all writes to disk.

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