SET TRANSACTION(7) SQL Commands SET TRANSACTION(7)NAME
SET TRANSACTION - set the characteristics of the current transaction
SYNOPSIS
SET TRANSACTION ISOLATION LEVEL { READ COMMITTED | SERIALIZABLE }
SET SESSION CHARACTERISTICS AS TRANSACTION ISOLATION LEVEL
{ READ COMMITTED | SERIALIZABLE }
DESCRIPTION
This command sets the transaction isolation level. The SET TRANSACTION command sets the characteristics for the current SQL-transaction. It
has no effect on any subsequent transactions. This command cannot be used after the first query or data-modification statement (SELECT,
INSERT, DELETE, UPDATE, FETCH, COPY) of a transaction has been executed. SET SESSION CHARACTERISTICS sets the default transaction isolation
level for each transaction for a session. SET TRANSACTION can override it for an individual transaction.
The isolation level of a transaction determines what data the transaction can see when other transactions are running concurrently.
READ COMMITTED
A statement can only see rows committed before it began. This is the default.
SERIALIZABLE
The current transaction can only see rows committed before first query or data-modification statement was executed in this transac-
tion.
Tip: Intuitively, serializable means that two concurrent transactions will leave the database in the same state as if the two has
been executed strictly after one another in either order.
NOTES
The session default transaction isolation level can also be set with the command
SET default_transaction_isolation = 'value'
and in the configuration file. Consult the Administrator's Guide for more information.
COMPATIBILITY
SQL92, SQL99
SERIALIZABLE is the default level in SQL. PostgreSQL does not provide the isolation levels READ UNCOMMITTED and REPEATABLE READ. Because of
multiversion concurrency control, the SERIALIZABLE level is not truly serializable. See the User's Guide for details.
In SQL there are two other transaction characteristics that can be set with these commands: whether the transaction is read-only and the
size of the diagnostics area. Neither of these concepts are supported in PostgreSQL.
SQL - Language Statements 2000-11-24 SET TRANSACTION(7)
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SET TRANSACTION(7) SQL Commands SET TRANSACTION(7)NAME
SET TRANSACTION - set the characteristics of the current transaction
SYNOPSIS
SET TRANSACTION transaction_mode [, ...]
SET SESSION CHARACTERISTICS AS TRANSACTION transaction_mode [, ...]
where transaction_mode is one of:
ISOLATION LEVEL { SERIALIZABLE | REPEATABLE READ | READ COMMITTED | READ UNCOMMITTED }
READ WRITE | READ ONLY
DESCRIPTION
The SET TRANSACTION command sets the characteristics of the current transaction. It has no effect on any subsequent transactions. SET SES-
SION CHARACTERISTICS sets the default transaction characteristics for subsequent transactions of a session. These defaults can be overrid-
den by SET TRANSACTION for an individual transaction.
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 what data the transaction can see when other transactions are running concurrently:
READ COMMITTED
A statement can only see rows committed before it began. This is the default.
SERIALIZABLE
All statements of the current transaction can only see rows committed before the first query or data-modification statement was exe-
cuted in this transaction.
The SQL standard defines two additional levels, READ UNCOMMITTED and REPEATABLE READ. In PostgreSQL READ UNCOMMITTED is treated as READ
COMMITTED, while REPEATABLE READ is treated as SERIALIZABLE.
The transaction isolation level cannot be changed after the first query or data-modification statement (SELECT, INSERT, DELETE, UPDATE,
FETCH, or COPY) of a transaction has been executed. See in the documentation for more information about transaction isolation and concur-
rency control.
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 are disallowed: INSERT, UPDATE, DELETE, and COPY FROM if the table they would write to is not a tem-
porary table; all CREATE, ALTER, and DROP commands; COMMENT, GRANT, REVOKE, TRUNCATE; and EXPLAIN ANALYZE and EXECUTE if the command they
would execute is among those listed. This is a high-level notion of read-only that does not prevent all writes to disk.
NOTES
If SET TRANSACTION is executed without a prior START TRANSACTION or BEGIN, it will appear to have no effect, since the transaction will
immediately end.
It is possible to dispense with SET TRANSACTION by instead specifying the desired transaction_modes in BEGIN or START TRANSACTION.
The session default transaction modes can also be set by setting the configuration parameters default_transaction_isolation and
default_transaction_read_only. (In fact SET SESSION CHARACTERISTICS is just a verbose equivalent for setting these variables with SET.)
This means the defaults can be set in the configuration file, via ALTER DATABASE, etc. Consult in the documentation for more information.
COMPATIBILITY
Both commands are defined in the SQL standard. SERIALIZABLE is the default transaction isolation level in the standard. In PostgreSQL the
default is ordinarily READ COMMITTED, but you can change it as mentioned above. Because of lack of predicate locking, the SERIALIZABLE
level is not truly serializable. See in the documentation for details.
In the SQL standard, there is one other transaction characteristic that can be set with these commands: the size of the diagnostics area.
This concept is specific to embedded SQL, and therefore is not implemented in the PostgreSQL server.
The SQL standard requires commas between successive transaction_modes, but for historical reasons PostgreSQL allows the commas to be omit-
ted.
SQL - Language Statements 2010-05-14 SET TRANSACTION(7)