set_transaction.7

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.\\" auto-generated by docbook2man-spec $Revision: 1.1.1.1 $
.TH "SET TRANSACTION" "" "2008-01-03" "SQL - Language Statements" "SQL Commands"
.SH NAME
SET TRANSACTION \- set the characteristics of the current transaction

.SH SYNOPSIS
.sp
.nf
SET TRANSACTION \fItransaction_mode\fR [, ...]
SET SESSION CHARACTERISTICS AS TRANSACTION \fItransaction_mode\fR [, ...]

where \fItransaction_mode\fR is one of:

    ISOLATION LEVEL { SERIALIZABLE | REPEATABLE READ | READ COMMITTED | READ UNCOMMITTED }
    READ WRITE | READ ONLY
.sp
.fi
.SH "DESCRIPTION"
.PP
The \fBSET TRANSACTION\fR command sets the
characteristics of the current transaction. It has no effect on any
subsequent transactions. \fBSET SESSION
CHARACTERISTICS\fR sets the default transaction
characteristics for subsequent transactions of a session. These
defaults can be overridden by \fBSET TRANSACTION\fR
for an individual transaction.
.PP
The available transaction characteristics are the transaction
isolation level and the transaction access mode (read/write or
read-only).
.PP
The isolation level of a transaction determines what data the
transaction can see when other transactions are running concurrently:
.TP
\fBREAD COMMITTED\fR
A statement can only see rows committed before it began. This
is the default.
.TP
\fBSERIALIZABLE\fR
All statements of the current transaction can only see rows committed
before the first query or data-modification statement was executed in
this transaction.
.PP
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.
.PP
The transaction isolation level cannot be changed after the first query or
data-modification statement (\fBSELECT\fR,
\fBINSERT\fR, \fBDELETE\fR,
\fBUPDATE\fR, \fBFETCH\fR, or
\fBCOPY\fR) of a transaction has been executed. See
in the documentation for more information about transaction
isolation and concurrency control.
.PP
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 temporary 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.
.SH "NOTES"
.PP
If \fBSET TRANSACTION\fR is executed without a prior
\fBSTART TRANSACTION\fR or \fBBEGIN\fR,
it will appear to have no effect, since the transaction will immediately
end.
.PP
It is possible to dispense with \fBSET TRANSACTION\fR
by instead specifying the desired \fItransaction_modes\fR in
\fBBEGIN\fR or \fBSTART TRANSACTION\fR.
.PP
The session default transaction modes can also be set by setting the
configuration parameters default_transaction_isolation
and default_transaction_read_only.
(In fact \fBSET SESSION CHARACTERISTICS\fR is just a
verbose equivalent for setting these variables with \fBSET\fR.)
This means the defaults can be set in the configuration file, via
\fBALTER DATABASE\fR, etc. Consult in the documentation
for more information.
.SH "COMPATIBILITY"
.PP
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.
.PP
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.
.PP
The SQL standard requires commas between successive \fItransaction_modes\fR, but for historical
reasons PostgreSQL allows the commas to be
omitted.