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📄 ALTER_COLLATION.7(2.8 KB)
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📄 ALTER_DATABASE.7(5.28 KB)
📄 ALTER_DEFAULT_PRIVILEGES.7(6.39 KB)
📄 ALTER_DOMAIN.7(6.79 KB)
📄 ALTER_EXTENSION.7(6.54 KB)
📄 ALTER_FOREIGN_DATA_WRAPPER.7(4.37 KB)
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📄 ALTER_FUNCTION.7(7.62 KB)
📄 ALTER_GROUP.7(3 KB)
📄 ALTER_INDEX.7(4.51 KB)
📄 ALTER_LANGUAGE.7(2.1 KB)
📄 ALTER_LARGE_OBJECT.7(1.97 KB)
📄 ALTER_OPERATOR.7(3.01 KB)
📄 ALTER_OPERATOR_CLASS.7(2.85 KB)
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📄 ALTER_ROLE.7(8.23 KB)
📄 ALTER_SCHEMA.7(2.31 KB)
📄 ALTER_SEQUENCE.7(7.52 KB)
📄 ALTER_SERVER.7(3.4 KB)
📄 ALTER_TABLE.7(30.51 KB)
📄 ALTER_TABLESPACE.7(3.27 KB)
📄 ALTER_TEXT_SEARCH_CONFIGURATION.7(4.8 KB)
📄 ALTER_TEXT_SEARCH_DICTIONARY.7(4.03 KB)
📄 ALTER_TEXT_SEARCH_PARSER.7(2.27 KB)
📄 ALTER_TEXT_SEARCH_TEMPLATE.7(2.31 KB)
📄 ALTER_TRIGGER.7(2.51 KB)
📄 ALTER_TYPE.7(6.87 KB)
📄 ALTER_USER.7(2.49 KB)
📄 ALTER_USER_MAPPING.7(3.28 KB)
📄 ALTER_VIEW.7(3.89 KB)
📄 ANALYZE.7(7.52 KB)
📄 BEGIN.7(4.08 KB)
📄 CHECKPOINT.7(2.32 KB)
📄 CLOSE.7(2.76 KB)
📄 CLUSTER.7(6.26 KB)
📄 COMMENT.7(9 KB)
📄 COMMIT.7(2.12 KB)
📄 COMMIT_PREPARED.7(2.61 KB)
📄 COPY.7(26.12 KB)
📄 CREATE_AGGREGATE.7(9.44 KB)
📄 CREATE_CAST.7(14.07 KB)
📄 CREATE_COLLATION.7(4.01 KB)
📄 CREATE_CONVERSION.7(3.82 KB)
📄 CREATE_DATABASE.7(7.86 KB)
📄 CREATE_DOMAIN.7(5.37 KB)
📄 CREATE_EXTENSION.7(5.67 KB)
📄 CREATE_FOREIGN_DATA_WRAPPER.7(5.08 KB)
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📄 CREATE_FUNCTION.7(21.62 KB)
📄 CREATE_GROUP.7(2.17 KB)
📄 CREATE_INDEX.7(18.72 KB)
📄 CREATE_LANGUAGE.7(9.86 KB)
📄 CREATE_OPERATOR.7(6.92 KB)
📄 CREATE_OPERATOR_CLASS.7(8.54 KB)
📄 CREATE_OPERATOR_FAMILY.7(3.64 KB)
📄 CREATE_ROLE.7(12.18 KB)
📄 CREATE_RULE.7(7.91 KB)
📄 CREATE_SCHEMA.7(5.4 KB)
📄 CREATE_SEQUENCE.7(9.11 KB)
📄 CREATE_SERVER.7(3.68 KB)
📄 CREATE_TABLE.7(39.83 KB)
📄 CREATE_TABLESPACE.7(3.52 KB)
📄 CREATE_TABLE_AS.7(7.94 KB)
📄 CREATE_TEXT_SEARCH_CONFIGURATION.7(3.25 KB)
📄 CREATE_TEXT_SEARCH_DICTIONARY.7(3.39 KB)
📄 CREATE_TEXT_SEARCH_PARSER.7(3.61 KB)
📄 CREATE_TEXT_SEARCH_TEMPLATE.7(3.43 KB)
📄 CREATE_TRIGGER.7(15.11 KB)
📄 CREATE_TYPE.7(28.33 KB)
📄 CREATE_USER.7(2.48 KB)
📄 CREATE_USER_MAPPING.7(3.31 KB)
📄 CREATE_VIEW.7(7.13 KB)
📄 DEALLOCATE.7(2.05 KB)
📄 DECLARE.7(10.26 KB)
📄 DELETE.7(7.39 KB)
📄 DISCARD.7(2.53 KB)
📄 DO.7(3.13 KB)
📄 DROP_AGGREGATE.7(2.58 KB)
📄 DROP_CAST.7(2.29 KB)
📄 DROP_COLLATION.7(2.34 KB)
📄 DROP_CONVERSION.7(2.4 KB)
📄 DROP_DATABASE.7(2.41 KB)
📄 DROP_DOMAIN.7(2.25 KB)
📄 DROP_EXTENSION.7(2.64 KB)
📄 DROP_FOREIGN_DATA_WRAPPER.7(2.52 KB)
📄 DROP_FOREIGN_TABLE.7(2.5 KB)
📄 DROP_FUNCTION.7(3.27 KB)
📄 DROP_GROUP.7(1.6 KB)
📄 DROP_INDEX.7(2.99 KB)
📄 DROP_LANGUAGE.7(2.77 KB)
📄 DROP_OPERATOR.7(2.9 KB)
📄 DROP_OPERATOR_CLASS.7(3.38 KB)
📄 DROP_OPERATOR_FAMILY.7(3.16 KB)
📄 DROP_OWNED.7(3.02 KB)
📄 DROP_ROLE.7(3.08 KB)
📄 DROP_RULE.7(2.2 KB)
📄 DROP_SCHEMA.7(2.55 KB)
📄 DROP_SEQUENCE.7(2.38 KB)
📄 DROP_SERVER.7(2.3 KB)
📄 DROP_TABLE.7(2.88 KB)
📄 DROP_TABLESPACE.7(2.51 KB)
📄 DROP_TEXT_SEARCH_CONFIGURATION.7(2.76 KB)
📄 DROP_TEXT_SEARCH_DICTIONARY.7(2.67 KB)
📄 DROP_TEXT_SEARCH_PARSER.7(2.58 KB)
📄 DROP_TEXT_SEARCH_TEMPLATE.7(2.61 KB)
📄 DROP_TRIGGER.7(2.52 KB)
📄 DROP_TYPE.7(2.43 KB)
📄 DROP_USER.7(1.68 KB)
📄 DROP_USER_MAPPING.7(2.59 KB)
📄 DROP_VIEW.7(2.34 KB)
📄 END.7(2.21 KB)
📄 EXECUTE.7(2.96 KB)
📄 EXPLAIN.7(12.89 KB)
📄 FETCH.7(8.52 KB)
📄 GRANT.7(21.41 KB)
📄 INSERT.7(8.74 KB)
📄 LISTEN.7(3.53 KB)
📄 LOAD.7(2.58 KB)
📄 LOCK.7(7.92 KB)
📄 MOVE.7(2.96 KB)
📄 NOTIFY.7(7.96 KB)
📄 PREPARE.7(6.2 KB)
📄 PREPARE_TRANSACTION.7(5.77 KB)
📄 REASSIGN_OWNED.7(3.08 KB)
📄 REINDEX.7(7.85 KB)
📄 RELEASE_SAVEPOINT.7(3.06 KB)
📄 RESET.7(2.96 KB)
📄 REVOKE.7(10.46 KB)
📄 ROLLBACK.7(2.12 KB)
📄 ROLLBACK_PREPARED.7(2.64 KB)
📄 ROLLBACK_TO_SAVEPOINT.7(4.01 KB)
📄 SAVEPOINT.7(3.55 KB)
📄 SECURITY_LABEL.7(5.29 KB)
📄 SELECT.7(52.77 KB)
📄 SELECT_INTO.7(4.53 KB)
📄 SET.7(7.46 KB)
📄 SET_CONSTRAINTS.7(4.56 KB)
📄 SET_ROLE.7(4.38 KB)
📄 SET_SESSION_AUTHORIZATION.7(3.92 KB)
📄 SET_TRANSACTION.7(8.44 KB)
📄 SHOW.7(4.25 KB)
📄 START_TRANSACTION.7(2.93 KB)
📄 TABLE.7(18 B)
📄 TRUNCATE.7(6.24 KB)
📄 UNLISTEN.7(2.77 KB)
📄 UPDATE.7(10.63 KB)
📄 VACUUM.7(7.09 KB)
📄 VALUES.7(6.97 KB)
📄 WITH.7(18 B)

Editing: SET_TRANSACTION.7

'\" t
.\"     Title: SET TRANSACTION
.\"    Author: The PostgreSQL Global Development Group
.\" Generator: DocBook XSL Stylesheets v1.78.1 <http://docbook.sf.net/>
.\"      Date: 2017-11-06
.\"    Manual: PostgreSQL 9.2.24 Documentation
.\"    Source: PostgreSQL 9.2.24
.\"  Language: English
.\"
.TH "SET TRANSACTION" "7" "2017-11-06" "PostgreSQL 9.2.24" "PostgreSQL 9.2.24 Documentation"
.\" -----------------------------------------------------------------
.\" * Define some portability stuff
.\" -----------------------------------------------------------------
.\" ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
.\" http://bugs.debian.org/507673
.\" http://lists.gnu.org/archive/html/groff/2009-02/msg00013.html
.\" ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
.ie \n(.g .ds Aq \(aq
.el       .ds Aq '
.\" -----------------------------------------------------------------
.\" * set default formatting
.\" -----------------------------------------------------------------
.\" disable hyphenation
.nh
.\" disable justification (adjust text to left margin only)
.ad l
.\" -----------------------------------------------------------------
.\" * MAIN CONTENT STARTS HERE *
.\" -----------------------------------------------------------------
.SH "NAME"
SET_TRANSACTION \- set the characteristics of the current transaction
.\" SET TRANSACTION
.\" transaction isolation level: setting
.\" read-only transaction: setting
.\" deferrable transaction: setting
.SH "SYNOPSIS"
.sp
.nf
SET TRANSACTION \fItransaction_mode\fR [, \&.\&.\&.]
SET TRANSACTION SNAPSHOT \fIsnapshot_id\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
    [ NOT ] DEFERRABLE
.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, the transaction access mode (read/write or read\-only), and the deferrable mode\&. In addition, a snapshot can be selected, though only for the current transaction, not as a session default\&.
.PP
The isolation level of a transaction determines what data the transaction can see when other transactions are running concurrently:
.PP
READ COMMITTED
.RS 4
A statement can only see rows committed before it began\&. This is the default\&.
.RE
.PP
REPEATABLE READ
.RS 4
All statements of the current transaction can only see rows committed before the first query or data\-modification statement was executed in this transaction\&.
.RE
.PP
SERIALIZABLE
.RS 4
All statements of the current transaction can only see rows committed before the first query or data\-modification statement was executed in this transaction\&. If a pattern of reads and writes among concurrent serializable transactions would create a situation which could not have occurred for any serial (one\-at\-a\-time) execution of those transactions, one of them will be rolled back with a
serialization_failure
error\&.
.RE
The SQL standard defines one additional level,
READ UNCOMMITTED\&. In
PostgreSQLREAD UNCOMMITTED
is treated as
READ COMMITTED\&.
.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
Chapter 13, Concurrency Control, 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\&.
.PP
The
DEFERRABLE
transaction property has no effect unless the transaction is also
SERIALIZABLE
and
READ ONLY\&. When all three of these properties are selected for a transaction, the transaction may block when first acquiring its snapshot, after which it is able to run without the normal overhead of a
SERIALIZABLE
transaction and without any risk of contributing to or being canceled by a serialization failure\&. This mode is well suited for long\-running reports or backups\&.
.PP
The
SET TRANSACTION SNAPSHOT
command allows a new transaction to run with the same
snapshot
as an existing transaction\&. The pre\-existing transaction must have exported its snapshot with the
pg_export_snapshot
function (see
Section 9.26.5, \(lqSnapshot Synchronization Functions\(rq, in the documentation)\&. That function returns a snapshot identifier, which must be given to
SET TRANSACTION SNAPSHOT
to specify which snapshot is to be imported\&. The identifier must be written as a string literal in this command, for example
\*(Aq000003A1\-1\*(Aq\&.
SET TRANSACTION SNAPSHOT
can only be executed at the start of a transaction, before the first query or data\-modification statement (\fBSELECT\fR,
\fBINSERT\fR,
\fBDELETE\fR,
\fBUPDATE\fR,
\fBFETCH\fR, or
\fBCOPY\fR) of the transaction\&. Furthermore, the transaction must already be set to
SERIALIZABLE
or
REPEATABLE READ
isolation level (otherwise, the snapshot would be discarded immediately, since
READ COMMITTED
mode takes a new snapshot for each command)\&. If the importing transaction uses
SERIALIZABLE
isolation level, then the transaction that exported the snapshot must also use that isolation level\&. Also, a non\-read\-only serializable transaction cannot import a snapshot from a read\-only transaction\&.
.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\&. But that option is not available for
\fBSET TRANSACTION SNAPSHOT\fR\&.
.PP
The session default transaction modes can also be set by setting the configuration parameters
default_transaction_isolation,
default_transaction_read_only, and
default_transaction_deferrable\&. (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
Chapter 18, Server Configuration, in the documentation
for more information\&.
.SH "EXAMPLES"
.PP
To begin a new transaction with the same snapshot as an already existing transaction, first export the snapshot from the existing transaction\&. That will return the snapshot identifier, for example:
.sp
.if n \{\
.RS 4
.\}
.nf
BEGIN TRANSACTION ISOLATION LEVEL REPEATABLE READ;
SELECT pg_export_snapshot();
 pg_export_snapshot
\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-
 000003A1\-1
(1 row)
.fi
.if n \{\
.RE
.\}
.sp
Then give the snapshot identifier in a
\fBSET TRANSACTION SNAPSHOT\fR
command at the beginning of the newly opened transaction:
.sp
.if n \{\
.RS 4
.\}
.nf
BEGIN TRANSACTION ISOLATION LEVEL REPEATABLE READ;
SET TRANSACTION SNAPSHOT \*(Aq000003A1\-1\*(Aq;
.fi
.if n \{\
.RE
.\}
.SH "COMPATIBILITY"
.PP
These commands are defined in the
SQL
standard, except for the
DEFERRABLE
transaction mode and the
\fBSET TRANSACTION SNAPSHOT\fR
form, which are
PostgreSQL
extensions\&.
.PP
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\&.
.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\&.

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