1SET TRANSACTION(7)       PostgreSQL 16.1 Documentation      SET TRANSACTION(7)
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NAME

6       SET_TRANSACTION - set the characteristics of the current transaction
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SYNOPSIS

9       SET TRANSACTION transaction_mode [, ...]
10       SET TRANSACTION SNAPSHOT snapshot_id
11       SET SESSION CHARACTERISTICS AS TRANSACTION transaction_mode [, ...]
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13       where transaction_mode is one of:
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15           ISOLATION LEVEL { SERIALIZABLE | REPEATABLE READ | READ COMMITTED | READ UNCOMMITTED }
16           READ WRITE | READ ONLY
17           [ NOT ] DEFERRABLE
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DESCRIPTION

20       The SET TRANSACTION command sets the characteristics of the current
21       transaction. It has no effect on any subsequent transactions.  SET
22       SESSION CHARACTERISTICS sets the default transaction characteristics
23       for subsequent transactions of a session. These defaults can be
24       overridden by SET TRANSACTION for an individual transaction.
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26       The available transaction characteristics are the transaction isolation
27       level, the transaction access mode (read/write or read-only), and the
28       deferrable mode. In addition, a snapshot can be selected, though only
29       for the current transaction, not as a session default.
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31       The isolation level of a transaction determines what data the
32       transaction can see when other transactions are running concurrently:
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34       READ COMMITTED
35           A statement can only see rows committed before it began. This is
36           the default.
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38       REPEATABLE READ
39           All statements of the current transaction can only see rows
40           committed before the first query or data-modification statement was
41           executed in this transaction.
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43       SERIALIZABLE
44           All statements of the current transaction can only see rows
45           committed before the first query or data-modification statement was
46           executed in this transaction. If a pattern of reads and writes
47           among concurrent serializable transactions would create a situation
48           which could not have occurred for any serial (one-at-a-time)
49           execution of those transactions, one of them will be rolled back
50           with a serialization_failure error.
51       The SQL standard defines one additional level, READ UNCOMMITTED. In
52       PostgreSQL READ UNCOMMITTED is treated as READ COMMITTED.
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54       The transaction isolation level cannot be changed after the first query
55       or data-modification statement (SELECT, INSERT, DELETE, UPDATE, MERGE,
56       FETCH, or COPY) of a transaction has been executed. See Chapter 13 for
57       more information about transaction isolation and concurrency control.
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59       The transaction access mode determines whether the transaction is
60       read/write or read-only. Read/write is the default. When a transaction
61       is read-only, the following SQL commands are disallowed: INSERT,
62       UPDATE, DELETE, MERGE, and COPY FROM if the table they would write to
63       is not a temporary table; all CREATE, ALTER, and DROP commands;
64       COMMENT, GRANT, REVOKE, TRUNCATE; and EXPLAIN ANALYZE and EXECUTE if
65       the command they would execute is among those listed. This is a
66       high-level notion of read-only that does not prevent all writes to
67       disk.
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69       The DEFERRABLE transaction property has no effect unless the
70       transaction is also SERIALIZABLE and READ ONLY. When all three of these
71       properties are selected for a transaction, the transaction may block
72       when first acquiring its snapshot, after which it is able to run
73       without the normal overhead of a SERIALIZABLE transaction and without
74       any risk of contributing to or being canceled by a serialization
75       failure. This mode is well suited for long-running reports or backups.
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77       The SET TRANSACTION SNAPSHOT command allows a new transaction to run
78       with the same snapshot as an existing transaction. The pre-existing
79       transaction must have exported its snapshot with the pg_export_snapshot
80       function (see Section 9.27.5). That function returns a snapshot
81       identifier, which must be given to SET TRANSACTION SNAPSHOT to specify
82       which snapshot is to be imported. The identifier must be written as a
83       string literal in this command, for example '00000003-0000001B-1'.  SET
84       TRANSACTION SNAPSHOT can only be executed at the start of a
85       transaction, before the first query or data-modification statement
86       (SELECT, INSERT, DELETE, UPDATE, MERGE, FETCH, or COPY) of the
87       transaction. Furthermore, the transaction must already be set to
88       SERIALIZABLE or REPEATABLE READ isolation level (otherwise, the
89       snapshot would be discarded immediately, since READ COMMITTED mode
90       takes a new snapshot for each command). If the importing transaction
91       uses SERIALIZABLE isolation level, then the transaction that exported
92       the snapshot must also use that isolation level. Also, a non-read-only
93       serializable transaction cannot import a snapshot from a read-only
94       transaction.
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NOTES

97       If SET TRANSACTION is executed without a prior START TRANSACTION or
98       BEGIN, it emits a warning and otherwise has no effect.
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100       It is possible to dispense with SET TRANSACTION by instead specifying
101       the desired transaction_modes in BEGIN or START TRANSACTION. But that
102       option is not available for SET TRANSACTION SNAPSHOT.
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104       The session default transaction modes can also be set or examined via
105       the configuration parameters default_transaction_isolation,
106       default_transaction_read_only, and default_transaction_deferrable. (In
107       fact SET SESSION CHARACTERISTICS is just a verbose equivalent for
108       setting these variables with SET.) This means the defaults can be set
109       in the configuration file, via ALTER DATABASE, etc. Consult Chapter 20
110       for more information.
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112       The current transaction's modes can similarly be set or examined via
113       the configuration parameters transaction_isolation,
114       transaction_read_only, and transaction_deferrable. Setting one of these
115       parameters acts the same as the corresponding SET TRANSACTION option,
116       with the same restrictions on when it can be done. However, these
117       parameters cannot be set in the configuration file, or from any source
118       other than live SQL.
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EXAMPLES

121       To begin a new transaction with the same snapshot as an already
122       existing transaction, first export the snapshot from the existing
123       transaction. That will return the snapshot identifier, for example:
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125           BEGIN TRANSACTION ISOLATION LEVEL REPEATABLE READ;
126           SELECT pg_export_snapshot();
127            pg_export_snapshot
128           ---------------------
129            00000003-0000001B-1
130           (1 row)
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132       Then give the snapshot identifier in a SET TRANSACTION SNAPSHOT command
133       at the beginning of the newly opened transaction:
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135           BEGIN TRANSACTION ISOLATION LEVEL REPEATABLE READ;
136           SET TRANSACTION SNAPSHOT '00000003-0000001B-1';
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COMPATIBILITY

139       These commands are defined in the SQL standard, except for the
140       DEFERRABLE transaction mode and the SET TRANSACTION SNAPSHOT form,
141       which are PostgreSQL extensions.
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143       SERIALIZABLE is the default transaction isolation level in the
144       standard. In PostgreSQL the default is ordinarily READ COMMITTED, but
145       you can change it as mentioned above.
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147       In the SQL standard, there is one other transaction characteristic that
148       can be set with these commands: the size of the diagnostics area. This
149       concept is specific to embedded SQL, and therefore is not implemented
150       in the PostgreSQL server.
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152       The SQL standard requires commas between successive transaction_modes,
153       but for historical reasons PostgreSQL allows the commas to be omitted.
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157PostgreSQL 16.1                      2023                   SET TRANSACTION(7)
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