1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
|
<!-- $PostgreSQL: pgsql/doc/src/sgml/ref/set_transaction.sgml,v 1.23.6.1 2006/07/31 01:13:58 alvherre Exp $ -->
<refentry id="SQL-SET-TRANSACTION">
<refmeta>
<refentrytitle id="SQL-SET-TRANSACTION-TITLE">SET TRANSACTION</refentrytitle>
<refmiscinfo>SQL - Language Statements</refmiscinfo>
</refmeta>
<refnamediv>
<refname>SET TRANSACTION</refname>
<refpurpose>set the characteristics of the current transaction</refpurpose>
</refnamediv>
<indexterm zone="sql-set-transaction">
<primary>SET TRANSACTION</primary>
</indexterm>
<refsynopsisdiv>
<synopsis>
SET TRANSACTION <replaceable class="parameter">transaction_mode</replaceable> [, ...]
SET SESSION CHARACTERISTICS AS TRANSACTION <replaceable class="parameter">transaction_mode</replaceable> [, ...]
where <replaceable class="parameter">transaction_mode</replaceable> is one of:
ISOLATION LEVEL { SERIALIZABLE | REPEATABLE READ | READ COMMITTED | READ UNCOMMITTED }
READ WRITE | READ ONLY
</synopsis>
</refsynopsisdiv>
<refsect1>
<title>Description</title>
<para>
The <command>SET TRANSACTION</command> command sets the
characteristics of the current transaction. It has no effect on any
subsequent transactions. <command>SET SESSION
CHARACTERISTICS</command> sets the default transaction
characteristics for subsequent transactions of a session. These
defaults can be overridden by <command>SET TRANSACTION</command>
for an individual transaction.
</para>
<para>
The available transaction characteristics are the transaction
isolation level and the transaction access mode (read/write or
read-only).
</para>
<para>
The isolation level of a transaction determines what data the
transaction can see when other transactions are running concurrently:
<variablelist>
<varlistentry>
<term><literal>READ COMMITTED</literal></term>
<listitem>
<para>
A statement can only see rows committed before it began. This
is the default.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>SERIALIZABLE</literal></term>
<listitem>
<para>
All statements of the current transaction can only see rows committed
before the first query or data-modification statement was executed in
this transaction.
</para>
</listitem>
</varlistentry>
</variablelist>
The SQL standard defines two additional levels, <literal>READ
UNCOMMITTED</literal> and <literal>REPEATABLE READ</literal>.
In <productname>PostgreSQL</productname> <literal>READ
UNCOMMITTED</literal> is treated as
<literal>READ COMMITTED</literal>, while <literal>REPEATABLE
READ</literal> is treated as <literal>SERIALIZABLE</literal>.
</para>
<para>
The transaction isolation level cannot be changed after the first query or
data-modification statement (<command>SELECT</command>,
<command>INSERT</command>, <command>DELETE</command>,
<command>UPDATE</command>, <command>FETCH</command>, or
<command>COPY</command>) of a transaction has been executed. See
<xref linkend="mvcc"> for more information about transaction
isolation and concurrency control.
</para>
<para>
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: <literal>INSERT</literal>, <literal>UPDATE</literal>,
<literal>DELETE</literal>, and <literal>COPY FROM</literal> if the
table they would write to is not a temporary table; all
<literal>CREATE</literal>, <literal>ALTER</literal>, and
<literal>DROP</literal> commands; <literal>COMMENT</literal>,
<literal>GRANT</literal>, <literal>REVOKE</literal>,
<literal>TRUNCATE</literal>; and <literal>EXPLAIN ANALYZE</literal>
and <literal>EXECUTE</literal> 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.
</para>
</refsect1>
<refsect1>
<title>Notes</title>
<para>
If <command>SET TRANSACTION</command> is executed without a prior
<command>START TRANSACTION</command> or <command>BEGIN</command>,
it will appear to have no effect, since the transaction will immediately
end.
</para>
<para>
It is possible to dispense with <command>SET TRANSACTION</command>
by instead specifying the desired <replaceable
class="parameter">transaction_modes</replaceable> in
<command>BEGIN</command> or <command>START TRANSACTION</command>.
</para>
<para>
The session default transaction modes can also be set by setting the
configuration parameters <xref linkend="guc-default-transaction-isolation">
and <xref linkend="guc-default-transaction-read-only">.
(In fact <command>SET SESSION CHARACTERISTICS</command> is just a
verbose equivalent for setting these variables with <command>SET</>.)
This means the defaults can be set in the configuration file, via
<command>ALTER DATABASE</>, etc. Consult <xref linkend="runtime-config">
for more information.
</para>
</refsect1>
<refsect1 id="R1-SQL-SET-TRANSACTION-3">
<title>Compatibility</title>
<para>
Both commands are defined in the <acronym>SQL</acronym> standard.
<literal>SERIALIZABLE</literal> is the default transaction
isolation level in the standard. In
<productname>PostgreSQL</productname> the default is ordinarily
<literal>READ COMMITTED</literal>, but you can change it as
mentioned above. Because of lack of predicate locking, the
<literal>SERIALIZABLE</literal> level is not truly
serializable. See <xref linkend="mvcc"> for details.
</para>
<para>
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 <productname>PostgreSQL</productname> server.
</para>
<para>
The SQL standard requires commas between successive <replaceable
class="parameter">transaction_modes</replaceable>, but for historical
reasons <productname>PostgreSQL</productname> allows the commas to be
omitted.
</para>
</refsect1>
</refentry>
<!-- Keep this comment at the end of the file
Local variables:
mode:sgml
sgml-omittag:nil
sgml-shorttag:t
sgml-minimize-attributes:nil
sgml-always-quote-attributes:t
sgml-indent-step:1
sgml-indent-data:t
sgml-parent-document:nil
sgml-default-dtd-file:"../reference.ced"
sgml-exposed-tags:nil
sgml-local-catalogs:("/usr/lib/sgml/catalog")
sgml-local-ecat-files:nil
End:
-->
|
