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
|
<!--
$PostgreSQL: pgsql/doc/src/sgml/ref/truncate.sgml,v 1.24 2007/05/11 19:40:08 neilc Exp $
PostgreSQL documentation
-->
<refentry id="SQL-TRUNCATE">
<refmeta>
<refentrytitle id="SQL-TRUNCATE-TITLE">TRUNCATE</refentrytitle>
<refmiscinfo>SQL - Language Statements</refmiscinfo>
</refmeta>
<refnamediv>
<refname>TRUNCATE</refname>
<refpurpose>empty a table or set of tables</refpurpose>
</refnamediv>
<indexterm zone="sql-truncate">
<primary>TRUNCATE</primary>
</indexterm>
<refsynopsisdiv>
<synopsis>
TRUNCATE [ TABLE ] <replaceable class="PARAMETER">name</replaceable> [, ...] [ CASCADE | RESTRICT ]
</synopsis>
</refsynopsisdiv>
<refsect1>
<title>Description</title>
<para>
<command>TRUNCATE</command> quickly removes all rows from a set of
tables. It has the same effect as an unqualified
<command>DELETE</command> on each table, but since it does not actually
scan the tables it is faster. Furthermore, it reclaims disk space
immediately, rather than requiring a subsequent <command>VACUUM</command>
operation. This is most useful on large tables.
</para>
</refsect1>
<refsect1>
<title>Parameters</title>
<variablelist>
<varlistentry>
<term><replaceable class="PARAMETER">name</replaceable></term>
<listitem>
<para>
The name (optionally schema-qualified) of a table to be truncated.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>CASCADE</literal></term>
<listitem>
<para>
Automatically truncate all tables that have foreign-key references
to any of the named tables, or to any tables added to the group
due to <literal>CASCADE</literal>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>RESTRICT</literal></term>
<listitem>
<para>
Refuse to truncate if any of the tables have foreign-key references
from tables that are not to be truncated. This is the default.
</para>
</listitem>
</varlistentry>
</variablelist>
</refsect1>
<refsect1>
<title>Notes</title>
<para>
Only the owner of a table can <command>TRUNCATE</> it.
</para>
<para>
<command>TRUNCATE</> cannot be used on a table that has foreign-key
references from other tables, unless all such tables are also truncated
in the same command. Checking validity in such cases would require table
scans, and the whole point is not to do one. The <literal>CASCADE</>
option can be used to automatically include all dependent tables —
but be very careful when using this option, or else you might lose data you
did not intend to!
</para>
<para>
<command>TRUNCATE</> will not run any <literal>ON DELETE</literal>
triggers that might exist for the tables.
</para>
<warning>
<para>
<command>TRUNCATE</> is not MVCC-safe (see <xref linkend="mvcc">
for general information about MVCC). After truncation, the table
will appear empty to all concurrent transactions, even if they
are using a snapshot taken before the truncation occurred. This
will only be an issue for a transaction that did not access the
truncated table before the truncation happened — any
transaction that has done so would hold at least an
<literal>ACCESS SHARE</literal> lock, which would block
<command>TRUNCATE</> until that transaction completes. So
truncation will not cause any apparent inconsistency in the table
contents for successive queries on the same table, but it could
cause visible inconsistency between the contents of the truncated
table and other tables in the database.
</para>
<para>
<command>TRUNCATE</> is transaction-safe, however: the truncation
will be safely rolled back if the surrounding transaction does not
commit.
</para>
</warning>
</refsect1>
<refsect1>
<title>Examples</title>
<para>
Truncate the tables <literal>bigtable</literal> and <literal>fattable</literal>:
<programlisting>
TRUNCATE bigtable, fattable;
</programlisting>
</para>
<para>
Truncate the table <literal>othertable</literal>, and cascade to any tables
that reference <literal>othertable</literal> via foreign-key
constraints:
<programlisting>
TRUNCATE othertable CASCADE;
</programlisting>
</para>
</refsect1>
<refsect1>
<title>Compatibility</title>
<para>
There is no <command>TRUNCATE</command> command in the SQL standard.
</para>
</refsect1>
</refentry>
|
