blob: df6e83a9236ea4e88a857b28e6a97763956ee761 (
plain)
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
|
<!-- doc/src/sgml/tsm-system-time.sgml -->
<sect1 id="tsm-system-time" xreflabel="tsm_system_time">
<title>tsm_system_time</title>
<indexterm zone="tsm-system-time">
<primary>tsm_system_time</primary>
</indexterm>
<para>
The <filename>tsm_system_time</filename> module provides the table sampling method
<literal>SYSTEM_TIME</literal>, which can be used in
the <literal>TABLESAMPLE</literal> clause of a <link linkend="sql-select"><command>SELECT</command></link>
command.
</para>
<para>
This table sampling method accepts a single floating-point argument that
is the maximum number of milliseconds to spend reading the table. This
gives you direct control over how long the query takes, at the price that
the size of the sample becomes hard to predict. The resulting sample will
contain as many rows as could be read in the specified time, unless the
whole table has been read first.
</para>
<para>
Like the built-in <literal>SYSTEM</literal> sampling
method, <literal>SYSTEM_TIME</literal> performs block-level sampling, so
that the sample is not completely random but may be subject to clustering
effects, especially if only a small number of rows are selected.
</para>
<para>
<literal>SYSTEM_TIME</literal> does not support
the <literal>REPEATABLE</literal> clause.
</para>
<para>
This module is considered <quote>trusted</quote>, that is, it can be
installed by non-superusers who have <literal>CREATE</literal> privilege
on the current database.
</para>
<sect2>
<title>Examples</title>
<para>
Here is an example of selecting a sample of a table with
<literal>SYSTEM_TIME</literal>. First install the extension:
</para>
<programlisting>
CREATE EXTENSION tsm_system_time;
</programlisting>
<para>
Then you can use it in a <command>SELECT</command> command, for instance:
<programlisting>
SELECT * FROM my_table TABLESAMPLE SYSTEM_TIME(1000);
</programlisting>
</para>
<para>
This command will return as large a sample of <structname>my_table</structname> as
it can read in 1 second (1000 milliseconds). Of course, if the whole
table can be read in under 1 second, all its rows will be returned.
</para>
</sect2>
</sect1>
|
