doc/src/sgml/tsm-system-time.sgml


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

<!-- doc/src/sgml/tsm-system-time.sgml -->

<sect1 id="tsm-system-time" xreflabel="tsm_system_time">
 <title>tsm_system_time &mdash;
   the <literal>SYSTEM_TIME</literal> sampling method for <literal>TABLESAMPLE</literal></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 id="tsm-system-time-examples">
  <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>