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
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
|
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<HTML
><HEAD
><TITLE
>Trigger Functions</TITLE
><META
NAME="GENERATOR"
CONTENT="Modular DocBook HTML Stylesheet Version 1.79"><LINK
REV="MADE"
HREF="mailto:pgsql-docs@postgresql.org"><LINK
REL="HOME"
TITLE="PostgreSQL 9.3.10 Documentation"
HREF="index.html"><LINK
REL="UP"
TITLE="Functions and Operators"
HREF="functions.html"><LINK
REL="PREVIOUS"
TITLE="System Administration Functions"
HREF="functions-admin.html"><LINK
REL="NEXT"
TITLE="Event Trigger Functions"
HREF="functions-event-triggers.html"><LINK
REL="STYLESHEET"
TYPE="text/css"
HREF="stylesheet.css"><META
HTTP-EQUIV="Content-Type"
CONTENT="text/html; charset=ISO-8859-1"><META
NAME="creation"
CONTENT="2015-10-05T19:31:56"></HEAD
><BODY
CLASS="SECT1"
><DIV
CLASS="NAVHEADER"
><TABLE
SUMMARY="Header navigation table"
WIDTH="100%"
BORDER="0"
CELLPADDING="0"
CELLSPACING="0"
><TR
><TH
COLSPAN="5"
ALIGN="center"
VALIGN="bottom"
><A
HREF="index.html"
>PostgreSQL 9.3.10 Documentation</A
></TH
></TR
><TR
><TD
WIDTH="10%"
ALIGN="left"
VALIGN="top"
><A
TITLE="System Administration Functions"
HREF="functions-admin.html"
ACCESSKEY="P"
>Prev</A
></TD
><TD
WIDTH="10%"
ALIGN="left"
VALIGN="top"
><A
HREF="functions.html"
ACCESSKEY="U"
>Up</A
></TD
><TD
WIDTH="60%"
ALIGN="center"
VALIGN="bottom"
>Chapter 9. Functions and Operators</TD
><TD
WIDTH="20%"
ALIGN="right"
VALIGN="top"
><A
TITLE="Event Trigger Functions"
HREF="functions-event-triggers.html"
ACCESSKEY="N"
>Next</A
></TD
></TR
></TABLE
><HR
ALIGN="LEFT"
WIDTH="100%"></DIV
><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="FUNCTIONS-TRIGGER"
>9.27. Trigger Functions</A
></H1
><P
> Currently <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> provides one built in trigger
function, <CODE
CLASS="FUNCTION"
>suppress_redundant_updates_trigger</CODE
>,
which will prevent any update
that does not actually change the data in the row from taking place, in
contrast to the normal behavior which always performs the update
regardless of whether or not the data has changed. (This normal behavior
makes updates run faster, since no checking is required, and is also
useful in certain cases.)
</P
><P
> Ideally, you should normally avoid running updates that don't actually
change the data in the record. Redundant updates can cost considerable
unnecessary time, especially if there are lots of indexes to alter,
and space in dead rows that will eventually have to be vacuumed.
However, detecting such situations in client code is not
always easy, or even possible, and writing expressions to detect
them can be error-prone. An alternative is to use
<CODE
CLASS="FUNCTION"
>suppress_redundant_updates_trigger</CODE
>, which will skip
updates that don't change the data. You should use this with care,
however. The trigger takes a small but non-trivial time for each record,
so if most of the records affected by an update are actually changed,
use of this trigger will actually make the update run slower.
</P
><P
> The <CODE
CLASS="FUNCTION"
>suppress_redundant_updates_trigger</CODE
> function can be
added to a table like this:
</P><PRE
CLASS="PROGRAMLISTING"
>CREATE TRIGGER z_min_update
BEFORE UPDATE ON tablename
FOR EACH ROW EXECUTE PROCEDURE suppress_redundant_updates_trigger();</PRE
><P>
In most cases, you would want to fire this trigger last for each row.
Bearing in mind that triggers fire in name order, you would then
choose a trigger name that comes after the name of any other trigger
you might have on the table.
</P
><P
> For more information about creating triggers, see
<A
HREF="sql-createtrigger.html"
>CREATE TRIGGER</A
>.
</P
></DIV
><DIV
CLASS="NAVFOOTER"
><HR
ALIGN="LEFT"
WIDTH="100%"><TABLE
SUMMARY="Footer navigation table"
WIDTH="100%"
BORDER="0"
CELLPADDING="0"
CELLSPACING="0"
><TR
><TD
WIDTH="33%"
ALIGN="left"
VALIGN="top"
><A
HREF="functions-admin.html"
ACCESSKEY="P"
>Prev</A
></TD
><TD
WIDTH="34%"
ALIGN="center"
VALIGN="top"
><A
HREF="index.html"
ACCESSKEY="H"
>Home</A
></TD
><TD
WIDTH="33%"
ALIGN="right"
VALIGN="top"
><A
HREF="functions-event-triggers.html"
ACCESSKEY="N"
>Next</A
></TD
></TR
><TR
><TD
WIDTH="33%"
ALIGN="left"
VALIGN="top"
>System Administration Functions</TD
><TD
WIDTH="34%"
ALIGN="center"
VALIGN="top"
><A
HREF="functions.html"
ACCESSKEY="U"
>Up</A
></TD
><TD
WIDTH="33%"
ALIGN="right"
VALIGN="top"
>Event Trigger Functions</TD
></TR
></TABLE
></DIV
></BODY
></HTML
>
|