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
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
|
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<HTML
><HEAD
><TITLE
>Visibility of Data Changes</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 9.1.15 Documentation"
HREF="index.html"><LINK
REL="UP"
TITLE="Triggers"
HREF="triggers.html"><LINK
REL="PREVIOUS"
TITLE="Overview of Trigger Behavior"
HREF="trigger-definition.html"><LINK
REL="NEXT"
TITLE="Writing Trigger Functions in C"
HREF="trigger-interface.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="2015-02-02T21:03:01"></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 9.1.15 Documentation</A
></TH
></TR
><TR
><TD
WIDTH="10%"
ALIGN="left"
VALIGN="top"
><A
TITLE="Overview of Trigger Behavior"
HREF="trigger-definition.html"
ACCESSKEY="P"
>Prev</A
></TD
><TD
WIDTH="10%"
ALIGN="left"
VALIGN="top"
><A
HREF="triggers.html"
ACCESSKEY="U"
>Up</A
></TD
><TD
WIDTH="60%"
ALIGN="center"
VALIGN="bottom"
>Chapter 36. Triggers</TD
><TD
WIDTH="20%"
ALIGN="right"
VALIGN="top"
><A
TITLE="Writing Trigger Functions in C"
HREF="trigger-interface.html"
ACCESSKEY="N"
>Next</A
></TD
></TR
></TABLE
><HR
ALIGN="LEFT"
WIDTH="100%"></DIV
><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="TRIGGER-DATACHANGES"
>36.2. Visibility of Data Changes</A
></H1
><P
> If you execute SQL commands in your trigger function, and these
commands access the table that the trigger is for, then
you need to be aware of the data visibility rules, because they determine
whether these SQL commands will see the data change that the trigger
is fired for. Briefly:
<P
></P
></P><UL
><LI
><P
> Statement-level triggers follow simple visibility rules: none of
the changes made by a statement are visible to statement-level
triggers that are invoked before the statement, whereas all
modifications are visible to statement-level <TT
CLASS="LITERAL"
>AFTER</TT
>
triggers.
</P
></LI
><LI
><P
> The data change (insertion, update, or deletion) causing the
trigger to fire is naturally <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>not</I
></SPAN
> visible
to SQL commands executed in a row-level <TT
CLASS="LITERAL"
>BEFORE</TT
> trigger,
because it hasn't happened yet.
</P
></LI
><LI
><P
> However, SQL commands executed in a row-level <TT
CLASS="LITERAL"
>BEFORE</TT
>
trigger <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>will</I
></SPAN
> see the effects of data
changes for rows previously processed in the same outer
command. This requires caution, since the ordering of these
change events is not in general predictable; a SQL command that
affects multiple rows can visit the rows in any order.
</P
></LI
><LI
><P
> Similarly, a row-level <TT
CLASS="LITERAL"
>INSTEAD OF</TT
> trigger will see the
effects of data changes made by previous firings of <TT
CLASS="LITERAL"
>INSTEAD
OF</TT
> triggers in the same outer command.
</P
></LI
><LI
><P
> When a row-level <TT
CLASS="LITERAL"
>AFTER</TT
> trigger is fired, all data
changes made
by the outer command are already complete, and are visible to
the invoked trigger function.
</P
></LI
></UL
><P>
</P
><P
> If your trigger function is written in any of the standard procedural
languages, then the above statements apply only if the function is
declared <TT
CLASS="LITERAL"
>VOLATILE</TT
>. Functions that are declared
<TT
CLASS="LITERAL"
>STABLE</TT
> or <TT
CLASS="LITERAL"
>IMMUTABLE</TT
> will not see changes made by
the calling command in any case.
</P
><P
> Further information about data visibility rules can be found in
<A
HREF="spi-visibility.html"
>Section 43.4</A
>. The example in <A
HREF="trigger-example.html"
>Section 36.4</A
> contains a demonstration of these rules.
</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="trigger-definition.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="trigger-interface.html"
ACCESSKEY="N"
>Next</A
></TD
></TR
><TR
><TD
WIDTH="33%"
ALIGN="left"
VALIGN="top"
>Overview of Trigger Behavior</TD
><TD
WIDTH="34%"
ALIGN="center"
VALIGN="top"
><A
HREF="triggers.html"
ACCESSKEY="U"
>Up</A
></TD
><TD
WIDTH="33%"
ALIGN="right"
VALIGN="top"
>Writing Trigger Functions in C</TD
></TR
></TABLE
></DIV
></BODY
></HTML
>
|
