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
|
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<HTML
><HEAD
><TITLE
>Visibility Map</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 9.3.12 Documentation"
HREF="index.html"><LINK
REL="UP"
TITLE="Database Physical Storage"
HREF="storage.html"><LINK
REL="PREVIOUS"
TITLE="Free Space Map"
HREF="storage-fsm.html"><LINK
REL="NEXT"
TITLE="The Initialization Fork"
HREF="storage-init.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="2016-03-28T20:28:06"></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 9.3.12 Documentation</A
></TH
></TR
><TR
><TD
WIDTH="10%"
ALIGN="left"
VALIGN="top"
><A
TITLE="Free Space Map"
HREF="storage-fsm.html"
ACCESSKEY="P"
>Prev</A
></TD
><TD
WIDTH="10%"
ALIGN="left"
VALIGN="top"
><A
HREF="storage.html"
ACCESSKEY="U"
>Up</A
></TD
><TD
WIDTH="60%"
ALIGN="center"
VALIGN="bottom"
>Chapter 58. Database Physical Storage</TD
><TD
WIDTH="20%"
ALIGN="right"
VALIGN="top"
><A
TITLE="The Initialization Fork"
HREF="storage-init.html"
ACCESSKEY="N"
>Next</A
></TD
></TR
></TABLE
><HR
ALIGN="LEFT"
WIDTH="100%"></DIV
><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="STORAGE-VM"
>58.4. Visibility Map</A
></H1
><P
>Each heap relation has a Visibility Map
(VM) to keep track of which pages contain only tuples that are known to be
visible to all active transactions. It's stored
alongside the main relation data in a separate relation fork, named after the
filenode number of the relation, plus a <TT
CLASS="LITERAL"
>_vm</TT
> suffix. For example,
if the filenode of a relation is 12345, the VM is stored in a file called
<TT
CLASS="FILENAME"
>12345_vm</TT
>, in the same directory as the main relation file.
Note that indexes do not have VMs.</P
><P
>The visibility map simply stores one bit per heap page. A set bit means
that all tuples on the page are known to be visible to all transactions.
This means that the page does not contain any tuples that need to be vacuumed.
This information can also be used by <I
CLASS="FIRSTTERM"
>index-only scans</I
> to answer
queries using only the index tuple.</P
><P
>The map is conservative in the sense that we make sure that whenever a bit is
set, we know the condition is true, but if a bit is not set, it might or
might not be true. Visibility map bits are only set by vacuum, but are
cleared by any data-modifying operations on a page.</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="storage-fsm.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="storage-init.html"
ACCESSKEY="N"
>Next</A
></TD
></TR
><TR
><TD
WIDTH="33%"
ALIGN="left"
VALIGN="top"
>Free Space Map</TD
><TD
WIDTH="34%"
ALIGN="center"
VALIGN="top"
><A
HREF="storage.html"
ACCESSKEY="U"
>Up</A
></TD
><TD
WIDTH="33%"
ALIGN="right"
VALIGN="top"
>The Initialization Fork</TD
></TR
></TABLE
></DIV
></BODY
></HTML
>
|