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
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
|
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<HTML
><HEAD
><TITLE
>Indices and Keys</TITLE
><META
NAME="GENERATOR"
CONTENT="Modular DocBook HTML Stylesheet version 1.19"><LINK
REL="HOME"
TITLE="PostgreSQL User's Guide"
HREF="user.html"><LINK
REL="PREVIOUS"
TITLE="UNION Queries"
HREF="typeconv2704.html"><LINK
REL="NEXT"
TITLE="Arrays"
HREF="arrays.html"></HEAD
><BODY
BGCOLOR="#FFFFFF"
TEXT="#000000"
><DIV
CLASS="NAVHEADER"
><TABLE
WIDTH="100%"
BORDER="0"
CELLPADDING="0"
CELLSPACING="0"
><TR
><TH
COLSPAN="3"
ALIGN="center"
>PostgreSQL User's Guide</TH
></TR
><TR
><TD
WIDTH="10%"
ALIGN="left"
VALIGN="bottom"
><A
HREF="typeconv2704.html"
>Prev</A
></TD
><TD
WIDTH="80%"
ALIGN="center"
VALIGN="bottom"
></TD
><TD
WIDTH="10%"
ALIGN="right"
VALIGN="bottom"
><A
HREF="arrays.html"
>Next</A
></TD
></TR
></TABLE
><HR
ALIGN="LEFT"
WIDTH="100%"></DIV
><H1
><A
NAME="KEYS"
>Chapter 9. Indices and Keys</A
></H1
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Author: </B
>Written by
<A
HREF="herouth@oumail.openu.ac.il"
TARGET="_top"
>Herouth Maoz</A
></P
></BLOCKQUOTE
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Editor's Note: </B
>This originally appeared on the mailing list
in response to the question:
"What is the difference between PRIMARY KEY and UNIQUE constraints?".</P
></BLOCKQUOTE
><PRE
CLASS="PROGRAMLISTING"
>Subject: Re: [QUESTIONS] PRIMARY KEY | UNIQUE
What's the difference between:
PRIMARY KEY(fields,...) and
UNIQUE (fields,...)
- Is this an alias?
- If PRIMARY KEY is already unique, then why
is there another kind of key named UNIQUE?</PRE
><P
>A primary key is the field(s) used to identify a specific row. For example,
Social Security numbers identifying a person.</P
><P
>A simply UNIQUE combination of fields has nothing to do with identifying
the row. It's simply an integrity constraint. For example, I have
collections of links. Each collection is identified by a unique number,
which is the primary key. This key is used in relations.</P
><P
>However, my application requires that each collection will also have a
unique name. Why? So that a human being who wants to modify a collection
will be able to identify it. It's much harder to know, if you have two
collections named "Life Science", the the one tagged 24433 is the one you
need, and the one tagged 29882 is not.</P
><P
>So, the user selects the collection by its name. We therefore make sure,
withing the database, that names are unique. However, no other table in the
database relates to the collections table by the collection Name. That
would be very inefficient.</P
><P
>Moreover, despite being unique, the collection name does not actually
define the collection! For example, if somebody decided to change the name
of the collection from "Life Science" to "Biology", it will still be the
same collection, only with a different name. As long as the name is unique,
that's OK.</P
><P
>So:
<P
></P
></P><UL
><LI
><P
>Primary key:
<P
></P
></P><UL
COMPACT="COMPACT"
><LI
STYLE="list-style-type: disc"
><P
>Is used for identifying the row and relating to it.</P
></LI
><LI
STYLE="list-style-type: disc"
><P
>Is impossible (or hard) to update.</P
></LI
><LI
STYLE="list-style-type: disc"
><P
>Should not allow NULLs.</P
></LI
></UL
><P> </P
></LI
><LI
><P
>Unique field(s):
<P
></P
></P><UL
COMPACT="COMPACT"
><LI
STYLE="list-style-type: disc"
><P
>Are used as an alternative access to the row.</P
></LI
><LI
STYLE="list-style-type: disc"
><P
>Are updateable, so long as they are kept unique.</P
></LI
><LI
STYLE="list-style-type: disc"
><P
>NULLs are acceptable.</P
></LI
></UL
><P></P
></LI
></UL
><P> </P
><P
>As for why no non-unique keys are defined explicitly in standard <SPAN
CLASS="ACRONYM"
>SQL</SPAN
> syntax?
Well, you
must understand that indices are implementation-dependent. <SPAN
CLASS="ACRONYM"
>SQL</SPAN
> does not
define the implementation, merely the relations between data in the
database. <SPAN
CLASS="PRODUCTNAME"
>Postgres</SPAN
> does allow non-unique indices, but indices
used to enforce <SPAN
CLASS="ACRONYM"
>SQL</SPAN
> keys are always unique.</P
><P
>Thus, you may query a table by any combination of its columns, despite the
fact that you don't have an index on these columns. The indexes are merely
an implementational aid which each <SPAN
CLASS="ACRONYM"
>RDBMS</SPAN
> offers you, in order to cause
commonly used queries to be done more efficiently. Some <SPAN
CLASS="ACRONYM"
>RDBMS</SPAN
> may give you
additional measures, such as keeping a key stored in main memory. They will
have a special command, for example
<PRE
CLASS="PROGRAMLISTING"
>CREATE MEMSTORE ON <table> COLUMNS <cols></PRE
>
(this is not an existing command, just an example).</P
><P
>In fact, when you create a primary key or a unique combination of fields,
nowhere in the <SPAN
CLASS="ACRONYM"
>SQL</SPAN
> specification does it say that an index is created, nor that
the retrieval of data by the key is going to be more efficient than a
sequential scan!</P
><P
>So, if you want to use a combination of fields which is not unique as a
secondary key, you really don't have to specify anything - just start
retrieving by that combination! However, if you want to make the retrieval
efficient, you'll have to resort to the means your <SPAN
CLASS="ACRONYM"
>RDBMS</SPAN
> provider gives you
- be it an index, my imaginary MEMSTORE command, or an intelligent <SPAN
CLASS="ACRONYM"
>RDBMS</SPAN
>
which creates indices without your knowledge based on the fact that you have
sent it many queries based on a specific combination of keys... (It learns
from experience).</P
><DIV
CLASS="NAVFOOTER"
><HR
ALIGN="LEFT"
WIDTH="100%"><TABLE
WIDTH="100%"
BORDER="0"
CELLPADDING="0"
CELLSPACING="0"
><TR
><TD
WIDTH="33%"
ALIGN="left"
VALIGN="top"
><A
HREF="typeconv2704.html"
>Prev</A
></TD
><TD
WIDTH="34%"
ALIGN="center"
VALIGN="top"
><A
HREF="user.html"
>Home</A
></TD
><TD
WIDTH="33%"
ALIGN="right"
VALIGN="top"
><A
HREF="arrays.html"
>Next</A
></TD
></TR
><TR
><TD
WIDTH="33%"
ALIGN="left"
VALIGN="top"
>UNION Queries</TD
><TD
WIDTH="34%"
ALIGN="center"
VALIGN="top"
>;</TD
><TD
WIDTH="33%"
ALIGN="right"
VALIGN="top"
>Arrays</TD
></TR
></TABLE
></DIV
></BODY
></HTML
>
|
