README.txt
1
2This directory contains source code for the SQLite "ICU" extension, an
3integration of the "International Components for Unicode" library with
4SQLite. Documentation follows.
5
6 1. Features
7
8 1.1 SQL Scalars upper() and lower()
9 1.2 Unicode Aware LIKE Operator
10 1.3 ICU Collation Sequences
11 1.4 SQL REGEXP Operator
12
13 2. Compilation and Usage
14
15 3. Bugs, Problems and Security Issues
16
17 3.1 The "case_sensitive_like" Pragma
18 3.2 The SQLITE_MAX_LIKE_PATTERN_LENGTH Macro
19 3.3 Collation Sequence Security Issue
20
21
221. FEATURES
23
24 1.1 SQL Scalars upper() and lower()
25
26 SQLite's built-in implementations of these two functions only
27 provide case mapping for the 26 letters used in the English
28 language. The ICU based functions provided by this extension
29 provide case mapping, where defined, for the full range of
30 unicode characters.
31
32 ICU provides two types of case mapping, "general" case mapping and
33 "language specific". Refer to ICU documentation for the differences
34 between the two. Specifically:
35
36 http://www.icu-project.org/userguide/caseMappings.html
37 http://www.icu-project.org/userguide/posix.html#case_mappings
38
39 To utilise "general" case mapping, the upper() or lower() scalar
40 functions are invoked with one argument:
41
42 upper('ABC') -> 'abc'
43 lower('abc') -> 'ABC'
44
45 To access ICU "language specific" case mapping, upper() or lower()
46 should be invoked with two arguments. The second argument is the name
47 of the locale to use. Passing an empty string ("") or SQL NULL value
48 as the second argument is the same as invoking the 1 argument version
49 of upper() or lower():
50
51 lower('I', 'en_us') -> 'i'
52 lower('I', 'tr_tr') -> 'ı' (small dotless i)
53
54 1.2 Unicode Aware LIKE Operator
55
56 Similarly to the upper() and lower() functions, the built-in SQLite LIKE
57 operator understands case equivalence for the 26 letters of the English
58 language alphabet. The implementation of LIKE included in this
59 extension uses the ICU function u_foldCase() to provide case
60 independent comparisons for the full range of unicode characters.
61
62 The U_FOLD_CASE_DEFAULT flag is passed to u_foldCase(), meaning the
63 dotless 'I' character used in the Turkish language is considered
64 to be in the same equivalence class as the dotted 'I' character
65 used by many languages (including English).
66
67 1.3 ICU Collation Sequences
68
69 A special SQL scalar function, icu_load_collation() is provided that
70 may be used to register ICU collation sequences with SQLite. It
71 is always called with exactly two arguments, the ICU locale
72 identifying the collation sequence to ICU, and the name of the
73 SQLite collation sequence to create. For example, to create an
74 SQLite collation sequence named "turkish" using Turkish language
75 sorting rules, the SQL statement:
76
77 SELECT icu_load_collation('tr_TR', 'turkish');
78
79 Or, for Australian English:
80
81 SELECT icu_load_collation('en_AU', 'australian');
82
83 The identifiers "turkish" and "australian" may then be used
84 as collation sequence identifiers in SQL statements:
85
86 CREATE TABLE aust_turkish_penpals(
87 australian_penpal_name TEXT COLLATE australian,
88 turkish_penpal_name TEXT COLLATE turkish
89 );
90
91 1.4 SQL REGEXP Operator
92
93 This extension provides an implementation of the SQL binary
94 comparision operator "REGEXP", based on the regular expression functions
95 provided by the ICU library. The syntax of the operator is as described
96 in SQLite documentation:
97
98 <string> REGEXP <re-pattern>
99
100 This extension uses the ICU defaults for regular expression matching
101 behaviour. Specifically, this means that:
102
103 * Matching is case-sensitive,
104 * Regular expression comments are not allowed within patterns, and
105 * The '^' and '$' characters match the beginning and end of the
106 <string> argument, not the beginning and end of lines within
107 the <string> argument.
108
109 Even more specifically, the value passed to the "flags" parameter
110 of ICU C function uregex_open() is 0.
111
112
1132 COMPILATION AND USAGE
114
115 The easiest way to compile and use the ICU extension is to build
116 and use it as a dynamically loadable SQLite extension. To do this
117 using gcc on *nix:
118
119 gcc -shared icu.c `icu-config --ldflags` -o libSqliteIcu.so
120
121 You may need to add "-I" flags so that gcc can find sqlite3ext.h
122 and sqlite3.h. The resulting shared lib, libSqliteIcu.so, may be
123 loaded into sqlite in the same way as any other dynamically loadable
124 extension.
125
126
1273 BUGS, PROBLEMS AND SECURITY ISSUES
128
129 3.1 The "case_sensitive_like" Pragma
130
131 This extension does not work well with the "case_sensitive_like"
132 pragma. If this pragma is used before the ICU extension is loaded,
133 then the pragma has no effect. If the pragma is used after the ICU
134 extension is loaded, then SQLite ignores the ICU implementation and
135 always uses the built-in LIKE operator.
136
137 The ICU extension LIKE operator is always case insensitive.
138
139 3.2 The SQLITE_MAX_LIKE_PATTERN_LENGTH Macro
140
141 Passing very long patterns to the built-in SQLite LIKE operator can
142 cause excessive CPU usage. To curb this problem, SQLite defines the
143 SQLITE_MAX_LIKE_PATTERN_LENGTH macro as the maximum length of a
144 pattern in bytes (irrespective of encoding). The default value is
145 defined in internal header file "limits.h".
146
147 The ICU extension LIKE implementation suffers from the same
148 problem and uses the same solution. However, since the ICU extension
149 code does not include the SQLite file "limits.h", modifying
150 the default value therein does not affect the ICU extension.
151 The default value of SQLITE_MAX_LIKE_PATTERN_LENGTH used by
152 the ICU extension LIKE operator is 50000, defined in source
153 file "icu.c".
154
155 3.3 Collation Sequence Security Issue
156
157 Internally, SQLite assumes that indices stored in database files
158 are sorted according to the collation sequence indicated by the
159 SQL schema. Changing the definition of a collation sequence after
160 an index has been built is therefore equivalent to database
161 corruption. The SQLite library is not very well tested under
162 these conditions, and may contain potential buffer overruns
163 or other programming errors that could be exploited by a malicious
164 programmer.
165
166 If the ICU extension is used in an environment where potentially
167 malicious users may execute arbitrary SQL (i.e. gears), they
168 should be prevented from invoking the icu_load_collation() function,
169 possibly using the authorisation callback.
170