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169 lines
6.6 KiB
Text
169 lines
6.6 KiB
Text
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This directory contains source code for the SQLite "ICU" extension, an
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integration of the "International Components for Unicode" library with
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SQLite. Documentation follows.
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1. Features
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1.1 SQL Scalars upper() and lower()
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1.2 Unicode Aware LIKE Operator
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1.3 ICU Collation Sequences
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1.4 SQL REGEXP Operator
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2. Compilation and Usage
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3. Bugs, Problems and Security Issues
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3.1 The "case_sensitive_like" Pragma
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3.2 The SQLITE_MAX_LIKE_PATTERN_LENGTH Macro
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3.3 Collation Sequence Security Issue
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1. FEATURES
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1.1 SQL Scalars upper() and lower()
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SQLite's built-in implementations of these two functions only
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provide case mapping for the 26 letters used in the English
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language. The ICU based functions provided by this extension
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provide case mapping, where defined, for the full range of
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unicode characters.
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ICU provides two types of case mapping, "general" case mapping and
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"language specific". Refer to ICU documentation for the differences
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between the two. Specifically:
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http://www.icu-project.org/userguide/caseMappings.html
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http://www.icu-project.org/userguide/posix.html#case_mappings
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To utilise "general" case mapping, the upper() or lower() scalar
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functions are invoked with one argument:
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upper('ABC') -> 'abc'
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lower('abc') -> 'ABC'
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To access ICU "language specific" case mapping, upper() or lower()
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should be invoked with two arguments. The second argument is the name
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of the locale to use. Passing an empty string ("") or SQL NULL value
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as the second argument is the same as invoking the 1 argument version
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of upper() or lower():
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lower('I', 'en_us') -> 'i'
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lower('I', 'tr_tr') -> 'ı' (small dotless i)
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1.2 Unicode Aware LIKE Operator
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Similarly to the upper() and lower() functions, the built-in SQLite LIKE
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operator understands case equivalence for the 26 letters of the English
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language alphabet. The implementation of LIKE included in this
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extension uses the ICU function u_foldCase() to provide case
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independent comparisons for the full range of unicode characters.
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The U_FOLD_CASE_DEFAULT flag is passed to u_foldCase(), meaning the
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dotless 'I' character used in the Turkish language is considered
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to be in the same equivalence class as the dotted 'I' character
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used by many languages (including English).
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1.3 ICU Collation Sequences
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A special SQL scalar function, icu_load_collation() is provided that
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may be used to register ICU collation sequences with SQLite. It
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is always called with exactly two arguments, the ICU locale
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identifying the collation sequence to ICU, and the name of the
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SQLite collation sequence to create. For example, to create an
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SQLite collation sequence named "turkish" using Turkish language
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sorting rules, the SQL statement:
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SELECT icu_load_collation('tr_TR', 'turkish');
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Or, for Australian English:
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SELECT icu_load_collation('en_AU', 'australian');
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The identifiers "turkish" and "australian" may then be used
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as collation sequence identifiers in SQL statements:
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CREATE TABLE aust_turkish_penpals(
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australian_penpal_name TEXT COLLATE australian,
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turkish_penpal_name TEXT COLLATE turkish
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);
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1.4 SQL REGEXP Operator
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This extension provides an implementation of the SQL binary
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comparision operator "REGEXP", based on the regular expression functions
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provided by the ICU library. The syntax of the operator is as described
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in SQLite documentation:
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<string> REGEXP <re-pattern>
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This extension uses the ICU defaults for regular expression matching
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behaviour. Specifically, this means that:
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* Matching is case-sensitive,
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* Regular expression comments are not allowed within patterns, and
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* The '^' and '$' characters match the beginning and end of the
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<string> argument, not the beginning and end of lines within
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the <string> argument.
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Even more specifically, the value passed to the "flags" parameter
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of ICU C function uregex_open() is 0.
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2 COMPILATION AND USAGE
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The easiest way to compile and use the ICU extension is to build
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and use it as a dynamically loadable SQLite extension. To do this
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using gcc on *nix:
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gcc -shared icu.c `icu-config --ldflags` -o libSqliteIcu.so
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You may need to add "-I" flags so that gcc can find sqlite3ext.h
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and sqlite3.h. The resulting shared lib, libSqliteIcu.so, may be
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loaded into sqlite in the same way as any other dynamically loadable
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extension.
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3 BUGS, PROBLEMS AND SECURITY ISSUES
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3.1 The "case_sensitive_like" Pragma
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This extension does not work well with the "case_sensitive_like"
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pragma. If this pragma is used before the ICU extension is loaded,
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then the pragma has no effect. If the pragma is used after the ICU
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extension is loaded, then SQLite ignores the ICU implementation and
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always uses the built-in LIKE operator.
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The ICU extension LIKE operator is always case insensitive.
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3.2 The SQLITE_MAX_LIKE_PATTERN_LENGTH Macro
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Passing very long patterns to the built-in SQLite LIKE operator can
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cause excessive CPU usage. To curb this problem, SQLite defines the
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SQLITE_MAX_LIKE_PATTERN_LENGTH macro as the maximum length of a
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pattern in bytes (irrespective of encoding). The default value is
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defined in internal header file "limits.h".
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The ICU extension LIKE implementation suffers from the same
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problem and uses the same solution. However, since the ICU extension
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code does not include the SQLite file "limits.h", modifying
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the default value therein does not affect the ICU extension.
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The default value of SQLITE_MAX_LIKE_PATTERN_LENGTH used by
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the ICU extension LIKE operator is 50000, defined in source
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file "icu.c".
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3.3 Collation Sequence Security Issue
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Internally, SQLite assumes that indices stored in database files
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are sorted according to the collation sequence indicated by the
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SQL schema. Changing the definition of a collation sequence after
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an index has been built is therefore equivalent to database
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corruption. The SQLite library is not very well tested under
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these conditions, and may contain potential buffer overruns
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or other programming errors that could be exploited by a malicious
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programmer.
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If the ICU extension is used in an environment where potentially
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malicious users may execute arbitrary SQL (i.e. gears), they
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should be prevented from invoking the icu_load_collation() function,
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possibly using the authorisation callback.
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