3.5. Character Data Types
For working with character data, Firebird has the fixed-length CHAR and the variable-length VARCHAR data types.
The maximum size of text data stored in these data types is 32,767 bytes for CHAR and 32,765 bytes for VARCHAR.
The maximum number of characters that will fit within these limits depends on the CHARACTER SET being used for the data under consideration.
The collation sequence does not affect this maximum, although it may affect the maximum size of any index that involves the column.
If no character set is explicitly specified when defining a character object, the default character set specified when the database was created will be used.
If the database does not have a default character set defined, the field gets the character set NONE.
3.5.1. Unicode
Most current development tools support Unicode, implemented in Firebird with the character sets UTF8 and UNICODE_FSS. UTF8 comes with collations for many languages.
UNICODE_FSS is more limited and is used mainly by Firebird internally for storing metadata.
Keep in mind that one UTF8 character occupies up to 4 bytes, thus limiting the size of CHAR fields to 8,191 characters (32,767/4).
The actual bytes per character
value depends on the range the character belongs to.
Non-accented Latin letters occupy 1 byte, Cyrillic letters from the WIN1251 encoding occupy 2 bytes in UTF8, characters from other encodings may occupy up to 4 bytes.
The UTF8 character set implemented in Firebird supports the latest version of the Unicode standard, thus recommending its use for international databases.
3.5.2. Client Character Set
While working with strings, it is essential to keep the character set of the client connection in mind.
If there is a mismatch between the character sets of the stored data and that of the client connection, the output results for string columns are automatically re-encoded, both when data are sent from the client to the server and when they are sent back from the server to the client.
For example, if the database was created in the WIN1251 encoding but KOI8R or UTF8 is specified in the client’s connection parameters, the mismatch will be transparent.
3.5.3. Special Character Sets
Character set NONEThe character set NONE is a special character set in Firebird.
It can be characterized such that each byte is a part of a string, but the string is stored in the system without any clues about what constitutes any character: character encoding, collation, case, etc. are simply unknown.
It is the responsibility of the client application to deal with the data and provide the means to interpret the string of bytes in some way that is meaningful to the application and the human user.
Character set OCTETSData in OCTETS encoding are treated as bytes that may not actually be interpreted as characters.
OCTETS provides a way to store binary data, which could be the results of some Firebird functions.
The database engine has no concept of what it is meant to do with a string of bits in OCTETS, other than just store it and retrieve it.
Again, the client side is responsible for validating the data, presenting them in formats that are meaningful to the application and its users and handling any exceptions arising from decoding and encoding them.
3.5.4. Collation Sequence
Each character set has a default collation sequence (COLLATE) that specifies the collation order.
Usually, it provides nothing more than ordering based on the numeric code of the characters and a basic mapping of upper- and lower-case characters.
If some behaviour is needed for strings that is not provided by the default collation sequence and a suitable alternative collation is supported for that character set, a COLLATE clause can be specified in the column definition.collation
A COLLATE clause can be applied in other contexts besides the column definition.
For greater-than/less-than comparison operations, it can be added in the collationWHERE clause of a SELECT statement.
If output needs to be sorted in a special alphabetic sequence, or case-insensitively, and the appropriate collation exists, then a COLLATE clause can be included with the ORDER BY clause when rows are being sorted on a character field and with the GROUP BY clause in case of grouping operations.
3.5.4.1. Case-Insensitive Searching
For a case-insensitive search, the UPPER function could be used to convert both the search argument and the searched strings to upper-case before attempting a match:
|…|where upper(name) = upper(:flt_name)
For strings in a character set that has a case-insensitive collation available, you can simply apply the collation, to compare the search argument and the searched strings directly.
For example, using the WIN1251 character set, the collation PXW_CYRL is case-insensitive for this purpose:
|…|WHERE FIRST_NAME COLLATE PXW_CYRL >= :FLT_NAME|…|ORDER BY NAME COLLATE PXW_CYRL
See alsoCONTAINING
3.5.4.2. UTF8 Collation Sequences
The following table shows the possible collation sequences for the UTF8 character set.
| Collation | Characteristics |
|---|---|
| Collation works according to the position of the character in the table (binary). Added in Firebird 2.0 |
| Collation works according to the UCA algorithm (Unicode Collation Algorithm) (alphabetical). Added in Firebird 2.0 |
| The default, binary collation, identical to |
| Case-insensitive collation, works without taking character case into account. Added in Firebird 2.1 |
| Case-insensitive, accent-insensitive collation, works alphabetically without taking character case or accents into account. Added in Firebird 2.5 |
ExampleAn example of collation for the UTF8 character set without taking into account the case or accentuation of characters (similar to COLLATE PXW_CYRL).
|...|ORDER BY NAME COLLATE UNICODE_CI_AI
3.5.5. Character Indexes
In Firebird earlier than version 2.0, a problem can occur with building an index for character columns that use a non-standard collation sequence: the length of an indexed field is limited to 252 bytes with no COLLATE specified or 84 bytes if COLLATE is specified.
Multi-byte character sets and compound indexes limit the size even further.
Starting from Firebird 2.0, the maximum length for an index equals one quarter of the page size, i.e. from 1,024 — for page size 4,096 — to 4,096 bytes — for page size 16,384. The maximum length of an indexed string is 9 bytes less than that quarter-page limit.
Calculating Maximum Length of an Indexed String FieldThe following formula calculates the maximum length of an indexed string (in characters):
|max_char_length = FLOOR((page_size / 4 - 9) / N)
where N is the number of bytes per character in the character set.
The table below shows the maximum length of an indexed string (in characters), according to page size and character set, calculated using this formula.
Page Size | Bytes per character | ||||
1 | 2 | 3 | 4 | 6 | |
4,096 | 1,015 | 507 | 338 | 253 | 169 |
8,192 | 2,039 | 1,019 | 679 | 509 | 339 |
16,384 | 4,087 | 2,043 | 1,362 | 1,021 | 682 |
With case-insensitive collations (_CI
), one character in the index will occupy not 4, but 6 (six) bytes, so the maximum key length for a page of — for example — 4,096 bytes, will be 169 characters.
See alsoCREATE DATABASE, Collation sequence, SELECT, WHERE, GROUP BY, ORDER BY
3.5.6. Character Types in Detail
3.5.6.1. CHAR
CHAR is a fixed-length data type.
If the entered number of characters is less than the declared length, trailing spaces will be added to the field.
Generally, the pad character does not have to be a space: it depends on the character set.
For example, the pad character for the OCTETS character set is zero.
The full name of this data type is CHARACTER, but there is no requirement to use full names and people rarely do so.
Fixed-length character data can be used to store codes whose length is standard and has a definite width
in directories.
An example of such a code is an EAN13 barcode — 13 characters, all filled.
Declaration Syntax
|
{ CHAR | CHARACTER } [ (length) ]
| [CHARACTER SET <set>] [COLLATE <name>]
If no length is specified, it is taken to be 1.
A valid length is from 1 to the maximum number of characters that can be accommodated within 32,767 bytes.
3.5.6.2. VARCHAR
VARCHAR is the basic string type for storing texts of variable length, up to a maximum of 32,765 bytes.
The stored structure is equal to the actual size of the data plus 2 bytes where the length of the data is recorded.
All characters that are sent from the client application to the database are considered meaningful, including the leading and trailing spaces. However, trailing spaces are not stored: they will be restored upon retrieval, up to the recorded length of the string.
The full name of this type is CHARACTER VARYING.
Another variant of the name is written as CHAR VARYING.
Syntax
|
{ VARCHAR | CHAR VARYING | CHARACTER VARYING } (length)
| [CHARACTER SET <set>] [COLLATE <name>]
3.5.6.3. NCHAR
NCHAR is a fixed-length character data type with the ISO8859_1 character set predefined.
In all other respects it is the same as CHAR.
Syntax
|{ NCHAR | NATIONAL { CHAR | CHARACTER } } [ (length) ]
A similar data type is available for the variable-length string type: NATIONAL CHARACTER VARYING.