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8.3. 字符类型

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表 8.4. 字符类型

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\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n
名字说明
character varying(n), varchar(n)有长度限制的变长
character(n), char(n), bpchar(n)定长，空格填充
bpchar无限长度的变长，忽略尾随空格
文本无限长度的变长
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表 8.4显示了在PostgreSQL里可用的一般用途的字符类型。

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SQL 定义了两种主要字符类型：character varying(n) 和 character(n)，其中 n 是正整数。这两种类型都可以存储长度最多为 n 个字符（而不是字节）的字符串。试图把更长的字符串存入这些类型的列时会报错，除非超出的字符全部都是空格，在这种情况下字符串会被截断到最大长度。（这一多少有些奇怪的例外是 SQL 标准要求的。）不过，如果显式把一个值转换为 character varying(n) 或 character(n)，那么超长的值会被截断为 n 个字符而不会报错。（这同样是 SQL 标准要求的。）如果要存储的字符串短于声明长度，character 类型的值会用空格填充；character varying 类型则只是简单地存储较短的字符串。

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此外，PostgreSQL 还提供 text 类型，用于存储任意长度的字符串。虽然 text 类型不在 SQL 标准中，但其他若干 SQL 数据库管理系统也提供了它。text 是 PostgreSQL 的原生字符串数据类型，因为大多数对字符串进行操作的内置函数都声明为接受或返回 text，而不是 character varying。在很多场景下，character varying 的行为就像是建立在 text 之上的一个域。

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类型名 varchar 是 character varying 的别名，而 bpchar（带长度说明时）和 char 是 character 的别名。varchar 和 char 这两个别名由 SQL 标准定义；bpchar 则是 PostgreSQL 的扩展。

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若指定长度 n，则它必须大于零且不能超过 10,485,760。若 character varying（或 varchar）没有长度限定，则接受任意长度的字符串。若 bpchar 没有长度限定，也接受任意长度的字符串，但尾部空格在语义上不重要。若 character（或 char）没有长度限定，则等价于 character(1)。

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character 类型的值在物理存储时会在右侧用空格填充到指定宽度 n，并且也会按这种形式存储和显示。但是，在比较两个 character 值时，尾随空格在语义上被视为不重要并会被忽略。在空白字符有区分意义的排序规则中，这种行为可能产生意外结果；例如，SELECT 'a '::CHAR(2) collate \"C\" < E'a\\n'::CHAR(2) 会返回真，即使 C 区域设置认为空格大于换行符。把 character 值转换为其他字符串类型之一时，尾随空格会被移除。请注意，在 character varying 和 text 值中，以及进行模式匹配（即 LIKE 和正则表达式）时，尾随空格在语义上是有意义的。

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可以存储在这些数据类型中的字符由数据库字符集确定，该数据库字符集在创建数据库时选择。无论特定的字符集是什么，都无法存储编码值为零的字符（有时称为NUL）。有关更多信息，请参阅第 23.3 节。

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短字符串（最多 126 字节）的存储空间为实际字符串大小加 1 字节；对于 character，实际字符串包含补齐的空格。更长的字符串开销为 4 字节，而非 1 字节。系统会自动压缩长字符串，因此实际磁盘占用可能更小。很长的值还会存储在后台表中，避免影响较短列值的快速访问。无论如何，可存储字符串的最大长度约为 1 GB。（类型声明中 n 的最大允许值更小。改变这一点没有意义，因为多字节编码中字符数与字节数可能相差很大。如果希望存储没有明确上限的长字符串，应使用 text 或不带长度的 character varying，而非随意指定一个长度上限。）

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提示

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这三种类型之间没有性能差别，除了使用空格填充类型时会占用更多存储空间，以及在写入带长度约束的列时需要少量额外 CPU 周期来检查长度。虽然在某些其他数据库系统中，character(n) 可能有一定性能优势，但在 PostgreSQL 中并不存在这种优势。事实上，由于额外的存储开销，character(n) 通常反而是三者中最慢的。大多数情况下，应优先使用 text 或 character varying。

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关于字符串常量的语法，请参见第 4.1.2.1 节；关于可用的操作符和函数，请参见第 9 章。

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例 8.1. 使用字符类型

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CREATE TABLE test1 (a character(4));\nINSERT INTO test1 VALUES ('ok');\nSELECT a, char_length(a) FROM test1; -- (1)\n\n  a   | char_length\n------+-------------\n ok   |           2\n\n\nCREATE TABLE test2 (b varchar(5));\nINSERT INTO test2 VALUES ('ok');\nINSERT INTO test2 VALUES ('good      ');\nINSERT INTO test2 VALUES ('too long');\nERROR:  value too long for type character varying(5)\nINSERT INTO test2 VALUES ('too long'::varchar(5)); -- explicit truncation\nSELECT b, char_length(b) FROM test2;\n\n   b   | char_length\n-------+-------------\n ok    |           2\n good  |           5\n too l |           5\n\n
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\n\n\n\n\n\n
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(1)

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函数char_length在第 9.4 节中讨论。

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PostgreSQL 还有另外两种定长字符类型，见表 8.5。它们只用于内部系统目录，不适合通用用途。name 用来存储标识符，当前长度定义为 64 字节，即 63 个可用字符加终止符，但 C 源码中应使用 NAMEDATALEN 常量引用。长度在编译时设定，因此可为特殊用途调整；默认最大长度可能在未来版本中变化。\"char\" 类型（注意双引号）与 char(1) 不同，只占一个字节，因此只能存储一个 ASCII 字符；它在系统目录中用作简单的枚举类型。

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表 8.5. 特殊字符类型

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\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n
名字存储大小说明
\"char\"1 字节单字节内部类型
name64 字节用于对象名的内部类型
\n
\n

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