例子：convert('text_in_utf8', 'UTF8', 'LATIN1') = text_in_utf8 represented in ISO 8859-1 encoding

函数：convert_from(string bytea, src_encoding name)

说明：Convert string to the database encoding. The original encoding is specified by src_encoding. The string must be valid in this encoding. 转换字符串编码，自己要指定源编码,目标编码默认为 数据库指定编码，

例子：convert_from('text_in_utf8', 'UTF8') = text_in_utf8 represented in the current database encoding

函数：convert_to(string text, dest_encoding name)

说明：Convert string to dest_encoding.转换字符串编码，源编码默认为数据库指定编码，自己要指定目标编码,

例子：convert_to('some text', 'UTF8') = some text represented in the UTF8 encoding

函数：decode(string text, type text)

说明：Decode binary data from string previously encoded with encode. Parameter type is same as in encode. 对字符串按指定的类型进行解码

例子：decode('MTIzAAE=', 'base64') = 123\000\001

函数：encode(data bytea, type text)

说明：Encode binary data to different representation. Supported types are: base64, hex, escape. Escape merely outputs null bytes as \000 and doubles backslashes. 与decode相反，对字符串按指定类型进行编码

例子：encode(E'123\\000\\001', 'base64') = MTIzAAE=

函数：initcap(string)

说明：Convert the first letter of each word to uppercase and the rest to lowercase. Words are sequences of alphanumeric characters separated by non-alphanumeric characters. 将字符串所有的单词进行格式化，首字母大写，其它为小写

例子：initcap('hi THOMAS') = Hi Thomas

函数：length(string)

说明：Number of characters in string 讲算字符串长度

例子：length('jose') = 4

函数：length(stringbytea, encoding name )

说明：Number of characters in string in the given encoding. The string must be valid in this encoding. 计算字符串长度，指定字符串使用的编码

例子：length('jose', 'UTF8') = 4

函数：lpad(string text, length int [, fill text])

说明：Fill up the string to length length by prepending the characters fill (a space by default). If the string is already longer than length then it is truncated (on the right). 对字符串左边进行某类字符自动填充，即不足某一长度，则在左边自动补上指定的字符串，直至达到指定长度，可同时指定多个自动填充的字符

例子：lpad('hi', 5, 'xy') = xyxhi

函数：ltrim(string text [, characters text])

说明：Remove the longest string containing only characters from characters (a space by default) from the start of string 删除字符串左边某一些的字符，可以时指定多个要删除的字符

例子：trim

函数：md5(string)

说明：Calculates the MD5 hash of string, returning the result in hexadecimal 将字符串进行md5编码

例子：md5('abc') = 900150983cd24fb0 d6963f7d28e17f72

函数：pg_client_encoding()

说明：Current client encoding name 得到pg客户端编码

例子：pg_client_encoding() = SQL_ASCII

函数：quote_ident(string text)

说明：Return the given string suitably quoted to be used as an identifier in an SQL statement string. Quotes are added only if necessary (i.e., if the string contains non-identifier characters or would be case-folded). Embedded quotes are properly doubled. 对某一字符串加上两引号

例子：quote_ident('Foo bar') = "Foo bar"

函数：quote_literal(string text)

说明：Return the given string suitably quoted to be used as a string literal in an SQL statement string. Embedded single-quotes and backslashes are properly doubled. 对字符串里两边加上单引号，如果字符串里面出现sql编码的单个单引号，则会被表达成两个单引号

例子：quote_literal('O\'Reilly') = 'O''Reilly'

函数：quote_literal(value anyelement)