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The PostgreSQL 9.0 Reference Manual - Volume 1A - SQL Language Reference
by The PostgreSQL Global Development Group
Paperback (6"x9"), 454 pages
ISBN 9781906966041
RRP £14.95 ($19.95)

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7.5 Binary String Functions and Operators

This section describes functions and operators for examining and manipulating values of type bytea.

SQL defines some string functions that use key words, rather than commas, to separate arguments. Details are in Table 7-8. PostgreSQL also provides versions of these functions that use the regular function invocation syntax (see Table 7-9).

Table 7-8: SQL Binary String Functions and Operators
Function Return Type Description
string || string bytea String concatenation
e.g. E'\\\\Post'::bytea || E'\\047gres\\000'::bytea => \\Post'gres\000
octet_length(string) int Number of bytes in binary string
e.g. octet_length(E'jo\\000se'::bytea) => 5
overlay(string placing string from int [for int]) bytea Replace substring
e.g. overlay(E'Th\\000omas'::bytea placing E'\\002\\003'::bytea from 2 for 3) => T\\002\\003mas
position(substring in string) int Location of specified substring
e.g. position(E'\\000om'::bytea in E'Th\\000omas'::bytea) => 3
substring(string [from int] [for int]) bytea Extract substring
e.g. substring(E'Th\\000omas'::bytea from 2 for 3) => h\000o
trim([both] bytes from string) bytea Remove the longest string containing only the bytes in bytes from the start and end of string
e.g. trim(E'\\000'::bytea from E'\\000Tom\\000'::bytea) => Tom

Additional binary string manipulation functions are available and are listed in Table 7-9. Some of them are used internally to implement the SQL-standard string functions listed in Table 7-8.

Table 7-9: Other Binary String Functions
Function Return Type Description
btrim(string bytea, bytes bytea) bytea Remove the longest string consisting only of bytes in bytes from the start and end of string
e.g. btrim(E'\\000trim\\000'::bytea, E'\\000'::bytea) => trim
decode(string text, type text) bytea Decode binary string from string previously encoded with encode. Parameter type is same as in encode.
e.g. decode(E'123\\000456', 'escape') => 123\000456
encode(string bytea, type text) text Encode binary string to ASCII-only representation. Supported types are: base64, hex, escape.
e.g. encode(E'123\\000456'::bytea, 'escape') => 123\000456
get_bit(string, offset) int Extract bit from string
e.g. get_bit(E'Th\\000omas'::bytea, 45) => 1
get_byte(string, offset) int Extract byte from string
e.g. get_byte(E'Th\\000omas'::bytea, 4) => 109
length(string) int Length of binary string
e.g. length(E'jo\\000se'::bytea) => 5
md5(string) text Calculates the MD5 hash of string, returning the result in hexadecimal
e.g. md5(E'Th\\000omas'::bytea) => 8ab2d3c9689aaf18 b4958c334c82d8b1
set_bit(string, offset, newvalue) bytea Set bit in string
e.g. set_bit(E'Th\\000omas'::bytea, 45, 0) => Th\000omAs
set_byte(string, offset, newvalue) bytea Set byte in string
e.g. set_byte(E'Th\\000omas'::bytea, 4, 64) => Th\000o@as

get_byte and set_byte number the first byte of a binary string as byte 0. get_bit and set_bit number bits from the right within each byte; for example bit 0 is the least significant bit of the first byte, and bit 15 is the most significant bit of the second byte.

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