9.5. Binary String Functions and Operators #
This section describes functions and operators for examining and manipulating binary strings, that is values of type bytea. Many of these are equivalent, in purpose and syntax, to the text-string functions described in the previous section.
SQL defines some string functions that use key words, rather than commas, to separate arguments. Details are in Table 9.11. Postgres Pro also provides versions of these functions that use the regular function invocation syntax (see Table 9.12).
Table 9.11. SQL Binary String Functions and Operators
Function/Operator Description Example(s) |
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bytea || bytea → bytea Concatenates the two binary strings. '\x123456'::bytea || '\x789a00bcde'::bytea → \x123456789a00bcde
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bit_length ( bytea ) → integer Returns number of bits in the binary string (8 times the octet_length). bit_length('\x123456'::bytea) → 24
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btrim ( bytes bytea, bytesremoved bytea ) → bytea Removes the longest string containing only bytes appearing in bytesremoved from the start and end of bytes. btrim('\x1234567890'::bytea, '\x9012'::bytea) → \x345678
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ltrim ( bytes bytea, bytesremoved bytea ) → bytea Removes the longest string containing only bytes appearing in bytesremoved from the start of bytes. ltrim('\x1234567890'::bytea, '\x9012'::bytea) → \x34567890
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octet_length ( bytea ) → integer Returns number of bytes in the binary string. octet_length('\x123456'::bytea) → 3
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overlay ( bytes bytea PLACING newsubstring bytea FROM start integer [ FOR count integer ] ) → bytea Replaces the substring of bytes that starts at the start'th byte and extends for count bytes with newsubstring. If count is omitted, it defaults to the length of newsubstring. overlay('\x1234567890'::bytea placing '\002\003'::bytea from 2 for 3) → \x12020390
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position ( substring bytea IN bytes bytea ) → integer Returns first starting index of the specified substring within bytes, or zero if it's not present. position('\x5678'::bytea in '\x1234567890'::bytea) → 3
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rtrim ( bytes bytea, bytesremoved bytea ) → bytea Removes the longest string containing only bytes appearing in bytesremoved from the end of bytes. rtrim('\x1234567890'::bytea, '\x9012'::bytea) → \x12345678
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substring ( bytes bytea [ FROM start integer ] [ FOR count integer ] ) → bytea Extracts the substring of bytes starting at the start'th byte if that is specified, and stopping after count bytes if that is specified. Provide at least one of start and count. substring('\x1234567890'::bytea from 3 for 2) → \x5678
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trim ( [ LEADING | TRAILING | BOTH ] bytesremoved bytea FROM bytes bytea ) → bytea Removes the longest string containing only bytes appearing in bytesremoved from the start, end, or both ends (BOTH is the default) of bytes. trim('\x9012'::bytea from '\x1234567890'::bytea) → \x345678
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trim ( [ LEADING | TRAILING | BOTH ] [ FROM ] bytes bytea, bytesremoved bytea ) → bytea
This is a non-standard syntax for trim(). trim(both from '\x1234567890'::bytea, '\x9012'::bytea) → \x345678
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Additional binary string manipulation functions are available and are listed in Table 9.12. Some of them are used internally to implement the SQL-standard string functions listed in Table 9.11.
Table 9.12. Other Binary String Functions
Function Description Example(s) |
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bit_count ( bytes bytea ) → bigint Returns the number of bits set in the binary string (also known as “popcount”). bit_count('\x1234567890'::bytea) → 15
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get_bit ( bytes bytea, n bigint ) → integer Extracts n'th bit from binary string. get_bit('\x1234567890'::bytea, 30) → 1
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get_byte ( bytes bytea, n integer ) → integer Extracts n'th byte from binary string. get_byte('\x1234567890'::bytea, 4) → 144
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length ( bytea ) → integer Returns the number of bytes in the binary string. length('\x1234567890'::bytea) → 5
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length ( bytes bytea, encoding name ) → integer
Returns the number of characters in the binary string, assuming that it is text in the given encoding. length('jose'::bytea, 'UTF8') → 4
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md5 ( bytea ) → text Computes the MD5 hash of the binary string, with the result written in hexadecimal. md5('Th\000omas'::bytea) → 8ab2d3c9689aaf18b4958c334c82d8b1
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set_bit ( bytes bytea, n bigint, newvalue integer ) → bytea Sets n'th bit in binary string to newvalue. set_bit('\x1234567890'::bytea, 30, 0) → \x1234563890
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set_byte ( bytes bytea, n integer, newvalue integer ) → bytea Sets n'th byte in binary string to newvalue. set_byte('\x1234567890'::bytea, 4, 64) → \x1234567840
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sha224 ( bytea ) → bytea Computes the SHA-224 hash of the binary string. sha224('abc'::bytea) → \x23097d223405d8228642a477bda255b32aadbce4bda0b3f7e36c9da7
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sha256 ( bytea ) → bytea Computes the SHA-256 hash of the binary string. sha256('abc'::bytea) → \xba7816bf8f01cfea414140de5dae2223b00361a396177a9cb410ff61f20015ad
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sha384 ( bytea ) → bytea Computes the SHA-384 hash of the binary string. sha384('abc'::bytea) → \xcb00753f45a35e8bb5a03d699ac65007272c32ab0eded1631a8b605a43ff5bed8086072ba1e7cc2358baeca134c825a7
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sha512 ( bytea ) → bytea Computes the SHA-512 hash of the binary string. sha512('abc'::bytea) → \xddaf35a193617abacc417349ae20413112e6fa4e89a97ea20a9eeee64b55d39a2192992a274fc1a836ba3c23a3feebbd454d4423643ce80e2a9ac94fa54ca49f
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substr ( bytes bytea, start integer [, count integer ] ) → bytea Extracts the substring of bytes starting at the start'th byte, and extending for count bytes if that is specified. (Same as substring(bytes from start for count).) substr('\x1234567890'::bytea, 3, 2) → \x5678
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Functions get_byte and set_byte number the first byte of a binary string as byte 0. Functions 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.
For historical reasons, the function md5 returns a hex-encoded value of type text whereas the SHA-2 functions return type bytea. Use the functions encode and decode to convert between the two. For example write encode(sha256('abc'), 'hex') to get a hex-encoded text representation, or decode(md5('abc'), 'hex') to get a bytea value.
Functions for converting strings between different character sets (encodings), and for representing arbitrary binary data in textual form, are shown in Table 9.13. For these functions, an argument or result of type text is expressed in the database's default encoding, while arguments or results of type bytea are in an encoding named by another argument.
Table 9.13. Text/Binary String Conversion Functions
Function Description Example(s) |
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convert ( bytes bytea, src_encoding name, dest_encoding name ) → bytea Converts a binary string representing text in encoding src_encoding to a binary string in encoding dest_encoding (see Section 22.3.4 for available conversions). convert('text_in_utf8', 'UTF8', 'LATIN1') → \x746578745f696e5f75746638
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convert_from ( bytes bytea, src_encoding name ) → text Converts a binary string representing text in encoding src_encoding to text in the database encoding (see Section 22.3.4 for available conversions). convert_from('text_in_utf8', 'UTF8') → text_in_utf8
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convert_to ( string text, dest_encoding name ) → bytea Converts a text string (in the database encoding) to a binary string encoded in encoding dest_encoding (see Section 22.3.4 for available conversions). convert_to('some_text', 'UTF8') → \x736f6d655f74657874
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encode ( bytes bytea, format text ) → text Encodes binary data into a textual representation; supported format values are: base64, escape, hex. encode('123\000\001', 'base64') → MTIzAAE=
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decode ( string text, format text ) → bytea Decodes binary data from a textual representation; supported format values are the same as for encode. decode('MTIzAAE=', 'base64') → \x3132330001
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The encode and decode functions support the following textual formats:
- base64 #
The base64 format is that of RFC 2045 Section 6.8. As per the RFC, encoded lines are broken at 76 characters. However instead of the MIME CRLF end-of-line marker, only a newline is used for end-of-line. The decode function ignores carriage-return, newline, space, and tab characters. Otherwise, an error is raised when decode is supplied invalid base64 data — including when trailing padding is incorrect.
- escape #
The escape format converts zero bytes and bytes with the high bit set into octal escape sequences (\nnn), and it doubles backslashes. Other byte values are represented literally. The decode function will raise an error if a backslash is not followed by either a second backslash or three octal digits; it accepts other byte values unchanged.
- hex #
The hex format represents each 4 bits of data as one hexadecimal digit, 0 through f, writing the higher-order digit of each byte first. The encode function outputs the a-f hex digits in lower case. Because the smallest unit of data is 8 bits, there are always an even number of characters returned by encode. The decode function accepts the a-f characters in either upper or lower case. An error is raised when decode is given invalid hex data — including when given an odd number of characters.
See also the aggregate function string_agg in Section 9.21 and the large object functions in Section 33.4.
