9.5. 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 9.11
.
PostgreSQL
also provides versions of these functions
that use the regular function invocation syntax
(see
Table 9.12
).
Note
The sample results shown on this page assume that the server parameter
bytea_output
is set
to
escape
(the traditional PostgreSQL format).
Table 9.11.
SQL
Binary String Functions and Operators
|
Function
|
Return Type
|
Description
|
Example
|
Result
|
string
||
string
|
bytea
|
String concatenation
|
'\\Post'::bytea || '\047gres\000'::bytea
|
\\Post'gres\000
|
octet_length(
string
)
|
int
|
Number of bytes in binary string
|
octet_length('jo\000se'::bytea)
|
5
|
overlay(
string
placing
string
from
int
[
for
int
])
|
bytea
|
Replace substring
|
overlay('Th\000omas'::bytea placing '\002\003'::bytea from 2 for 3)
|
T\\002\\003mas
|
position(
substring
in
string
)
|
int
|
Location of specified substring
|
position('\000om'::bytea in 'Th\000omas'::bytea)
|
3
|
substring(
string
[
from
int
] [
for
int
])
|
bytea
|
Extract substring
|
substring('Th\000omas'::bytea from 2 for 3)
|
h\000o
|
trim([
both
]
bytes
from
string
)
|
bytea
|
Remove the longest string containing only bytes appearing in
bytes
from the start
and end of
string
|
trim('\000\001'::bytea from '\000Tom\001'::bytea)
|
Tom
|
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
|
Return Type
|
Description
|
Example
|
Result
|
btrim(
string
bytea
,
bytes
bytea
)
|
bytea
|
Remove the longest string containing only bytes appearing in
bytes
from the start and end of
string
|
btrim('\000trim\001'::bytea, '\000\001'::bytea)
|
trim
|
decode(
string
text
,
format
text
)
|
bytea
|
Decode binary data from textual representation in
string
.
Options for
format
are same as in
encode
.
|
decode('123\000456', 'escape')
|
123\000456
|
encode(
data
bytea
,
format
text
)
|
text
|
Encode binary data into a textual representation. Supported
formats are:
base64
,
hex
,
escape
.
escape
converts zero bytes and high-bit-set bytes to
octal sequences (
\
nnn
) and
doubles backslashes.
|
encode('123\000456'::bytea, 'escape')
|
123\000456
|
get_bit(
string
,
offset
)
|
int
|
Extract bit from string
|
get_bit('Th\000omas'::bytea, 45)
|
1
|
get_byte(
string
,
offset
)
|
int
|
Extract byte from string
|
get_byte('Th\000omas'::bytea, 4)
|
109
|
length(
string
)
|
int
|
Length of binary string
|
length('jo\000se'::bytea)
|
5
|
md5(
string
)
|
text
|
Calculates the MD5 hash of
string
,
returning the result in hexadecimal
|
md5('Th\000omas'::bytea)
|
8ab2d3c9689aaf18 b4958c334c82d8b1
|
set_bit(
string
,
offset
,
newvalue
)
|
bytea
|
Set bit in string
|
set_bit('Th\000omas'::bytea, 45, 0)
|
Th\000omAs
|
set_byte(
string
,
offset
,
newvalue
)
|
bytea
|
Set byte in string
|
set_byte('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.
See also the aggregate function
string_agg
in
Section 9.20
and the large object functions
in
Section 34.4
.