Postgresql 中文操作指南
9.6. Bit String Functions and Operators #
本节介绍用于检查和操作位串的函数和运算符，即类型为 _bit_和 _bit varying_的值。（虽然这些表中只提到了 _bit_类型，但可以使用 _bit varying_类型的值交替使用。）位串支持 Table 9.1中显示的常规比较运算符，以及 Table 9.14中显示的运算符。
This section describes functions and operators for examining and manipulating bit strings, that is values of the types bit and bit varying. (While only type bit is mentioned in these tables, values of type bit varying can be used interchangeably.) Bit strings support the usual comparison operators shown in Table 9.1, as well as the operators shown in Table 9.14.
Table 9.14. Bit String Operators
Operator Description Example(s) |
bit _ |
_ bit → bit Concatenation _B'10001' |
B'011'_ → 10001011 |
bit & bit → bit Bitwise AND (inputs must be of equal length) B'10001' & B'01101' → 00001 |
bit _ |
_ bit → bit Bitwise OR (inputs must be of equal length) _B'10001' |
B'01101'_ → 11101 |
bit # bit → bit Bitwise exclusive OR (inputs must be of equal length) B'10001' # B'01101' → 11100 |
~ bit → bit Bitwise NOT ~ B'10001' → 01110 |
bit << integer → bit Bitwise shift left (string length is preserved) B'10001' << 3 → 01000 |
bit >> integer → bit Bitwise shift right (string length is preserved) B'10001' >> 2 → 00100 |
Table 9.15中显示的某些可用于二进制串的函数也可用于位串。
Some of the functions available for binary strings are also available for bit strings, as shown in Table 9.15.
Table 9.15. Bit String Functions
Function Description Example(s) |
bit_count ( bit ) → bigint Returns the number of bits set in the bit string (also known as “popcount”). bit_count(B'10111') → 4 |
bit_length ( bit ) → integer Returns number of bits in the bit string. bit_length(B'10111') → 5 |
length ( bit ) → integer Returns number of bits in the bit string. length(B'10111') → 5 |
octet_length ( bit ) → integer Returns number of bytes in the bit string. octet_length(B'1011111011') → 2 |
overlay ( bits bit PLACING newsubstring bit FROM start integer [ FOR count integer ] ) → bit Replaces the substring of bits that starts at the start'th bit and extends for count bits with newsubstring. If count is omitted, it defaults to the length of newsubstring. overlay(B'01010101010101010' placing B'11111' from 2 for 3) → 0111110101010101010 |
position ( substring bit IN bits bit ) → integer Returns first starting index of the specified substring within bits, or zero if it’s not present. position(B'010' in B'000001101011') → 8 |
substring ( bits bit [ FROM start integer ] [ FOR count integer ] ) → bit Extracts the substring of bits starting at the start'th bit if that is specified, and stopping after count bits if that is specified. Provide at least one of start and count. substring(B'110010111111' from 3 for 2) → 00 |
get_bit ( bits bit, n integer ) → integer Extracts n'th bit from bit string; the first (leftmost) bit is bit 0. get_bit(B'101010101010101010', 6) → 1 |
set_bit ( bits bit, n integer, newvalue integer ) → bit Sets n'th bit in bit string to newvalue; the first (leftmost) bit is bit 0. set_bit(B'101010101010101010', 6, 0) → 101010001010101010 |
另外，可以将整数值强制转换为 bit(n) 类型或从 bit(n) 类型强制转换。将整数强制转换为 n 会复制最右边的 bit(1) 个比特。将整数强制转换为比整数自身更宽的比特串宽度将会在左侧扩展变量。一些示例：
In addition, it is possible to cast integral values to and from type bit. Casting an integer to bit(n) copies the rightmost n bits. Casting an integer to a bit string width wider than the integer itself will sign-extend on the left. Some examples:
44::bit(10) 0000101100
44::bit(3) 100
cast(-44 as bit(12)) 111111010100
'1110'::bit(4)::integer 14请注意，仅强制转换为“bit”表示强制转换为 amcostestimate，因此只会传递整数的最低有效位。
Note that casting to just “bit” means casting to bit(1), and so will deliver only the least significant bit of the integer.