oracle中dump函式

轉自：http://blog.vsharing.com/nimrod/A654847.html

DUMP函式的輸出格式類似:

型別 ，符號/指數位 [數字1，數字2，數字3，......，數字20]

各位的含義如下:

1.型別: Number型,Type=2 (型別程式碼可以從Oracle的文件上查到)

2.長度:指儲存的位元組數

3.符號/指數位

在儲存上,Oracle對正數和負數分別進行儲存轉換:

正數：加1儲存(為了避免Null)
負數：被101減,如果總長度小於21個位元組，最後加一個102(是為了排序的需要)

指數位換算:

正數：指數=符號/指數位 - 193 (最高位為1是代表正數)
負數：指數=62 - 第一位元組

4.從開始是有效的資料位

從開始是最高有效位,所儲存的數值計算方法為：

將下面計算的結果加起來：

每個乘以100^(指數-N) (N是有效位數的順序位，第一個有效位的N=0)

5、舉例說明

SQL> select dump(123456.789) from dual;

DUMP(123456.789)
-------------------------------
Typ=2 Len=6: 195,13,35,57,79,91

:     195 - 193 = 2
      13 - 1      = 12 *100^(2-0) 120000
      35 - 1      = 34 *100^(2-1) 3400
      57 - 1      = 56 *100^(2-2) 56
      79 - 1      = 78 *100^(2-3) .78
      91 - 1      = 90 *100^(2-4) .009
　                             123456.789

SQL> select dump(-123456.789) from dual;

DUMP(-123456.789)
----------------------------------
Typ=2 Len=7: 60,89,67,45,23,11,102

       62 - 60 = 2(最高位是0，代表為負數)
101 - 89 = 12 *100^(2-0) 120000
101 - 67 = 34 *100^(2-1) 3400
101 - 45 = 56 *100^(2-2) 56
101 - 23 = 78 *100^(2-3) .78
101 - 11 = 90 *100^(2-4) .009
　                               123456.789(-)

現在再考慮一下為什麼在最後加102是為了排序的需要，-123456.789在資料庫中實際儲存為

60,89,67,45,23,11

而-123456.78901在資料庫中實際儲存為

60,89,67,45,23,11,91

可見，如果不在最後加上102，在排序時會出現-123456.789

轉自

Internal representation of the NUMBER datatype

As with other datatypes, stored numbers are preceded by a length byte which stores the size of the datum in bytes, or 0xFF for NULLs. The actual data bytes for non-null numbers represent the value in scientific notation. For example, the number 12.3 is represented as +0.123 * 10². The high order bit of the first byte represents the sign. The sign bit is set for positive numbers; and clear for negative numbers. The remainder of the first byte represents the exponent, and then up to 20 bytes may be used to represent the significant digits excluding trailing zeros. This is sometimes called the mantissa.

Each byte of the mantissa normally represents two decimal digits. For positive numbers an offset of 1 is added to avoid null bytes, while for negative numbers an offset of 101 is added to the negated digit pair. Thus a mantissa byte with the decimal value of 100 might represent the digit pair "99" in a positive number, or the digit pair "01" in a negative number. The interpretation must be based on the sign bit. Negative numbers with less than 20 mantissa bytes also have a byte with the (impossible) decimal value 102 appended. I don't know what purpose this serves.

If there are an odd number of significant digits before the decimal point, the first mantissa byte can only represent 1 digit because the decimal exponent must be even. In this case, the 20-byte mantissa can represent at most 39 decimal digits. However, the last digit may not be accurate if a more precise value has been truncated for storage. This is why the maximum guaranteed precision for Oracle numbers is 38 decimal digits, even though 40 digits can be represented.

The decimal exponent is guaranteed to be even by the alignment of the mantissa. Thus the value stored for the exponent is always halved and is expressed such that the decimal point falls before the first digit of the mantissa. It again represents a pair of decimal digits, this time with an offset of 64 for positive numbers, and 63 for the negated exponent of negative numbers. Thus a set of exponent bits with the decimal value of 65 might represent the exponent +2 in a positive number, or the exponent -4 in a negative number. Please note that the encoding of the exponent is based on the sign of the number, and not on the sign of the exponent itself.

Finally, there are special encodings for zero, and . Zero is represented by the single byte 0x80. Negative infinity is represented by 0x00, and positive infinity is represented by the two bytes 0xFF65. These are illustrated in the listing below.

SQL> select n, dump(n,16) from special_numbers; N DUMP(N,16) --- ------------------------------------------ 0 Typ=2 Len=1: 80 -~ Typ=2 Len=1: 0 ~ Typ=2 Len=2: ff,65

For the rest, the best way to familiarize yourself further with the internal representation of numbers is to use the dump function to examine the representation of some sample values. This is simulated below. Just type a number and then press "Enter" to check out its representation. For example, try to find out why 110 takes one more byte of storage than 1100 despite being a smaller number.

Normalized number: +0.1230 * 10^2
Sign bit:          1
Exponent bits:     65 = (64 + 2/2)
First byte:        193
Mantissa digits:   12,30
Mantissa bytes:    13,31