Использование CHECKSUM для создания хэш-индексов

Август 21, 2022

Главная
Информатика
Использование CHECKSUM для создания хэш-индексов

Содержание

2. CHECKSUM_AGG This function returns the checksum of the values in a group CHECKSUM_AGG can detect changes
3. CHECKSUM_AGG Returns the checksum of all expression values as int The CHECKSUM_AGG result does not depend
4. CHECKSUM_AGG
5. CHECKSUM The CHECKSUM function returns the checksum value computed over a table row, or over an
6. CHECKSUM Return types INT CHECKSUM computes a hash value, called the checksum, over its argument list.
7. CHECKSUM
8. CHECKSUM
9. CHECKSUM
10. ЗАДАНИЕ Создайте собственную процедуру по аналогии с предыдущим слайдом для таблицы CHECKS Проверьте контрольную сумму Закомментируйте
11. Использование CHECKSUM для создания хэш-индексов
13. Скачать презентацию

Слайд 2

CHECKSUM_AGG
This function returns the checksum of the values in a

group
CHECKSUM_AGG can detect changes in a table
CHECKSUM_AGG ( [ ALL | DISTINCT ] expression )
ALL: Applies the aggregate function to all values. ALL is the default argument.
DISTINCT: Specifies that CHECKSUM_AGG returns the checksum of unique values.
Expression: An integer expression. CHECKSUM_AGG does not allow use of aggregate functions or subqueries.

Слайд 3

CHECKSUM_AGG
Returns the checksum of all expression values as int
The CHECKSUM_AGG

result does not depend on the order of the rows in the table
CHECKSUM_AGG ignores null values
If an expression list value changes, the list checksum value list will also probably change. However, a small possibility exists that the calculated checksum will not change

Слайд 4

CHECKSUM_AGG

Слайд 5

CHECKSUM
The CHECKSUM function returns the checksum value computed over a table

row, or over an expression list
Use CHECKSUM to build hash indexes
CHECKSUM ( * | expression [ ,...n ] )
*
This argument specifies that the checksum computation covers all table columns. CHECKSUM returns an error if any column has a noncomparable data type. Noncomparable data types include: cursor, image, ntext, text, XML
Expression
An expression of any type, except a noncomparable data type.

Слайд 6

CHECKSUM
Return types INT
CHECKSUM computes a hash value, called the checksum, over

its argument list. Use this hash value to build hash indexes. A hash index will result if the CHECKSUM function has column arguments, and an index is built over the computed CHECKSUM value. This can be used for equality searches over the columns.
CHECKSUM ignores the nchar and nvarchar dash character (N'-' or nchar(45)). Therefore, a hash collision is guaranteed for any two strings where the only differences are dashes.

Слайд 7