Здравствуйте!
помогите, пожалуйста, решить задачу из курса 2073 модуль 7:
я не понимаю, как же получить такие ответы. Моя преподша толком объяснить не смогла, но она говорила про то, что надо взять длину всех записей что ли и делить на длину одной записи (это 38 байт)

1 charge_no numeric_id (int) 4 0
0 member_no numeric_id (int) 4 0
0 provider_no numeric_id (int) 4 0
0 category_no numeric_id (int) 4 0
0 charge_dt datetime 8 0
0 charge_amt money 8 0
0 statement_no numeric_id (int) 4 0
0 charge_code status_code (char) 2 0

Задача:

To determine selectivity
In this procedure, you will execute a series of SELECT statements that select all rows from the Charge table. A table scan is performed for each SELECT statement. For each SELECT statement, you will first select the query and then view the estimated execution plan. You will modify the WHERE clause so that the query optimizer uses an index to retrieve the rows, and then you execute that query. After executing the query, you will record and evaluate the maximum number of rows that can be returned by using an index.
USE credit
SELECT * FROM charge
WHERE charge_no BETWEEN 1 AND 100000

USE credit
SELECT * FROM charge
WHERE member_no BETWEEN 1 AND 10000

USE credit
SELECT * FROM charge
WHERE provider_no BETWEEN 1 AND 500

USE credit
SELECT * FROM charge
WHERE category_no between 1 AND 10
1. Select the first statement, but do not execute it.
2. In the query window, on the Query menu, click Display Estimated Execution Plan.
Notice the query plan for the statement.
3. Modify the range in the SELECT statements so that the query optimizer uses an index to retrieve the rows, rather than the optimizer using a table scan or full index scan.
When choosing a range, remember that:
• Each page has approximately 172 charges.
• Older members and providers are less active than newer ones.
• All categories are equally popular.
• Execute the statements.
• In the following table, record the maximum number of rows that can be returned by using an index.
WHERE clause Approximate number of rows
WHERE charge_no BETWEEN 1 AND n 116
WHERE member_no BETWEEN 1 AND n 222
WHERE provider_no BETWEEN 1 AND n 70
WHERE category_no BETWEEN 1 AND n 0
• Repeat steps 1 through 5 for the remaining SELECT statements.
Note You will notice that it is not easy to predict the selectivity of a query, even when you know the values of all of the arguments. It is best to let the query optimizer decide how to execute the query.
• Is the number of rows accessed by the query optimizer the same for all indexes? Why or why not?

ну вот например первый запрос. В таблице 100 тыс строк, и в запросе мы все их хотим получить. Как получить, что именно при 116 сканирование пойдет по индексу? Мне говорили, что это можно вычислить прям по формуле

оппа, спасибо, сейчас буду втыкать=)