1. In each of the situations below, the users are accessing the same data at the same time. Which of the situations could give rise to concurrency problems?
| Situation | Yes | No |
|---|---|---|
3 users executing SELECT
statements |
||
2 users executing SELECT
statements 1 user executing an INSERT statement |
||
1 user executing a SELECT
statement 1 user executing an INSERT statement1 user executing a DELETE statement |
||
2 users executing SELECT
statements 1 user executing an UPDATE statement |
||
3 users executing UPDATE
statements |
| Situation | Yes | No |
|---|---|---|
3 users executing SELECT
statements |
no | |
2 users executing SELECT
statements 1 user executing an INSERT statement |
yes | |
1 user executing a SELECT
statement 1 user executing an INSERT statement1 user executing a DELETE statement |
yes | |
2 users executing SELECT
statements 1 user executing an UPDATE statement |
yes | |
3 users executing UPDATE
statements |
yes |
2. Why do these situations give rise to possible concurrency problems?
As soon as one user wants to change the data (with an INSERT,
UPDATE or DELETE statement) then possible
concurrency problems can occur.
The first situation, the 3 SELECT statements, is the
only one where only read access is wanted by the users. Multiple
readers (without a writer) will never have concurrency issues. As soon
as there is one or more writers then concurrency issues occur.