|
| UR
| CS
| RS
| RR
|
| Can the application see uncommitted changes made by other application
processes?
| Yes
| No
| No
| No
|
| Can the application update uncommitted changes made by other application
processes?
| No
| No
| No
| No
|
| Can the re-execution of a statement be affected by other application
processes? See phenomenon P3 (phantom) below.
| Yes
| Yes
| Yes
| No
|
| Can "updated" rows be updated by other application processes?
| No
| No
| No
| No
|
| Can "updated" rows be read by other application processes that
are running at an isolation level other than UR?
| No
| No
| No
| No
|
| Can "updated" rows be read by other application processes that
are running at the UR isolation level?
| Yes
| Yes
| Yes
| Yes
|
| Can "accessed" rows be updated by other application processes?
See phenomenon P2 (nonrepeatable read) below.
| Yes
| Yes
| No
| No
|
| Can "accessed" rows be read by other application processes?
| Yes
| Yes
| Yes
| Yes
|
| Can "current" row be updated or deleted by other application
processes? See phenomenon P1 (dirty-read) below.
| See Note below
| See Note below
| No
| No
|
|
Note:
- If the cursor is not updatable, with CS the current row may be updated or
deleted by other application processes in some cases.
|
| Examples of Phenomena: |
- P1
- Dirty Read. Unit of work UW1 modifies a row. Unit
of work UW2 reads that row before UW1 performs a COMMIT. If UW1 then
performs a ROLLBACK, UW2 has read a nonexistent row.
- P2
- Nonrepeatable Read. Unit of work UW1 reads a row.
Unit of work UW2 modifies that row and performs a COMMIT. If UW1 then
re-reads the row, it might receive a modified value.
- P3
- Phantom. Unit of work UW1 reads the set of n
rows that satisfies some search condition. Unit of work UW2 then
INSERTs one or more rows that satisfies the search condition. If UW1
then repeats the initial read with the same search condition, it obtains the
original rows plus the inserted rows.
|
|