| Isolation level |
Characteristics |
Isolation properties |
Concurrency |
| Serializable |
- No compromises on isolation.
|
- Execution of concurrent transactions is really
equivalent to some serial execution.
|
- Lowest level of concurrency.
- Often unacceptable in practice.
|
| Repeatable Reads |
- A row that has been read must look the same each time
it is read.
- But a query that is run multiple times may
return additional rows on each run.
- These are phantom reads resulting from committed
inserts by other transactions.
|
|
- More concurrency than serializable.
|
| Read Committed |
- Reads are non-repeatable. Multiple reads of the same row
might look different.
- These differences are due to updates from recently committed transactions.
- Phantom reads may occur.
- Rows may disappear on a subsequent read (due to a committed delete).
|
- Lowest level of isolation.
|
- More concurrency than repeatable read.
|
| Read Uncommitted |
- Reads may see uncommitted data!
- These are dirty reads.
- "Yes, my code is buggy, but at least it's fast."
|
- No isolation!
- Read uncommitted changes!
|
- Fast and out of control.
- All bets are off.
- Don't use this!
|