READ UNCOMMITTED: UserA will see the change made by UserB. This isolation level is called dirty reads, which means that read data is not consistent with other parts of the table or the query, and may not yet have been committed.
READ COMMITTED: UserA will not see the change made by UserB. This is because the rows returned by a query are the rows that were committed when the query was started. The change made by UserB was not present when the query started, and therefore will not be included in the query result.
REPEATABLE READ: UserA will not see the change made by UserB. This is because the rows returned by a query are the rows that were committed when the transaction was started. The change made by UserB was not present when the transaction was started, and therefore will not be included in the query result.
This means that “All consistent reads within the same transaction read the snapshot established by the first read”.
SERIALIZABLE: This isolation level specifies that all transactions occur in a completely isolated fashion, meaning as if all transactions in the system were executed serially, one after the other. The DBMS can execute two or more transactions at the same time only if the illusion of serial execution can be maintained.
In practice, SERIALIZABLE is similar to REPEATABLE READ, but uses a different implementation for each database engine. In Oracle, the REPEATABLE READ level is not supported and SERIALIZABLE provides the highest isolation level. This level is similar to REPEATABLE READ, but InnoDB implicitly converts all plain SELECT statements to “SELECT … LOCK IN SHARE MODE
The default isolation level in MySQL’s InnoDB is REPEATABLE READ, which provides a rather high isolation.
Since old values of row data are required for current queries, databases use a special segment to store old row values and snapshots. MySQL calls this segment a Rollback Segment.
Read full article from Database Isolation Levels And Their Effects on Performance and Scalability - High Scalability -
READ COMMITTED: UserA will not see the change made by UserB. This is because the rows returned by a query are the rows that were committed when the query was started. The change made by UserB was not present when the query started, and therefore will not be included in the query result.
REPEATABLE READ: UserA will not see the change made by UserB. This is because the rows returned by a query are the rows that were committed when the transaction was started. The change made by UserB was not present when the transaction was started, and therefore will not be included in the query result.
This means that “All consistent reads within the same transaction read the snapshot established by the first read”.
SERIALIZABLE: This isolation level specifies that all transactions occur in a completely isolated fashion, meaning as if all transactions in the system were executed serially, one after the other. The DBMS can execute two or more transactions at the same time only if the illusion of serial execution can be maintained.
In practice, SERIALIZABLE is similar to REPEATABLE READ, but uses a different implementation for each database engine. In Oracle, the REPEATABLE READ level is not supported and SERIALIZABLE provides the highest isolation level. This level is similar to REPEATABLE READ, but InnoDB implicitly converts all plain SELECT statements to “SELECT … LOCK IN SHARE MODE
The default isolation level in MySQL’s InnoDB is REPEATABLE READ, which provides a rather high isolation.
Since old values of row data are required for current queries, databases use a special segment to store old row values and snapshots. MySQL calls this segment a Rollback Segment.
Read full article from Database Isolation Levels And Their Effects on Performance and Scalability - High Scalability -
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