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This graphic is explained in the accompanying text Phenomena Locate the document in its SAP Library structure

If several transactions access the same database object, such as a table, at the same time, this can result in inconsistencies in the results. The following phenomena can occur:

·        Dirty Read

Transaction T1 changes a row. Transaction T2 reads this row before T1 ends the transaction with COMMIT. T1 then performs a ROLLBACK.

In this case, T2 reads a row that never really existed.

·        Non Repeatable Read

Transaction T1 reads a row. Transaction T2 then changes or deletes this row and closes the transaction with COMMIT. If T1 subsequently reads the row again, then T1 receives either the changed row or the message that the row no longer exists.

·        Phantom

Transaction T1 performs an SQL statement S with a search condition that returns a result set M of rows. Transaction T2 then creates at least one additional row that meets this search condition, for example, by adding or changing data. If T1 subsequently executes S again, then the new result set is different to M.

You can configure which phenomena are possible with the isolation level.

Relationship between the isolation level and phenomena

Phenomena

Isolation Level

0

1 or 10

2 or 20

3 or 30

Dirty Read

yes

no

no

no

Non Repeatable Read

yes

yes

no

no

Phantom

yes

yes

yes

no

 

 

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