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Handling Transactions


Transaction control statements in Mimer SQL are:

SQL Statement Restrictions in Transactions

The following SQL statements may not be used inside a transaction:

ENTER
SET SHADOW
LEAVE
SET TRANSACTION
SET DATABANK
START BACKUP
SET DATABASE
START TRANSACTION
SET SESSION

Data definition statements (e.g ALTER, DROP, UPDATE STATISTICS) may be used inside a transaction provided they are the only statement executed in that transaction.

Optimizing Transactions

It is strongly recommended that the SET TRANSACTION READ ONLY option be used for each transaction that does not perform updates to the database and that the SET TRANSACTION READ WRITE option be used only when a transaction performs updates.

Taking a little extra care to set these options appropriately will ensure the transaction performance remains optimal at all times.

The default transaction read option can be defined by using SET SESSION, see Default Transaction Options. If this has not been used to set the default transaction read option, the default is READ WRITE.

Consistency Within a Transaction

The SET TRANSACTION ISOLATION LEVEL options are provided to control the degree to which the updates performed by one transaction are affected by the updates performed by other transactions which are executing concurrently.

The default isolation level can be defined by using SET SESSION, see Default Transaction Options. If this has not been used to set a default isolation level, the default is REPEATABLE READ. This isolation level guarantees that the end result of the operations performed by two or more concurrent transactions is the same as if the transactions had been executed in a serial fashion, except that an effect known as `Phantoms' may occur.

This is where one transaction reads a set of rows that satisfy some search condition. Another transaction then performs an update which generates one or more new rows that satisfy that search condition. If the original query is repeated (using exactly the same search condition), extra rows appear in the result-set that were previously not found.

The other isolation levels are: READ UNCOMMITTED, READ COMMITTED and SERIALIZABLE.

All four isolation levels guarantee that each transaction will be executed completely or not at all and that no updates will be lost.

Refer to Mimer SQL Reference Manual, SET TRANSACTION, for a full description of the effects that are possible, or guaranteed never to occur, at each of the four isolation levels.

Default Transaction Options


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