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SAVEPOINT

Available in: DSQL

Added in: 1.5

Description

Creates an SQL-99 compliant savepoint, to which you can later rollback your work without rolling back the entire transaction. Savepoint mechanisms are also known as "nested transactions".

Syntax

 SAVEPOINT <name>

 <name> ::= a user-chosen identifier, unique within the transaction

If the supplied name exists already within the same transaction, the existing savepoint is deleted and a new one is created with the same name.

If you later want to rollback your work to the point where the savepoint was created, use:

 ROLLBACK [WORK] TO [SAVEPOINT] name

ROLLBACK TO SAVEPOINT performs the following operations:

  • All the database mutations performed within the transaction since the savepoint was created are undone. User variables set with RDB$SET_CONTEXT() remain unchanged.
  • All savepoints created after the one named are destroyed. All earlier savepoints are preserved, as is the savepoint itself. This means that you can rollback to the same savepoint several times.
  • All implicit and explicit record locks acquired since the savepoint are released. Other transactions that have requested access to rows locked after the savepoint must continue to wait until the transaction is committed or rolled back. Other transactions that have not already requested the rows can request and access the unlocked rows immediately.

The internal savepoint bookkeeping can consume huge amounts of memory, especially if you update the same records multiple times in one transaction. If you don't need a savepoint anymore but you're not yet ready to end the transaction, you can delete the savepoint and free the resources it uses with:

 RELEASE SAVEPOINT name [ONLY]

With ONLY, the named savepoint is the only one that gets released. Without it, all savepoints created after it are released as well.

Example DSQL session using a savepoint

 create table test (id integer);
 commit;
 insert into test values (1);
 commit;
 insert into test values (2);
 savepoint y;
 delete from test;
 select * from test; -- returns no rows
 rollback to y;
 select * from test; -- returns two rows
 rollback;
 select * from test; -- returns one row

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Internal savepoints

By default, the engine uses an automatic transaction-level system savepoint to perform transaction rollback. When you issue a ROLLBACK statement, all changes performed in this transaction are backed out via a transaction-level savepoint and the transaction is then committed. This logic reduces the amount of garbage collection caused by rolled back transactions.

When the volume of changes performed under a transaction-level savepoint is getting large (104106 records affected), the engine releases the transaction-level savepoint and uses the TIP mechanism to roll back the transaction if needed.

Tip: If you expect the volume of changes in your transaction to be large, you can specify the NO AUTO UNDO option in your SET TRANSACTION statement, or if you use the API set the TPB flag isc_tpb_no_auto_undo. Both prevent the transaction-level savepoint from being created.

Savepoints and PSQL

Transaction control statements are not allowed in PSQL, as that would break the atomicity of the statement that calls the procedure. But Firebird does support the raising and handling of exceptions in PSQL, so that actions performed in stored procedures and triggers can be selectively undone without the entire procedure failing.

Internally, automatic savepoints are used to:

  • undo all actions in a BEGIN...END block where an exception occurs;
  • undo all actions performed by the SP/trigger (or, in the case of a selectable SP, all actions performed since the last SUSPEND) when it terminates prematurely due to an uncaught error or exception.

Each PSQL exception handling block is also bounded by automatic system savepoints.

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