Data consistency and concurrency

Informix^®

Informix uses a locking mechanism to handle data consistency and concurrency. When a process changes database information with UPDATE, INSERT or DELETE, an exclusive lock is set on the touched rows. The lock remains active until the end of the transaction. Statements performed outside a transaction are treated as a transaction containing a single operation and therefore release the locks immediately after execution. SELECT statements can set shared locks according to the isolation level. In case of locking conflicts (for example, when two processes want to acquire an exclusive lock on the same row for modification or when a writer is trying to modify data protected by a shared lock), the behavior of a process can be changed by setting the lock wait mode.

Control:

• Lock wait mode: SET LOCK MODE TO ...
• Isolation level: SET ISOLATION TO ...
• Locking granularity: CREATE TABLE ... LOCK MODE {PAGE|ROW}
• Explicit exclusive lock: SELECT ... FOR UPDATE

Defaults:

• The default isolation level is read committed.
• The default lock wait mode is "not wait".
• The default locking granularity is per page.

PostgreSQL

When data is modified, exclusive locks are set and held until the end of the transaction. For data consistency, PostgreSQL uses a multi-version consistency model: A copy of the original row is kept for readers before performing writer modifications. Readers do not have to wait for writers as in Informix. The simplest way to think of the PostgreSQL implementation of read consistency is to imagine each user operating a private copy of the database, hence the multi-version consistency model. The lock wait mode cannot be changed as in Informix. Locks are set at the row level in PostgreSQL and this cannot be changed.

Control:

• No lock wait mode control is provided.
• Isolation level: SET TRANSACTION ISOLATION LEVEL ...
• Explicit exclusive lock: SELECT ... FOR UPDATE

Defaults:

• The default isolation level is Read Committed.

The main difference between Informix and PostgreSQL is that readers do not have to wait for writers in PostgreSQL.

Solution

The SET ISOLATION TO ... Informix syntax is replaced by SET SESSION CHARACTERISTICS AS TRANSACTION ISOLATION LEVEL ... in PostgreSQL. The next table shows the isolation level mappings done by the PostgreSQL database driver:

For portability, it is recommended that you work with Informix in the read committed isolation level, make processes wait for each other (lock mode wait), and create tables with the "lock mode row" option.

See the Informix and PostgreSQL documentation for more details about data consistency, concurrency and locking mechanisms.