While executing the SQL statements that comprise one transaction, the database server can execute some or all of the statements in other transactions. Transactions processed at the same time are said to be concurrent.
Sybase IQ's approach to concurrency is designed especially for the data warehouse. Typically, in a data warehouse environment, many users need to read from the database, but only the DBA needs to update it. However, there is often a need to be able to make those updates while other users continue to request and receive query results.
Sybase IQ allows many simultaneous connections by many users to one database. It can also process transactions from more than one connected user or application concurrently.
Sybase IQ ensures that all database operations occur within a transaction, and that these operations do not interfere with each other. It does so by setting access restrictions at the table level, and by using a technique called snapshot versioning, described in “Introduction to versioning.” On a given table, IQ allows concurrent processing of multiple read transactions, but only one write transaction. This approach maintains the internal consistency of the database.
