Relational databases for conditions data and event selection in ATLAS

The ATLAS experiment at LHC will make extensive use of relational databases in both online and offline contexts, running to O(TBytes) per year. Two of the most challenging applications in terms of data volume and access patterns are conditions data, making use of the LHC conditions database, COOL, and the TAG database, that stores summary event quantities allowing a rapid selection of interesting events. Both of these databases are being replicated to regional computing centres using Oracle Streams technology, in collaboration with the LCG 3D project. Database optimisation, performance tests and first user experience with these applications will be described, together with plans for first LHC data-taking and future prospects.