(Portland)Fourth ACM SIGACT-SIGMOD Symposium on Principles of Database Systems (Cambridge)ACM Sigact-Sigmed Symposium on Principles of Database Systems ACM Sigact-Sigmod Symposium on Principles of Database Systems ACM Sigact-Sigmed Symposium on Principles of Database Systems Fourth ACM SIGACT-SIGMOD Symposium on Principles of Database Systems ACM SIGACT-SIGMOD Symposium on Principles of Database Systems. ACM Sigact-Sigmod Symposium on Principles of Database Systems: Proceedings