#555555;">Every so often a DSS query that usually takes 10 minutes ends up taking over an hour. (or one that takes an hour never seems to finish)
#555555;">Why would this happen?
#555555;">When investigating the DSS query, perhaps with wait event tracing, one finds that the query which is doing full table scans and should be doing large multi-block reads and waiting for “db file scattered read” is instead waiting for single block reads, ie “db file sequential read”. What the heck is going on?
#555555;">Sequential reads during a full table scan scattered read query is a classic sign of reading rollback and reading rollback can make that minute(s) full table scan take hours.