does postgreSQL have any mechanism like Oracle's Redo-log or the 'logupdate' option of MySQL which log only those queries that cause really happened changes to databases? In postgreSQL, how to do things like MySQL's "flush logs" and Oracle's "alter database archivelog"? what's the information in $PGDATA/pg_log? how to browse it? how to delete rows having 'invalid' timestamp value? i use "delete from tablename where timestampColName = 'invalid';",but can't delete them.(My postgreSQL is version 6.5)