Rob, I'd estimate the priority of this feature as very low. There are some reasons why this is less important for Virtuoso than for Oracle:
1. Virtuoso integrates web server and the core of RDBMS in a single process, so page generation can be made by a Virtuoso/PL stored procedure from beginning to end. Hence the total size of result set in bytes is not as important as in inter-process communication case. 2. If data should be returned to the client then either a procedure view or user-defined aggregate can incapsulate any appropriate logic, including logic of "TOP m,n". It is not a problem for a procedure view to return, say, all even-numbered rows or rows #10-#19, plus a total count. It is not a problem for user-define aggregate to return a single value that is a vector of required rows in a single column, so the result-set select my_funny_aggregate (fields, filtering_arguments), count (*) from ... will return a single row with all required data. 3. Virtuoso hackers may cheat and tweak parameters of opened cursors between fetches. Consider a Virtuoso/PL procedure, a local variable start_key and an opened cursor for SELECT such that WHERE clause contains condition like (primary_key_column > start_value) . It is safe to open cursor and make few fetches with one value of start_key, then decide that some rows may be skipped and increase the value of start_key so next fetches will probably ignore rows with keys less than new value of start_key. The most popular use case of this ugly dirty hack is an "intersection join" at application level. Some brain-damaging examples can be found in procedures "CatFilter_GET_RDF_INVERSE_HITS_DISTVALS" and "CatFilter_GET_RDF_INVERSE_HITS_RES_IDS" in binsrc/dav/DET_Catfilter.sql (the growing value of res_id_max variable is used as a parameter of numerous opened cursors). Don't use this hack; or at least don't use it without a reason :) Best Regards, Ivan Mikhailov, OpenLink Software. On Thu, 2008-05-01 at 15:17 -0700, rob bygrave wrote: > Does Virtuoso use an MVCC architecture? > > I wondering if it has support (or planned support) for something like > Oracle Flash back queries. > > Ala ... select ... from ... AS OF SCN xxx > > http://www.singlequery.com/?cat=10 > ... is an interesting example of a use of flash back queries. > > Thanks, Rob.
