2011/12/15 Alexandr Němec <[email protected]> > > Dear all, > > just a quick question, I did not find the answer in the various technical > documents. I have two identical tables with a id INTEGER as a primary key, > which means that SELECTions ORDERed BY id are very fast. Now if I do SELECT > * FROM table1 UNION ALL SELECT * FROM table2 ORDER by id, rows from both > tables appear in the resultset. But I was surprised that the speed of the > sorting is still the same (as for one indexed column). Does it mean, that > SQLite can (somehow) use the index to speed up the sorting when UNIONing > several tables? >
Yes. The ORDER BY applies to the complete result of the UNION ALL. So what SQLite does in this case is run both subqueries in a parallel, delivering the results of each subquery in sorted order (which is easy since the source key is the primary key), and merge the results together. > > Thanks > Alex > _______________________________________________ > sqlite-users mailing list > [email protected] > http://sqlite.org:8080/cgi-bin/mailman/listinfo/sqlite-users > -- D. Richard Hipp [email protected] _______________________________________________ sqlite-users mailing list [email protected] http://sqlite.org:8080/cgi-bin/mailman/listinfo/sqlite-users
