Hi, of course there is in general a difference between syntax complexity and performance but unfortunately not in this case. And the „just“ is very often the most difficult part.
Regards, Hartwig > Am 2016-06-07 um 07:39 schrieb James K. Lowden <jklow...@schemamania.org>: > > On Sat, 4 Jun 2016 18:18:36 +0200 > skywind mailing lists <mailingli...@skywind.eu> wrote: > >> At the moment I have to run something like: >> >> UPDATE A SET item1=(SELECT B.item FROM B WHERE B.ID=A.ID),... >> itemN=... WHERE EXISTS (SELECT 1 FROM B WHERE B.ID=A.ID); >> >> Using a FROM clause I just need one scan through B (at least in >> principle). Now, I need N+1 scans. > > Nonsense. SQL provides no instruction to the implementation on how to > organize or traverse the data. SQLite is free to scan B once, twice, > or not at all. > > Syntax has nothing to do with performance. A correlated subquery is an > expression of logic; it's not meant to be taken literally, and often > isn't. This particular form "just" needs to be recognized by the > optimizer. > > --jkl > _______________________________________________ > sqlite-users mailing list > sqlite-users@mailinglists.sqlite.org > http://mailinglists.sqlite.org/cgi-bin/mailman/listinfo/sqlite-users > _______________________________________________ sqlite-users mailing list sqlite-users@mailinglists.sqlite.org http://mailinglists.sqlite.org/cgi-bin/mailman/listinfo/sqlite-users
