On Tue, Mar 7, 2017 at 9:36 AM, Clemens Ladisch <clem...@ladisch.de> wrote:
> Vermes Mátyás wrote: > > http://comfirm.hu/pub/sqlite3-regression.rb > > > db.execute("select szamla,megnevezes from proba") do |row| > > ... > > db.execute( "update proba set megnevezes=? where szamla=?", > row[1]+"*", row[0] ) > > The equivalent Python code would be: > > for row in db.execute("select ... from proba"): > db.execute("update proba ...") > > > The linked ruby script demonstrates a feature of the newer sqlite3 > libraries, which may be a regression. > > I do not know what you expect to happen, or what actually happens, but > changing a table and reading it through a query at the same time has an > unspecified result. In particular, the database might either fetch the > next result on demand from the actual table or have some result rows > already precomputed, so it is undefined whether continuing the SELECT > sees the old values or the new values or both or neither. > Right, unless you use WAL mode for MVCC, and do the updates on a separate connection and transaction. That way you iterate on the old rows at the time (snapshot, SCN, etc...) the select transaction starts, and the updates (on that other connection, in a different transaction) do not affect that select. At least that's the conceptual model I have in my head for SQLite in WAL mode. If that's incorrect, please let me know. Thanks, --DD _______________________________________________ sqlite-users mailing list sqlite-users@mailinglists.sqlite.org http://mailinglists.sqlite.org/cgi-bin/mailman/listinfo/sqlite-users
