On Sun, Oct 25, 2009 at 10:56 AM, Kristoffer Danielsson <kristoffer.daniels...@live.se> wrote: > > Sure I can test this specific case, but my question is still relevant. Does > SQLite or does it not optimize repeated expressions? > >
You are correct. Stated as above, your question is very valid as it is trying to ascertain the way SQLite works internally, which may be known to a few of the developers. > > Am I supposed to brute-force all possible expression repetitions to find it > out, when a simple "yes" or "no" from anybody - who knows about the inner > workings of SQLite - would suffice? > >> Date: Sun, 25 Oct 2009 10:08:01 -0500 >> From: punk.k...@gmail.com >> To: sqlite-users@sqlite.org >> Subject: Re: [sqlite] How does SQLite treat repeated expressions? >> >> On Sun, Oct 25, 2009 at 9:49 AM, Igor Tandetnik <itandet...@mvps.org> wrote: >> > Kristoffer Danielsson wrote: >> >> A: SELECT COUNT(DataID) FROM Data GROUP BY DataID ORDER BY COUNT(DataID); >> >> >> >> B: SELECT COUNT(DataID) AS X FROM Data GROUP BY DataID ORDER BY X; >> >> >> >> Is statement B faster than A? Why? >> > >> > Last time I checked, SQLite didn't perform any kind of common >> > subexpression elimitation. Its expression evaluator was very >> > simple-minded, closely following the syntax tree. >> > >> > Things might have improved since then, but I wouldn't be surprised if >> > statement A calls count() twice per row while statement B only once. I >> > also think that any resulting difference in performance is likely to be >> > immeasurably small. Why don't you test it and find out, if you are really >> > curious? >> > >> >> >> +1 re "Why don't you test it and find out, if you are really curious?" >> >> While I can certainly understand a question of the sort, >> >> "I did A and I did B, and I found B to be faster than A; why?" >> >> I really can't understand the point of questions such as, >> >> "Here is A and B; which one will be faster?" >> >> since the best person to answer the latter question is the person >> posing the question -- just test it and find out. >> >> Now, it could be that implementing A or B just for testing purposes >> might be so difficult that the poster might want to get a sense >> *before* diving into the test. That is understandable, but, >> implementing something based solely on the unmeasured advice of others >> is itself questionable. >> >> >> >> >> -- >> Puneet Kishor http://www.punkish.org >> Carbon Model http://carbonmodel.org >> Charter Member, Open Source Geospatial Foundation http://www.osgeo.org >> Science Commons Fellow, http://sciencecommons.org/about/whoweare/kishor >> Nelson Institute, UW-Madison http://www.nelson.wisc.edu >> ----------------------------------------------------------------------- >> Assertions are politics; backing up assertions with evidence is science >> ======================================================================= >> Sent from Madison, WI, United States _______________________________________________ sqlite-users mailing list sqlite-users@sqlite.org http://sqlite.org:8080/cgi-bin/mailman/listinfo/sqlite-users
