I think you are optimizing the performance of a conceptually inefficient query.

If you are looking for a recipe that contains apples, do you read the entire 
cook book, checking each recipe for apples? Maybe it is much more efficient to 
look up apples in the index of ingredients and retrieve only the recipes that 
actually contain them.

-----Urspr√ľngliche Nachricht-----
Von: sqlite-users [mailto:sqlite-users-boun...@mailinglists.sqlite.org] Im 
Auftrag von John Found
Gesendet: Montag, 05. Februar 2018 15:55
An: sqlite-users@mailinglists.sqlite.org
Betreff: [EXTERNAL] Re: [sqlite] Optimization - don't understand.

It is clear now. But should I define an index that contains all fields used in 
the query?

Something like:

    create index idxPostsComplex on posts(threadid, userid, Content, postTime, 
ReadCount);

Actually I tried and the query uses this index without problems (and the 
performance seems to be good).

But what are the disadvantages of such approach? (except the bigger database 
size, of course)

On Mon, 5 Feb 2018 09:24:51 -0500
Richard Hipp <d...@sqlite.org> wrote:

> On 2/5/18, John Found <johnfo...@asm32.info> wrote:
> > The following query:
> >
> >     explain query plan
> >     select
> >       U.nick,
> >       U.id,
> >       U.av_time,
> >       T.Caption,
> >       P.id,
> >     --  P.ReadCount,
> >     --  P.Content,
> >     --  P.postTime,l
> >       T.Caption
> >     from Posts P
> >     left join Threads T on P.threadID = T.id
> >     left join ThreadTags TT on TT.threadID = T.id
> >     left join Users U on P.userID = U.id
> >     where TT.Tag = ?1;
> >
> > ...returns:
> >
> >     0 0 0 SCAN TABLE Posts AS P USING COVERING INDEX idxPostsThreadUser
> >     0 1 1 SEARCH TABLE Threads AS T USING INTEGER PRIMARY KEY (rowid=?)
> >     0 2 2 SEARCH TABLE ThreadTags AS TT USING COVERING INDEX
> > idxThreadTagsUnique (ThreadID=? AND Tag=?)
> >     0 3 3 SEARCH TABLE Users AS U USING INTEGER PRIMARY KEY
> > (rowid=?)
> >
> > But uncommenting any of the commented fields, turns the result into:
> >
> >     0 0 0 SCAN TABLE Posts AS P
> >     0 1 1 SEARCH TABLE Threads AS T USING INTEGER PRIMARY KEY (rowid=?)
> >     0 2 2 SEARCH TABLE ThreadTags AS TT USING COVERING INDEX
> > idxThreadTagsUnique (ThreadID=? AND Tag=?)
> >     0 3 3 SEARCH TABLE Users AS U USING INTEGER PRIMARY KEY
> > (rowid=?)
> >
> > ... and significantly degrades the performance.
> >
> > The index idxPostsThreadUser is defined following way:
> >
> >     create index idxPostsThreadUser on Posts(threadid, userid);
> >
> > IMHO, the change of the selected columns should not affect the query
> > plan, but maybe I am wrong somehow.
> >
> > What I am missing?
> >
>
> SQLite prefers to scan the index rather than the original table,
> because the index is usually smaller (since it contains less data) and
> hence there is less I/O required to scan the whole thing.
>
> But the index only provides access to the id, threadid, and userid
> columns.  If content of other columns is needed, then the whole table
> must be scanned instead.
> --
> D. Richard Hipp
> d...@sqlite.org
> _______________________________________________
> sqlite-users mailing list
> sqlite-users@mailinglists.sqlite.org
> http://mailinglists.sqlite.org/cgi-bin/mailman/listinfo/sqlite-users


--
http://fresh.flatassembler.net
http://asm32.info
John Found <johnfo...@asm32.info>
_______________________________________________
sqlite-users mailing list
sqlite-users@mailinglists.sqlite.org
http://mailinglists.sqlite.org/cgi-bin/mailman/listinfo/sqlite-users


___________________________________________
 Gunter Hick | Software Engineer | Scientific Games International GmbH | 
Klitschgasse 2-4, A-1130 Vienna | FN 157284 a, HG Wien, DVR: 0430013 | (O) +43 
1 80100 - 0

May be privileged. May be confidential. Please delete if not the addressee.
_______________________________________________
sqlite-users mailing list
sqlite-users@mailinglists.sqlite.org
http://mailinglists.sqlite.org/cgi-bin/mailman/listinfo/sqlite-users

Reply via email to