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 <[email protected]> wrote:
> On 2/5/18, John Found <[email protected]> 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
> [email protected]
> _______________________________________________
> sqlite-users mailing list
> [email protected]
> http://mailinglists.sqlite.org/cgi-bin/mailman/listinfo/sqlite-users
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John Found <[email protected]>
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