> -----Original Message-----
> From: Hyrum K Wright [mailto:hy...@hyrumwright.org]
> Sent: maandag 16 mei 2011 9:39
> To: Branko Čibej
> Cc: dev@subversion.apache.org
> Subject: Re: SQLite and the LIKE operator
>
> 2011/5/16 Branko Čibej <br...@e-reka.si>:
> > On 16.05.2011 03:13, Hyrum K Wright wrote:
> >> Several places in wc_db we use the following pattern to select all
> >> nodes with a common tree ancestor:
> >> WHERE wc_id = ?1 AND (local_relpath = ?2 OR local_relpath LIKE ?3
> ESCAPE '#')
> >>
> >> While this works, there was some concern about whether or not SQLite
> >> was using the proper indicies when executing this query. By examining
> >> the output for 'EXPLAIN QUERY PLAN' on some of the relevant SELECT
> >> statements, I believe it does use the indicies as intended.
> >>
> >> However, I stumbled across an alternate implementation which I believe
> >> has some merit. Instead of the above clause, we could use:
> >> WHERE wc_id = ?1 AND substr(local_relpath, 1, length(?2)) = ?2
This also needs a table scan, as SQLite can't look through the substr to find
that it can use the index for the result.
My guess is that
WHERE wc_id = ?1 AND ((local_relpath = ?2) OR (local_relpath > ?2 || '/' AND
local_relpath < ?2 || '0'))
is most likely the most efficient form we can get in SQLite as the constant
string directly map to an index, but we should really create a few tests to
verify these guesses.
I'm going to the Elego office now. See you there? :)
Bert
