Re: svn commit: r1081484 - in /subversion/trunk/subversion/libsvn_wc: wc-queries.sql wc_db.c

On Mar 28, 2011 7:32 AM, "Stefan Sperling" <stsp_at_elego.de> wrote:
>
> On Sat, Mar 26, 2011 at 09:46:48AM -0400, Greg Stein wrote:
> > If it only needs to detect a single row, then LIMIT 1 should be in the
> > query.
>
> Is there really a significant difference between LIMIT 1 and not stepping
> through more than one result row?
>
> I'd rather keep the queries reusable for contexts where a complete
> list of modified nodes is required, instead of adding COUNT() (as
> suggested elsewhere) or LIMIT 1.

Think about how the query optimizer/planner would work. Consider whether a
result set needs to be constructed.

... a LIMIT 1 provides a ton of help to the SQL engine about what needs to
be fetched (or not).

There is also a semantic element to it. Readers of the code will see that
you're looking for basic presence rather than a set of results.

If/when you need a query containing the set of nodes with mods, then you can
consider another query. Until that time, the LIMIT is helpful/best.

Cheers,
-g
Received on 2011-03-28 13:42:28 CEST