Thread

1. [PATCH] Make skipped sort explicit in EXPLAIN ANALYZE

   Marti Raudsepp <marti@juffo.org> — 2012-05-25T13:30:01Z

   Hi,
   
   This is just a small clarity improvement. tuplesort_performsort()
   skips sorting entirely when the result set has 0 or 1 tuples, but
   EXPLAIN still says it's using "quicksort". The patch changes that to
   "skipped"
   
   For example:
   
   db=# explain analyze select * from now() order by 1;
    Sort  (cost=0.02..0.03 rows=1 width=8) (actual time=0.126..0.126
   rows=1 loops=1)
      Sort Key: now
      Sort Method: skipped  Memory: 25kB
      ->  Function Scan on now  (cost=0.00..0.01 rows=1 width=8) (actual
   time=0.032..0.033 rows=1 loops=1)
   
   Patch attached.
   
   Regards,
   Marti
   

2. Re: [PATCH] Make skipped sort explicit in EXPLAIN ANALYZE

   Tom Lane <tgl@sss.pgh.pa.us> — 2012-05-25T13:49:18Z

   Marti Raudsepp <marti@juffo.org> writes:
   > This is just a small clarity improvement. tuplesort_performsort()
   > skips sorting entirely when the result set has 0 or 1 tuples, but
   > EXPLAIN still says it's using "quicksort". The patch changes that to
   > "skipped"
   
   I'm not convinced this is an improvement; it's just one more value
   to confuse people with.  And any implementation of quicksort is likely
   to have a fast exit path for 0 or 1 item, so I don't think the existing
   display is incorrect either.
   
   			regards, tom lane
   
   

3. Re: [PATCH] Make skipped sort explicit in EXPLAIN ANALYZE

   Marti Raudsepp <marti@juffo.org> — 2012-05-25T14:19:28Z

   On Fri, May 25, 2012 at 4:49 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
   > I'm not convinced this is an improvement; it's just one more value
   > to confuse people with.
   
   I understand where you're coming from, but personally I think the
   current output is more confusing: "Gee Postgres is stupid, it's
   sorting when there's nothing to sort!"
   
   But let's wait for a third opinion.
   
   Regards,
   Marti
   
   

4. Re: [PATCH] Make skipped sort explicit in EXPLAIN ANALYZE

   Peter Geoghegan <peter@2ndquadrant.com> — 2012-05-25T14:39:24Z

   On 25 May 2012 15:19, Marti Raudsepp <marti@juffo.org> wrote:
   > I understand where you're coming from, but personally I think the
   > current output is more confusing: "Gee Postgres is stupid, it's
   > sorting when there's nothing to sort!"
   >
   > But let's wait for a third opinion.
   
   I agree with Tom. The idea that you "sort when there's nothing to
   sort" is not confusing. Textbook implementations of recursive sorting
   algorithms explicitly have a notion of sorting one element, by simply
   recognising that one element must already be sorted. For example, look
   at the quicksort pseudocode here:
   
   http://en.wikipedia.org/wiki/Quicksort
   
   -- 
   Peter Geoghegan       http://www.2ndQuadrant.com/
   PostgreSQL Development, 24x7 Support, Training and Services