Обсуждение: Query performance. 7.2.3 Vs. 7.3
While playing with one of my DBs under 7.3 to make use of its better explain features, I came across a query that runs significantly slower under 7.3 than 7.2.3. At first, I thought it would be a hardware issue, so i installed both versions on the same box. 7.2.3 tends to run the query in 80% of the time 7.3 does. Explain output can be found at http://arch.wavefire.com/72v73a.txt Please don't hesitate to drop me a line if you require more info.-Wade Klaver
wade <wade@wavefire.com> writes: > While playing with one of my DBs under 7.3 to make use of its better > explain features, I came across a query that runs significantly slower > under 7.3 than > 7.2.3. At first, I thought it would be a hardware issue, so i installed both > versions on the same box. > 7.2.3 tends to run the query in 80% of the time 7.3 does. > Explain output can be found at http://arch.wavefire.com/72v73a.txt The difference evidently is that 7.3 chooses a mergejoin where 7.2 picks a hashjoin. AFAICT this must be a consequence of the reduction in mergejoin estimated costs associated with this patch: 2002-02-28 23:09 tgl * src/: backend/executor/nodeMergejoin.c,backend/optimizer/path/costsize.c, backend/utils/adt/selfuncs.c,backend/utils/cache/lsyscache.c,include/utils/lsyscache.h,include/utils/selfuncs.h: Teach plannerabout the idea that amergejoin won't necessarily read both input streams to the end. Ifone variable's range is muchless than the other, anindexscan-based merge can win by not scanning all of the othertable. Per example from ReinhardMax. since we really didn't do anything else in 7.3 that changed the behavior of costsize.c. I can't get totally excited about a 20% estimation error (if the planner was never off by more than that, I'd be overjoyed ;-)) ... but if you want to dig into the statistics and try to figure out why this added logic is misestimating in your particular case, I'd be interested to hear. Probably the first thing to look at is why the estimated row counts are off by almost a factor of 3 for that join. regards, tom lane
On Thu, 2002-11-28 at 21:23, Tom Lane wrote: > wade <wade@wavefire.com> writes: > > Explain output can be found at http://arch.wavefire.com/72v73a.txt > > The difference evidently is that 7.3 chooses a mergejoin where 7.2 > picks a hashjoin. I was looking at this a bit in IRC, and I was more concerned by the fact that 7.3 was 20% than 7.2 on the same hardware, when they both used the same query plan (consider the data at the end of the URL above, after the execution of 'SET enable_mergejoin = off;'). Also, is it expected that the cardinality estimates for join steps won't be very accurate, right? (estimated: 19 rows, actual: 765 rows) Cheers, Neil -- Neil Conway <neilc@samurai.com> || PGP Key ID: DB3C29FC
Neil Conway <neilc@samurai.com> writes:
> I was looking at this a bit in IRC, and I was more concerned by the fact
> that 7.3 was 20% than 7.2 on the same hardware, when they both used the
> same query plan (consider the data at the end of the URL above, after
> the execution of 'SET enable_mergejoin = off;').
Hm. Are we sure that both versions were built with the same
optimization level, etc? (My private bet is that Wade's 7.2 didn't
have multibyte or locale support --- but that's a long shot when we
don't know the datatypes of the columns being joined on...)
> Also, is it expected that the cardinality estimates for join steps won't
> be very accurate, right? (estimated: 19 rows, actual: 765 rows)
Well, it'd be nice to do better --- I was hoping Wade would look into
why the row estimates were off so much.
regards, tom lane
At 09:58 PM 11/28/02 -0500, you wrote: >Hm. Are we sure that both versions were built with the same >optimization level, etc? (My private bet is that Wade's 7.2 didn't >have multibyte or locale support --- but that's a long shot when we >don't know the datatypes of the columns being joined on...) > >> Also, is it expected that the cardinality estimates for join steps won't >> be very accurate, right? (estimated: 19 rows, actual: 765 rows) OK, I've updated the link http://arch.wavefire.com/72v73a.txt to include the table schema for those involved in the query. As far as locale suport et al, I can tell you that both are built using a straigh, out-of-the-box ./configure. > >Well, it'd be nice to do better --- I was hoping Wade would look into >why the row estimates were off so much. I'd love to :). But where to start? Can you point me at a thread where a similar procedure was explained to someone else?-Wade > > regards, tom lane > >---------------------------(end of broadcast)--------------------------- >TIP 4: Don't 'kill -9' the postmaster > >