On 10/11/2016 04:07 AM, Andres Freund wrote:
On 2016-10-10 17:46:22 -0700, Andres Freund wrote:
TPC-DS (tpcds.ods)

In this case, I'd say the results are less convincing. There are quite a few
queries that got slower by ~10%, which is well above - for example queries
22 and 67. There are of course queries that got ~10% faster, and in total
the patched version executed more queries (so overall the result is slightly
positive, but not significantly).

That's interesting. I wonder whether that's plan changes just due to the
changing memory estimates, or what's causing that. I'll look into it.

Hm. Based on an initial look those queries aren't planned with any of
the affected codepaths.  Could this primarily be a question of
randomness? Would it perhaps make sense to run the tests in a comparable
order? Looking at tpcds.py and the output files, it seems that the query
order differes between the runs, that can easily explain bigger
difference than the above. For me a scale=1 run creates a database of
approximately 4.5GB, thus with shared_buffers=1GB execution order is
likely to have a significant performance impact.

Yes, I see similar plans (no bitmap index scans or hash aggregates). But the difference is there, even when running the query alone (so it's not merely due to the randomized ordering).

I wonder whether this is again due to compiler moving stuff around.


Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

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