[HACKERS] DBT-3 Query 2 EXPLAIN ANALYZE differences

[Mark Wong]

I was doing some testing with DBT-3 on our 8-way STP systems and
noticed a significant difference in the execution of Query 2 using
8.0beta3.

Here is the query template we're using:
select s_acctbal, s_name, n_name, p_partkey, p_mfgr, s_address, s_phone,
   s_comment
from part, supplier, partsupp, nation, region
where p_partkey = ps_partkey
  and s_suppkey = ps_suppkey
  and p_size = :1
  and p_type like '%:2'
  and s_nationkey = n_nationkey
  and n_regionkey = r_regionkey
  and r_name = ':3'
  and ps_supplycost = ( select min(ps_supplycost)
from partsupp, supplier, nation, region
where p_partkey = ps_partkey
  and s_suppkey = ps_suppkey
  and s_nationkey = n_nationkey
  and n_regionkey = r_regionkey
  and r_name = ':3'
  )
order by s_acctbal desc, n_name, s_name, p_partkey

This first run executes it pretty fast:
http://khack.osdl.org/stp/298338/  1555.414 ms

This second run executes it relatively slow:
http://khack.osdl.org/stp/298340/  42532.855 ms

The plans are different and I suspect thats where the differences lie.
For brevity (and readability) I won't copy the plans here but I'll
provide the links.  Search for 'PERF1.POWER.Q2' in the file, it's the
second query executed and you'll notice the differences under the
SubPlan:

The first run:
http://khack.osdl.org/stp/298338/results/run/power_query.result

The second run:
http://khack.osdl.org/stp/298340/results/run/power_query.result

I know Jenny has previously presented a problem that was solved by
doing a setseed(0), but I noticed the kit doesn't do that anymore.
Anyone know if this might be the same or related issues?

Thanks,
Mark

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Re: [HACKERS] DBT-3 Query 2 EXPLAIN ANALYZE differences

[Josh Berkus]

Mark,

 The plans are different and I suspect thats where the differences lie.
 For brevity (and readability) I won't copy the plans here but I'll
 provide the links. Search for 'PERF1.POWER.Q2' in the file, it's the
 second query executed and you'll notice the differences under the
 SubPlan:

Yeah, the difference seems to be an arbitrary choice of which table to join on 
first.The error is only +10 ms, but when it's being executed 3000 
times ...

Will delve into this when I can re-format it, and try to figure out why the 
planner is choosing what it is.

-- 
--Josh

Josh Berkus
Aglio Database Solutions
San Francisco

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