Re: [HACKERS] Project proposal/comments please - query optimization

Kim Bisgaard Fri, 12 Aug 2005 01:20:08 -0700

Tom Lane wrote:

Kim Bisgaard <[EMAIL PROTECTED]> writes:

I have noticed a deficiency in the current query optimizer related to 
"full outer joins". Tom Lane has confirmed to me that it will not be 8.1 
material.


... There
are related issues involving N-way joins that we're still not very
good at.
...


Consider this version of Kim Bisgaard's example:
	SELECT FROM a join (b full join c using (id)) using (id)
If A is small and B,C have indexes on ID then it is interesting to consider
a plan like
	Nest Loop
		Scan A
		Merge Full Join
			Indexscan B using id = outer.id
			Indexscan C using id = outer.id
We are fairly far from being able to do this. generate_outer_join_implications
could easily be modified to generate derived equalities (I think it works to
allow a deduction against any clause not overlapping the outerjoin itself)
but the planner would want to evaluate them at the wrong level, and the
executor doesn't have support for passing the outer variable down more than
one level of join.  This is why the existing hack works only for equalities
to pseudoconstants.  We could maybe mark join RestrictInfos as "valid only
below xxx" and ignore them when processing a join that includes all of the
indicated rels?  Still not clear how you get the planner to recognize the
above as an inner indexscan situation though.

The query samples I gave was the smallest test I could find to provoke the behavior. Tom is right in that the full case I am ideally want solved is of the form above with lots (below 20) of full outer joined tables.

I am still a little intrigued by the effect of substituting "full" with "left" in my examples; maybe an alternative to Toms idea could be to work in the direction of treating "full" more like "left/right"

There are some examples of my problems (on nightly builds of yesterday) at the bottom of the mail.

Regards,
Kim.

obsdb=> explain analyse select wmo_id,timeobs,temp_dry_at_2m,temp_grass
from station
         join (temp_grass
                 full join temp_dry_at_2m using (station_id, timeobs)
         ) using (station_id)
where wmo_id=6065 and '2004-1-2 6:0' between startdate and enddate
and timeobs = '2004-1-2 06:0:0';

                                                                                      QUERY PLAN
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Hash Join (cost=5.04..372928.92 rows=1349 width=28) (actual time=23986.480..46301.966 rows=1 loops=1)
   Hash Cond: (COALESCE("outer".station_id, "outer".station_id) = "inner".station_id)
   -> Merge Full Join (cost=0.00..338124.90 rows=6957100 width=32) (actual time=23965.761..46281.380 rows=76 loops=1)
         Merge Cond: (("outer".timeobs = "inner".timeobs) AND ("outer".station_id = "inner".station_id))
         Filter: (COALESCE("outer".timeobs, "inner".timeobs) = '2004-01-02 06:00:00'::timestamp without time zone)
         -> Index Scan using temp_grass_idx on temp_grass (cost=0.00..75312.59 rows=2406292 width=16) (actual time=12.436..4916.043 rows=2406292 loops=1)
         -> Index Scan using temp_dry_at_2m_idx on temp_dry_at_2m (cost=0.00..210390.85 rows=6957100 width=16) (actual time=13.696..21363.054 rows=6956994 loops=1)
   -> Hash (cost=5.03..5.03 rows=1 width=8) (actual time=19.612..19.612 rows=0 loops=1)
         -> Index Scan using wmo_idx on station (cost=0.00..5.03 rows=1 width=8) (actual time=19.586..19.591 rows=1 loops=1)
               Index Cond: ((wmo_id = 6065) AND ('2004-01-02 06:00:00'::timestamp without time zone >= startdate) AND ('2004-01-02 06:00:00'::timestamp without time zone <= enddate))
Total runtime: 46302.208 ms
(11 rows)

obsdb=> explain analyse select wmo_id,timeobs,wind_dir_10m,temp_dry_at_2m,temp_grass
from station
join (temp_grass
full join temp_dry_at_2m using (station_id, timeobs)
full join wind_dir_10m using (station_id, timeobs)
) using (station_id)
where wmo_id=6065 and '2004-1-2 6:0' between startdate and enddate
and timeobs = '2004-1-2 06:0:0';

                                                                                      QUERY PLAN
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Hash Join (cost=1249443.54..1700082.70 rows=1392 width=40) (actual time=331437.803..384389.174 rows=1 loops=1)
   Hash Cond: (COALESCE(COALESCE("outer".station_id, "outer".station_id), "outer".station_id) = "inner".station_id)
   -> Merge Full Join (cost=1249438.51..1664170.44 rows=7178660 width=48) (actual time=291406.613..384359.322 rows=82 loops=1)
         Merge Cond: (("outer".station_id = "inner"."?column7?") AND ("outer".timeobs = "inner"."?column8?"))
         Filter: (COALESCE("inner"."?column8?", "outer".timeobs) = '2004-01-02 06:00:00'::timestamp without time zone)
         -> Index Scan using wind_dir_10m_idx on wind_dir_10m (cost=0.00..299534.38 rows=7178660 width=16) (actual time=15.194..52605.785 rows=7178639 loops=1)
         -> Sort (cost=1249438.51..1266831.26 rows=6957100 width=32) (actual time=290255.163..302211.520 rows=8743745 loops=1)
               Sort Key: COALESCE(temp_grass.station_id, temp_dry_at_2m.station_id), COALESCE(temp_grass.timeobs, temp_dry_at_2m.timeobs)
               -> Merge Full Join (cost=0.00..337004.00 rows=6957100 width=32) (actual time=28.909..73512.533 rows=8743745 loops=1)
                     Merge Cond: (("outer".timeobs = "inner".timeobs) AND ("outer".station_id = "inner".station_id))
                     -> Index Scan using temp_grass_idx on temp_grass (cost=0.00..75312.59 rows=2406292 width=16) (actual time=15.178..6088.732 rows=2406292 loops=1)
                     -> Index Scan using temp_dry_at_2m_idx on temp_dry_at_2m (cost=0.00..210390.85 rows=6957100 width=16) (actual time=13.694..25573.509 rows=6956994 loops=1)
   -> Hash (cost=5.03..5.03 rows=1 width=8) (actual time=28.609..28.609 rows=0 loops=1)
         -> Index Scan using wmo_idx on station (cost=0.00..5.03 rows=1 width=8) (actual time=28.581..28.585 rows=1 loops=1)
               Index Cond: ((wmo_id = 6065) AND ('2004-01-02 06:00:00'::timestamp without time zone >= startdate) AND ('2004-01-02 06:00:00'::timestamp without time zone <= enddate))
Total runtime: 384979.287 ms
(16 rows)

Re: [HACKERS] Project proposal/comments please - query optimization

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