Re: Plan for relatively simple query seems to be very inefficient

Arjen van der Meijden <acmmailing(at)vulcanus(dot)its(dot)tudelft(dot)nl> writes:
> On 6-4-2005 19:42, Tom Lane wrote:
>> Wrong index ... what you probably could use here is an index on
>> data_main.range, so that the query could run with postcodes as the
>> outer side. I get such a plan by default with empty tables:
>> but I'm not sure if the planner would prefer it with the tables loaded
>> up. (It might not be the right thing anyway ... but seems worth
>> trying.)

> No it didn't prefer it.

Planner error ... because it doesn't have any good way to estimate the
number of matching rows, it thinks that way is a bit more expensive than
data_main as the outside, but in reality it seems a good deal cheaper:

arjen=# set enable_seqscan TO 1;
SET
arjen=# explain analyze
arjen-# SELECT COUNT(*) FROM data_main AS dm, postcodes AS p WHERE dm.range BETWEEN p.range_from AND p.range_till;
QUERY PLAN
-------------------------------------------------------------------------------------------------------------------------------
Aggregate (cost=332586.85..332586.85 rows=1 width=0) (actual time=143999.678..143999.683 rows=1 loops=1)
-> Nested Loop (cost=3.76..328945.96 rows=1456356 width=0) (actual time=0.211..143549.461 rows=82688 loops=1)
Join Filter: (("outer".range >= "inner".range_from) AND ("outer".range <= "inner".range_till))
-> Seq Scan on data_main dm (cost=0.00..1262.20 rows=81920 width=2) (actual time=0.059..663.065 rows=81920 loops=1)
-> Materialize (cost=3.76..5.36 rows=160 width=4) (actual time=0.004..0.695 rows=160 loops=81920)
-> Seq Scan on postcodes p (cost=0.00..3.60 rows=160 width=4) (actual time=0.028..1.589 rows=160 loops=1)
Total runtime: 144000.415 ms
(7 rows)

arjen=# set enable_seqscan TO 0;
SET
arjen=# explain analyze
arjen-# SELECT COUNT(*) FROM data_main AS dm, postcodes AS p WHERE dm.range BETWEEN p.range_from AND p.range_till;
QUERY PLAN
---------------------------------------------------------------------------------------------------------------------------------------------
Aggregate (cost=100336307.18..100336307.18 rows=1 width=0) (actual time=2367.097..2367.102 rows=1 loops=1)
-> Nested Loop (cost=100000000.00..100332666.28 rows=1456356 width=0) (actual time=0.279..1918.890 rows=82688 loops=1)
-> Seq Scan on postcodes p (cost=100000000.00..100000003.60 rows=160 width=4) (actual time=0.060..1.381 rows=160 loops=1)
-> Index Scan using dm_range on data_main dm (cost=0.00..1942.60 rows=9103 width=2) (actual time=0.034..7.511 rows=517 loops=160)
Index Cond: ((dm.range >= "outer".range_from) AND (dm.range <= "outer".range_till))
Total runtime: 2368.056 ms
(6 rows)

(this machine is slower than yours, plus I have profiling enabled still...)

regards, tom lane