On 10/8/17 2:34 PM, Tom Lane wrote:
Jim Nasby <jim@xxxxxxxxx> writes:
I've got a query that's regressed from 9.4 to 9.6. I suspect it has
something to do with the work done around bad plans from single-row
estimates.
Why has this indexscan's cost estimate changed so much?
-> Index Scan using bdata_filed_departuretime on bdata_forks (cost=0.57..50807.51 rows=1 width=36) (actual time=979.381..3207.777 rows=508 loops=1)
-> Index Scan using bdata_filed_departuretime on bdata_forks (cost=0.57..14894236.06 rows=1 width=36) (actual time=892.664..3025.653 rows=508 loops=1)
I think the reason it's discarding the preferable plan is that, with this
huge increment in the estimated cost getting added to both alternatives,
the two nestloop plans have fuzzily the same total cost, and it's picking
the one you don't want on the basis of some secondary criterion.
Great question... the only thing that sticks out is the coalesce(). Let
me see if an analyze with a higher stats target changes anything. FWIW,
the 9.6 database is copied from the 9.4 one once a week and then
pg_upgraded. I'm pretty sure an ANALYZE is part of that process.
9.4:
-> Index Scan using bdata_filed_departuretime on bdata_forks (cost=0.57..50807.51 rows=1 width=36) (actual time=979.381..3207.777 rows=508 loops=1)
Index Cond: ((filed_departuretime >= '2017-07-20 05:00:00'::timestamp without time zone) AND (filed_departuretime <= '2017-07-30 04:59:59'::timestamp without time zone))
Filter: (((view_www IS NULL) OR (view_www IS TRUE)) AND (sch_block_out IS NOT NULL) AND (diverted IS NOT TRUE) AND (true_cancel IS NOT TRUE) AND (sch_block_out >= '2017-07-23 05:00:00'::timestamp without time zone) AND (sch_block_out <= '2017-07-24 04:59:59'::timestamp without time zone) AND (COALESCE(actualarrivaltime, cancellation) >= actualdeparturetime) AND ((act_block_out - sch_block_out) >= '00:15:00'::interval) AND (((SubPlan 2))::text = 'KORD'::text))
Rows Removed by Filter: 2696593
SubPlan 2
-> Index Scan using bd_airport_pkey on bd_airport bd_airport_1 (cost=0.56..4.58 rows=1 width=20) (actual time=0.003..0.003 rows=1 loops=21652)
Index Cond: (id = bdata_forks.origin_id)
9.6:
-> Index Scan using bdata_filed_departuretime on bdata_forks (cost=0.57..14894236.06 rows=1 width=36) (actual time=892.664..3025.653 rows=508 loops=1)
Index Cond: ((filed_departuretime >= '2017-07-20 05:00:00'::timestamp without time zone) AND (filed_departuretime <= '2017-07-30 04:59:59'::timestamp without time zone))
Filter: (((view_www IS NULL) OR (view_www IS TRUE)) AND (sch_block_out IS NOT NULL) AND (diverted IS NOT TRUE) AND (true_cancel IS NOT TRUE) AND (sch_block_out >= '2017-07-23 05:00:00'::timestamp without time zone) AND (sch_block_out <= '2017-07-24 04:59:59'::timestamp without time zone) AND (COALESCE(actualarrivaltime, cancellation) >= actualdeparturetime) AND ((act_block_out - sch_block_out) >= '00:15:00'::interval) AND (((SubPlan 2))::text = 'KORD'::text))
Rows Removed by Filter: 2696592
SubPlan 2
-> Index Scan using bd_airport_pkey on bd_airport bd_airport_1 (cost=0.56..4.58 rows=1 width=20) (actual time=0.004..0.004 rows=1 loops=21652)
Index Cond: (id = bdata_forks.origin_id)
--
Jim C. Nasby, Data Architect jim@xxxxxxxxx
512.569.9461 (cell) http://jim.nasby.net
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