Re: [PERFORM] Regression from 9.4-9.6

On 10/8/17 3:37 PM, Tom Lane wrote:
> Jim Nasby <jim@nasby.net> writes:
>> On 10/8/17 2:34 PM, Tom Lane wrote:
>>> Why has this indexscan's cost estimate changed so much?
> 
>> 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.
> 
> Hm, now that I see the SubPlan in there, I wonder whether 9.6 is
> accounting more conservatively for the cost of the subplan.  It
> probably is assuming that the subplan gets run for each row fetched
> from the index, although the loops and rows-removed counts show
> that the previous filter conditions reject 99% of the fetched rows.
> 
> But that code looks the same in 9.4, so I don't understand why
> the 9.4 estimate isn't equally large ...

Besides the analyze issue, the other part of this is

asdidata@graceful.hou/20106> select 
pg_size_pretty(pg_relation_size('bdata_forks')); pg_size_pretty
---------------- 106 GB
(1 row)

asdidata@graceful.hou/20106> select relpages::bigint*8192/reltuples from 
pg_class where relname='bdata_forks';     ?column?
------------------ 185.559397863791
(1 row)

With an effective_cache_size of 200GB that's not really helping things. 
But it's also another example of the planner's reluctance towards index 
scans.
-- 
Jim C. Nasby, Data Architect                       jim@nasby.net
512.569.9461 (cell)                         http://jim.nasby.net


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