Re: Strange choice of general index over partial index

On 16/01/15 15:32, Mark Kirkwood wrote:
> On 16/01/15 13:37, Mark Kirkwood wrote:
>> On 16/01/15 11:30, Josh Berkus wrote:
>>> This is an obfuscation and mock up, but:
>>>
>>> table files (
>>> id serial pk,
>>> filename text not null,
>>> state varchar(20) not null
>>> ... 18 more columns
>>> )
>>>
>>> index file_state on (state)
>>> (35GB in size)
>>> index file_in_flight_state (state) where state in (
>>> 'waiting','assigning', 'processing' )
>>> (600MB in size)
>>> ... 10 more indexes
>>>
>>> More important facts:
>>> * state = 'done' 95% of the time. thereform the partial index
>>> represents only 5% of the table
>>> * all indexes and the table are very bloated
>>> * server has 128GB RAM
>>> * Version 9.2.
>>>
>>> Given this setup, I would expect the planner to *always* choose
>>> file_in_flight_state over file_state for this query:
>>>
>>> SELECT id, filename FROM files WHERE state = 'waiting';
>>>
>>> ... and yet it keeps selecting file_state based on extremely small
>>> changes to the stats. This is important because the same query, using
>>> file_state, is 20X to 50X slower, because that index frequently gets
>>> pushed out of memory.
>>>
>>> What am I missing? Or is this potentially a planner bug for costing?
>>>
>>
>> Are you seeing a bitmapscan access plan? If so see if disabling it gets
>> you a plan on the files_in_flight index. I'm seeing this scenario with a
>> fake/generated dataset a bit like yours in 9.2 (9.5 uses the
>> files_in_flight w/o any coercing).
>>
>
> FWIW: For me 9.2 and 9.3 (default config) generate plans like:
> state=# EXPLAIN ANALYZE
> SELECT id, filename
> FROM files
> WHERE state = 'processing';
> QUERY PLAN
> ----------------------------------------------------------------------------------------------------------------------------------
>
> Bitmap Heap Scan on files (cost=3102.02..89228.68 rows=164333
> width=15) (actual time=26.629..803.507 rows=166696 loops=1)
> Recheck Cond: ((state)::text = 'processing'::text)
> Rows Removed by Index Recheck: 7714304
> -> Bitmap Index Scan on file_state (cost=0.00..3060.93 rows=164333
> width=0) (actual time=25.682..25.682 rows=166696 loops=1)
> Index Cond: ((state)::text = 'processing'::text)
> Total runtime: 808.662 ms
> (6 rows)
>
>
> whereas 9.4 and 9.5 get:
>
> QUERY PLAN
>
> ---------------------------------------------------------------------------------------------------------------------------------------
>
> Index Scan using file_in_flight on files (cost=0.42..62857.39
> rows=158330 width=15) (actual time=0.055..202.732 rows=166696 loops=1)
> Index Cond: ((state)::text = 'processing'::text)
> Planning time: 24.203 ms
> Execution time: 208.926 ms
> (4 rows)
>
>
> This is with each version loading exactly the same dataset (generated by
> the attached scripty). Obviously this is a vast simplification of what
> Josh is looking at - but it is (hopefully) interesting that these later
> versions are doing so much better...
>

A bit more poking about shows that the major factor (which this fake
dataset anyway) is the default for effective_cache_size (changes from
128MB to 4GB in 9.4). Increasing this makes 9.2 start using the
files_in_flight index in a plain index scan too.

Josh - might be worth experimenting with this parameter.

regards

Mark