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
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