Vladimir Sitnikov wrote:
> Here's the simplified testcase:
> https://gist.github.com/vlsi/df08cbef370b2e86a5c1
>
> It reproduces the problem in both 9.4.4 and 9.5rc1.
> It is reproducible via both psql and pgjdbc.
>
> I use a single table, however my production case includes a join of
> two tables and the query is like
> select ... from foo, bar where foo.skewed=? and bar.non_skewed=? and
> foo.bar_id=bar.id
>
> Note: my application _always_ sends *the same* *bad* value for skewed
> column (it effectively is used as a filtering column in the particular
> query).
> Unfortunately, on 6th execution backend switches to the plan that uses
> skewed index access.
>
> Is it something that can be fixed/improved?
>
> Good plan (the first 5 executions):
> Index Scan using non_skewed__flipper on plan_flipper
> (cost=0.43..42.77 rows=10 width=113) (actual time=0.030..0.072 rows=10
> loops=1)
> Index Cond: (non_skewed = 42)
> Filter: (skewed = 0)
> Rows Removed by Filter: 10
> Buffers: shared hit=20 read=3
> Execution time: 0.094 ms
>
> Bad plan (all the subsequent executions):
> Index Scan using skewed__flipper on plan_flipper (cost=0.43..6.77
> rows=1 width=113) (actual time=0.067..355.867 rows=10 loops=1)
> Index Cond: (skewed = $1)
> Filter: (non_skewed = $2)
> Rows Removed by Filter: 999990
> Buffers: shared hit=18182 read=2735
> Execution time: 355.901 ms
The problem is that the index "skewed__flipper" is more selective than
"non_skewed__flipper" except when "skewed = 0", so the generic plan prefers it.
I don't know if there is a good solution except disabling server prepared statements.
Yours,
Laurenz Albe
