Re: planner not choosing fastest estimated plan

Hello

pls, can you send EXPLAIN ANALYZE result?

Regards

Pavel Stehule

2013/7/6 Jeff Janes <jeff.janes@gmail.com>:
> I have a weird case where the planner doesn't choose the plan that it
> itself believes to be the fastest plan.  If I disable seqscan, it then
> chooses a faster plan (faster in estimate and faster in reality) than
> the one chosen when all options were open to it.  I can't figure out
> how this can be anything other than a bug.  The *collapse_limit
> parameters are not restraining things.
>
> I've created a dummy self-contained test case that is a simple
> self-join of a partitioned table, with a function-based index.
>
> If I analyze the tables after the function-based indexes are in place,
> then the problems goes away.  And that is the production solution.
> But still, a bug is a bug, even if there is a work around.
>
> This is using the default configuration with LANG=C in 9.2, 9.3, and 9.4.
>
> It was introduced in commit 5b7b5518d0ea56c422a19787, "Revise
> parameterized-path mechanism to fix assorted issues"
>
> I've tried compiling under OPTIMIZER_DEBUG, but the output did not
> mean anything to me.  It looks like the only RELOPTINFO corresponding
> to the join, "RELOPTINFO (1 2)" only lists the HashJoin when
> enable_seqscan=on, and only contains NestLoop when enable_seqscan=off.
>  I don't know why it doesn't list both in both cases and then choose
> the faster.
>
>
>
> create table foo1 as select lpad(g::text,7,'0') as val1, g as num1
> from generate_series(1,100000) g;
> create table foo2 as select lpad(g::text,7,'0') as val1, g as num1
> from generate_series(100001,1000000) g;
> alter table foo2 inherit foo1;
> create index on foo1(num1 );
> create index on foo2(num1 );
> analyze foo1; analyze foo2;
> create language plperl;
> CREATE OR REPLACE FUNCTION perlupper(text)
>  RETURNS text
>  LANGUAGE plperl
>  IMMUTABLE COST 1000
> AS $function$
>   return uc($_[0]);
> $function$;
> create index on foo1 (perlupper(val1));
> create index on foo2 (perlupper(val1));
>
>
> jjanes=# explain select a.num1 from foo1 a, foo1 b where
> perlupper(a.val1)=perlupper(b.val1) and b.num1 = 987845;
>                                        QUERY PLAN
> ----------------------------------------------------------------------------------------
>  Hash Join  (cost=32789.00..538040.65 rows=10000 width=4)
>    Hash Cond: (perlupper(b.val1) = perlupper(a.val1))
>    ->  Append  (cost=0.00..16.64 rows=2 width=8)
>          ->  Index Scan using foo1_num1_idx on foo1 b
> (cost=0.00..8.28 rows=1 width=8)
>                Index Cond: (num1 = 987845)
>          ->  Index Scan using foo2_num1_idx on foo2 b
> (cost=0.00..8.37 rows=1 width=8)
>                Index Cond: (num1 = 987845)
>    ->  Hash  (cost=15406.00..15406.00 rows=1000000 width=12)
>          ->  Append  (cost=0.00..15406.00 rows=1000000 width=12)
>                ->  Seq Scan on foo1 a  (cost=0.00..1541.00 rows=100000 width=12)
>                ->  Seq Scan on foo2 a  (cost=0.00..13865.00
> rows=900000 width=12)
>
> jjanes=# set enable_seqscan TO off;
> jjanes=# explain select a.num1 from foo1 a, foo1 b where
> perlupper(a.val1)=perlupper(b.val1) and b.num1 = 987845;
>                                            QUERY PLAN
> -------------------------------------------------------------------------------------------------
>  Nested Loop  (cost=17.14..60438.19 rows=10000 width=4)
>    ->  Append  (cost=0.00..16.64 rows=2 width=8)
>          ->  Index Scan using foo1_num1_idx on foo1 b
> (cost=0.00..8.28 rows=1 width=8)
>                Index Cond: (num1 = 987845)
>          ->  Index Scan using foo2_num1_idx on foo2 b
> (cost=0.00..8.37 rows=1 width=8)
>                Index Cond: (num1 = 987845)
>    ->  Append  (cost=17.14..30160.77 rows=5000 width=12)
>          ->  Bitmap Heap Scan on foo1 a  (cost=17.14..3022.65 rows=500 width=12)
>                Recheck Cond: (perlupper(val1) = perlupper(b.val1))
>                ->  Bitmap Index Scan on foo1_perlupper_idx
> (cost=0.00..17.01 rows=500 width=0)
>                      Index Cond: (perlupper(val1) = perlupper(b.val1))
>          ->  Bitmap Heap Scan on foo2 a  (cost=92.22..27138.12
> rows=4500 width=12)
>                Recheck Cond: (perlupper(val1) = perlupper(b.val1))
>                ->  Bitmap Index Scan on foo2_perlupper_idx
> (cost=0.00..91.10 rows=4500 width=0)
>                      Index Cond: (perlupper(val1) = perlupper(b.val1))
>
> Cheers,
>
> Jeff
>
>
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