Message-ID: <55070FC9.4010307@agliodbs.com>
Date: Mon, 16 Mar 2015 10:15:53 -0700
From: Josh Berkus <josh@agliodbs.com>
Organization: PostgreSQL Experts Inc.
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To: pgsql-performance@postgresql.org
Subject: Re: Bad cost estimate with FALSE filter condition
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So ... should I assume my diagnosis is correct?  Haven't heard any other
suggestions.

On 02/27/2015 05:28 PM, Josh Berkus wrote:
> All:
> 
> This got posted to pgsql-bugs, but got no attention there[1], so I'm
> sending it to this list.
> 
> Test case:
> 
> createdb bench
> pgbench -i -s bench
> \c bench
> 
> bench=# explain select * from pgbench_accounts where aid = 2;
>                                           QUERY PLAN
> ---------------------------------------------------------------
>  Index Scan using pgbench_accounts_pkey on pgbench_accounts
> (cost=0.42..8.44 rows=1 width=97)
>    Index Cond: (aid = 2)
> (2 rows)
> 
> bench=# explain select * from pgbench_accounts where aid = 2 and false;
> 
>                                 QUERY PLAN
> -------------------------------------------------
>  Result  (cost=0.00..26394.00 rows=1 width=97)
>    One-Time Filter: false
>    ->  Seq Scan on pgbench_accounts  (cost=0.00..26394.00 rows=1 width=97)
> (3 rows)
> 
> This seems like a special case of the "aborted plan cost", that is, when
> the planner expects to abort a plan early, it nevertheless returns the
> full cost for the non-aborted version of the query, rather than the
> working cost, which is based on the abort.
> 
> For example:
> 
> bench=# create index on pgbench_accounts(bid);
> CREATE INDEX
> bench=# explain select * from pgbench_accounts where bid = 2;
>                                                 QUERY PLAN
> 
> ----------------------------------------------------------------------------------------------------------
>  Index Scan using pgbench_accounts_bid_idx on pgbench_accounts
> (cost=0.42..4612.10 rows=102667 width=97)
>    Index Cond: (bid = 2)
> (2 rows)
> 
> bench=# explain select * from pgbench_accounts where bid = 2 limit 1;
>                                    QUERY PLAN
> 
> --------------------------------------------------------------------------------
>  Limit  (cost=0.00..0.28 rows=1 width=97)
>    ->  Seq Scan on pgbench_accounts  (cost=0.00..28894.00 rows=102667
> width=97)
>          Filter: (bid = 2)
> (3 rows)
> 
> So in this case, the top-level node returns a lower cost because the
> planner knows that it will find a row with bid=2 fairly quickly in the
> seq scan.  But in the WHERE FALSE example, that scan *is* the top-level
> node, so the planner returns a fictitious cost for the whole query.
> 
> Or is there something else at work here?
> 
> [1]
> http://www.postgresql.org/message-id/20150225194953.2546.86836@wrigleys.postgresql.org
> 


-- 
Josh Berkus
PostgreSQL Experts Inc.
http://pgexperts.com


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