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Re: unexpected plan for unused table (8.1dev)

From: Hannu Krosing <hannu(at)skype(dot)net>
To: Oleg Bartunov <oleg(at)sai(dot)msu(dot)su>
Cc: Pgsql Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: unexpected plan for unused table (8.1dev)
Date: 2005-08-29 06:37:11
Message-ID: 1125297431.4897.3.camel@fuji.krosing.net (view raw or flat)
Thread:
Lists: pgsql-hackers
On E, 2005-08-29 at 09:50 +0400, Oleg Bartunov wrote:
> Hi there,
> 
> I'm a bit surprised how bad could be plan for unused table.
> I had to cancel second query :)
...
> tp=# explain analyze select tp_rewrite_substitute(query, 
> 'select p.name_tsquery, p.name_alias_tsquery from place p') 
> from place, to_tsquery('new&york&hotel') as query;
>       ^^^^^
> 
> Here is plan without analyze:
> 
> tp=# explain select tp_rewrite_substitute(query, 
> 'select p.name_tsquery, p.name_alias_tsquery from place p') 
> from place, to_tsquery('new&york&hotel') as query;

Actually you are asking the query to be evaluated once for each row of
table "place".

>                                  QUERY PLAN 
> ---------------------------------------------------------------------------
>   Nested Loop  (cost=13.50..44280620.90 rows=1964340000 width=32)
>     ->  Seq Scan on place  (cost=0.00..82957.40 rows=1964340 width=0)
>     ->  Materialize  (cost=13.50..23.50 rows=1000 width=32)
>           ->  Function Scan on query  (cost=0.00..12.50 rows=1000 width=32)
> (4 rows)
> 
> 
> 
> The only difference is unused table 'place' in FROM-clause. I typed in by
> mistake, but I think optimizer could figure out not to take into account
> this table.

This is valid SQL and afaik exactly what you asked for in the query.
Postgresql can't possibly know that you made a mistake :)

-- 
Hannu Krosing <hannu(at)skype(dot)net>


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