Обсуждение: unexpected plan for unused table (8.1dev)

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 to_tsquery('new&york&hotel') as query;
y;                                                  QUERY PLAN 
---------------------------------------------------------------------------------------------------------------
FunctionScan on query  (cost=0.00..15.00 rows=1000 width=32) (actual time=3801.069..3801.071 rows=1 loops=1) Total
runtime:3801.107 ms
 
(2 rows)

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;                                QUERY PLAN 
--------------------------------------------------------------------------- Nested Loop  (cost=13.50..44280620.90
rows=1964340000width=32)   ->  Seq Scan on place  (cost=0.00..82957.40 rows=1964340 width=0)   ->  Materialize
(cost=13.50..23.50rows=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.

    Regards,        Oleg
_____________________________________________________________
Oleg Bartunov, sci.researcher, hostmaster of AstroNet,
Sternberg Astronomical Institute, Moscow University (Russia)
Internet: oleg@sai.msu.su, http://www.sai.msu.su/~megera/
phone: +007(095)939-16-83, +007(095)939-23-83

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@skype.net>

On Mon, 29 Aug 2005, Hannu Krosing wrote:

>>
>> 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 :)

Oh, yes, you're right, of course !

>
>
    Regards,        Oleg
_____________________________________________________________
Oleg Bartunov, sci.researcher, hostmaster of AstroNet,
Sternberg Astronomical Institute, Moscow University (Russia)
Internet: oleg@sai.msu.su, http://www.sai.msu.su/~megera/
phone: +007(095)939-16-83, +007(095)939-23-83