Re: [PERFORM] How to force an Index ?

[Josh Berkus]

Rhaoni,

 I could .. but this way I wont be used because Oracle doesn't accept such
 sintax ! I changed gsames00.ano_mes from varchar to text ! But it still not
 fast enough to take Oracle's place !!!
 I still trying to do so ...

Well, your basic problem is that performance tuning for *any* database often 
requires use of database-specific syntax.   You would be having the same 
problem, in the opposite direction, if you were trying to port a PostgreSQL 
app to Oracle without changing any syntax.

Here's syntax Oracle should accept:

... gsames00.ano_mes = (CAST(to_char(ftnfco00.data_emissao,'MM') AS 
VARCHAR)) AND 
...


-- 
Josh Berkus
Aglio Database Solutions
San Francisco

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Re: [PERFORM] How to force an Index ?

[Rhaoni Chiu Pereira]

I solve this problem doing this:

 create function date_to_mm( timestamp ) returns gsames00.ano_mes%type as
 'select to_char($1, ''MM'');
 ' language sql immutable strict;

And changing the SQL where clause:

 ... gsames00.ano_mes = to_char(ftnfco00.data_emissao,'MM') AND ...

to:

 ... gsames00.ano_mes = date_to_mm(ftnfco00.data_emissao) AND ...

   Then it uses the gsames00 index instead of a SeqScan 'cuz it is camparing
same data type, but .. I don't want to create this function 'cuz this aplication
is used with Oracle too. 
I need to know if there is a way to set the to_char output to varchar instead of
text !
  Any Idea ? So, this way I wont have to change my aplication source.



Atenciosamente,

Rhaoni Chiu Pereira
Sistêmica Computadores

Visite-nos na Web: http://sistemica.info
Fone/Fax : +55 51 3328 1122



Citando Josh Berkus [EMAIL PROTECTED]:

 Rhaoni,
 
 First off, thanks for posting such complete info.
 
 ... gsames00.ano_mes = to_char(ftnfco00.data_emissao,'MM') AND ...
  
 ftnfco00.data_emissao is a timestamp. When I run the explain analyze it
 
 says:
  
  ...
   -  Seq Scan on gsames00  (cost=1.00..10006.72 rows=372 
 width=10) 
  (actual time=0.01..0.96 rows=372 loops=19923)
  ...
 
 Your problem is that you're comparing against a calculated expression based
 on 
 ftnfco00, which is being filtered in about 18 other ways.  As a result, the
 
 planner doesn't know what to estimate (see the cost estimate of 1, 
 which is a blind guess values) and goes for a seq scan.
 
  Can I ask you to try this workaround, to create an expressional index on 
 ftnfco00 (assuming that data_emmisao is of type DATE)
 
 create function date_to_mm( date ) returns text as
 'select to_char($1, ''MM'');
 ' language sql immutable strict;
 
 create index idx_data_mm on ftnfco00(date_to_mm(data_emmisao));
 
 -- 
 -Josh Berkus
  Aglio Database Solutions
  San Francisco
 
 
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Re: [PERFORM] How to force an Index ?

[Tom Lane]

Rhaoni Chiu Pereira [EMAIL PROTECTED] writes:
 I need to know if there is a way to set the to_char output to varchar instead of
 text !

Why don't you change the datatype of ano_mes to text, instead?  It's
unlikely your application would notice the difference.  (You could set
a CHECK constraint on the length if you really want to duplicate the
behavior of varchar(6).)

Alternatively, try 7.4 beta. I believe this issue goes away in 7.4,
because varchar no longer has separate comparison operators.

Of course there's also the option of modifying to_char's result type
in pg_proc, but I won't promise that doing so wouldn't break things.

regards, tom lane

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