Re: [SQL] From with case
Quoth pavel.steh...@gmail.com (Pavel Stehule):
> Dne 25.3.2013 23:51 "Ben Morrow" napsal(a):
> >
> > I would use a view for this:
> >
> > create view vale_any as
> > select 'P'::text "type", v.adiant, v.desc_per, v.cod
> > from valepag v
> > union all
> > select 'R', v.adiant, v.desc_per, v.cod
> > from valerec v;
> >
> > then
> >
> > for rSql in
> > select a.adiant, a.desc_per
> > from vale_any a
> > where a.type = cTip and a.cod = 2
> > loop
>
> This design has a performance problem. You read both tables everywhere -
> for large tables can be bad
You would think so, but, in general, Pg is cleverer than that. For the
simple case of queries with constants in (so, a client-submitted query
like
select * from vale_any a where a.type = 'P' and a.cod = 2
or the equivalent with bound placeholders) the planner won't even plan
the parts of the view which don't get used. Try some experiments with
EXPLAIN to see what I mean: the unused sections of the Append (that is,
the UNION ALL) are either omitted entirely or get replaced with
Result
One-Time Filter: false
(I'm not entirely sure what makes the difference, though it seems to be
to do with how complicated the individual parts of the UNION are).
PL/pgSQL is a bit more complicated, because (unless you use EXECUTE) it
pre-plans all its statements, so the condition on a.type is not constant
at planning time. However, if you PREPARE a statement like
prepare v as select * from vale_any a
where a.type = $1 and a.cod = $2
and then run it with EXPLAIN ANALYZE EXECUTE v ('P', 2) you will see
that although the plan includes the parts of the view that don't get
used they are all marked '(never executed)' by EXPLAIN ANALYZE, because
the executor had enough information to work out they could never return
any rows. Skipping those parts of the plan at execute time does have a
small cost--for small tables you will see the total query time go up a
little for a prepared statement--but nothing like the cost of scanning a
large table. I would expect it's about the same as the cost of a
PL/pgSQL IF/THEN/ELSE.
It's worth noting at this point that if you know the rows of a UNION
will be distinct it's worth making it a UNION ALL, since otherwise Pg
has to add a sort-and-uniq step which can be expensive.
Ben
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Re: [SQL] From with case
This design has a performance problem. You read both tables everywhere - for large tables can be bad Dne 25.3.2013 23:51 "Ben Morrow" napsal(a): > > Quoth c...@sygecom.com.br (Mauricio Cruz): > > > > I'm working in a PL/SQL and I'd like to use the same > > PL for 2 kinds of tables... > > > > I have "valepag" and "valerec" both tables > > have the same columns, but one is for debit and the other one is for > > credit, the PL will work for both cases > > > > with the unique diference for > > the name of the table... > > > > So I thought to use something like this: > > ... > > > > For rSql in select a.adiant, > > a.desc_per > > from case > > when > > cTip='P' > > then valapag > > else valerec > > end > > where cod=2 Loop > > > > ... > > > > But > > it just dont work... does some one have other solution for this case ? > > I would use a view for this: > > create view vale_any as > select 'P'::text "type", v.adiant, v.desc_per, v.cod > from valepag v > union all > select 'R', v.adiant, v.desc_per, v.cod > from valerec v; > > then > > for rSql in > select a.adiant, a.desc_per > from vale_any a > where a.type = cTip and a.cod = 2 > loop > > You need to cast the constant in the view definition, otherwise Pg > complains about its type being ambiguous. You should use the same type > as cTip will be. > > Ben > > > > -- > Sent via pgsql-sql mailing list (pgsql-sql@postgresql.org) > To make changes to your subscription: > http://www.postgresql.org/mailpref/pgsql-sql
Re: [SQL] From with case
Quoth c...@sygecom.com.br (Mauricio Cruz): > > I'm working in a PL/SQL and I'd like to use the same > PL for 2 kinds of tables... > > I have "valepag" and "valerec" both tables > have the same columns, but one is for debit and the other one is for > credit, the PL will work for both cases > > with the unique diference for > the name of the table... > > So I thought to use something like this: > ... > > For rSql in select a.adiant, > a.desc_per > from case > when > cTip='P' > then valapag > else valerec > end > where cod=2 Loop > > ... > > But > it just dont work... does some one have other solution for this case ? I would use a view for this: create view vale_any as select 'P'::text "type", v.adiant, v.desc_per, v.cod from valepag v union all select 'R', v.adiant, v.desc_per, v.cod from valerec v; then for rSql in select a.adiant, a.desc_per from vale_any a where a.type = cTip and a.cod = 2 loop You need to cast the constant in the view definition, otherwise Pg complains about its type being ambiguous. You should use the same type as cTip will be. Ben -- Sent via pgsql-sql mailing list (pgsql-sql@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-sql
Re: [SQL] From with case
Thanks very much Pavel, the dynamic SQL ideia has work perfect in my
case.
Thanks so much.
--
Grato,
Mauricio Cruz
Sygecom Informática
51 3442-3975 / 3442-2345
On Mon, 25 Mar 2013 15:01:09 +0100, Pavel Stehule
wrote:
> Hello
>
>
>> For rSql in select a.adiant,
>>a.desc_per
>> from case
>> when cTip='P'
>> then valapag
>> else valerec
>>end
>> where cod=2 Loop
>
>
> you can use a dynamic SQL, but it is not best solution usually. In
> this case I usually prefer
>
> IF cTip = 'P' THEN
> FOR r IN SELECT .. FROM valapag LOOP
> PERFORM proc(r);
> END LOOP;
> ELSE
> FOR r IN SELECT .. FROM valerec LOOP
> PERFORM proc(r);
> END LOOP;
> END IF;
>
> with dynamic SQL
>
> FOR r IN EXECUTE format('SELECT .. FROM %I ..', CASE WHEN ctip = 'P'
> THEN 'valapag' ELSE 'valerec' END)
> LOOP
> ..
> END LOOP;
>
> Regards
>
> Pavel Stehule
>
>
> 2013/3/25 Mauricio Cruz :
>> Hi everyone,
>>
>>
>>
>> I'm working in a PL/SQL and I'd like to use the same PL for 2 kinds of
>> tables...
>>
>> I have "valepag" and "valerec" both tables have the same columns, but one is
>> for debit and the other one is for credit, the PL will work for both cases
>>
>> with the unique diference for the name of the table...
>>
>>
>>
>> So I thought to use something like this:
>>
>> ...
>>
>> For rSql in select a.adiant,
>>a.desc_per
>> from case
>> when cTip='P'
>> then valapag
>> else valerec
>>end
>> where cod=2 Loop
>>
>>
>>
>> ...
>>
>>
>> But it just dont work... does some one have other solution for this case ?
>>
>>
>>
>> Thanks guys.
>>
>>
>>
>> --
>> Grato,
>> Mauricio Cruz
>> Sygecom Informática
>> 51 3442-3975 / 3442-2345
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Re: [SQL] From with case
Hello
> For rSql in select a.adiant,
>a.desc_per
> from case
> when cTip='P'
> then valapag
> else valerec
>end
> where cod=2 Loop
you can use a dynamic SQL, but it is not best solution usually. In
this case I usually prefer
IF cTip = 'P' THEN
FOR r IN SELECT .. FROM valapag LOOP
PERFORM proc(r);
END LOOP;
ELSE
FOR r IN SELECT .. FROM valerec LOOP
PERFORM proc(r);
END LOOP;
END IF;
with dynamic SQL
FOR r IN EXECUTE format('SELECT .. FROM %I ..', CASE WHEN ctip = 'P'
THEN 'valapag' ELSE 'valerec' END)
LOOP
..
END LOOP;
Regards
Pavel Stehule
2013/3/25 Mauricio Cruz :
> Hi everyone,
>
>
>
> I'm working in a PL/SQL and I'd like to use the same PL for 2 kinds of
> tables...
>
> I have "valepag" and "valerec" both tables have the same columns, but one is
> for debit and the other one is for credit, the PL will work for both cases
>
> with the unique diference for the name of the table...
>
>
>
> So I thought to use something like this:
>
> ...
>
> For rSql in select a.adiant,
>a.desc_per
> from case
> when cTip='P'
> then valapag
> else valerec
>end
> where cod=2 Loop
>
>
>
> ...
>
>
> But it just dont work... does some one have other solution for this case ?
>
>
>
> Thanks guys.
>
>
>
> --
> Grato,
> Mauricio Cruz
> Sygecom Informática
> 51 3442-3975 / 3442-2345
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[SQL] From with case
Hi everyone, I'm working in a PL/SQL and I'd like to use the same PL for 2 kinds of tables... I have "valepag" and "valerec" both tables have the same columns, but one is for debit and the other one is for credit, the PL will work for both cases with the unique diference for the name of the table... So I thought to use something like this: ... For rSql in select a.adiant, a.desc_per from case when cTip='P' then valapag else valerec end where cod=2 Loop ... But it just dont work... does some one have other solution for this case ? Thanks guys. -- Grato, Mauricio Cruz Sygecom Informática [1] 51 3442-3975 / 3442-2345 Links: -- [1] http://www.sygecom.com.br
