Обсуждение: Problem with ALTER TYPE, Indexes and cast

Hello,

I plan to replace some btree indexes through btree_gin on some timed tables(e.g. monthly tables)
For this, I first need to change the data type from character(n) to varchar,
but I can't afford it on historical tables as this would be too time consuming, so only new tables should get the
varchartype. 

Now I have generated queries that include cast information in order to ensure that the indexes get used.

e.g.:  WHERE month1.foo = cast('XY' as character(2))

with mixed type, this should become something like:

 SELECT ... FROM month1
 WHERE month1.foo = cast('XY' as character(2))
 UNION ALL
 SELECT... FROM month2
 WHERE month2.foo = cast('XY' as varchar)

 which is quite complicated to resolve in our "query builder framework"


 There seems to be no way to have dynamic casting, something like:

  WHERE month2.foo = cast('XY' as 'month2.foo'::regtype)

 Is there a way for it ?


 regards,
 Marc Mamin

Marc Mamin <M.Mamin@intershop.de> writes:
> Now I have generated queries that include cast information in order to ensure that the indexes get used.

> e.g.:  WHERE month1.foo = cast('XY' as character(2))

> with mixed type, this should become something like:

>  SELECT ... FROM month1
>  WHERE month1.foo = cast('XY' as character(2))
>  UNION ALL
>  SELECT... FROM month2
>  WHERE month2.foo = cast('XY' as varchar)

>  which is quite complicated to resolve in our "query builder framework"

 >  There seems to be no way to have dynamic casting, something like:

>   WHERE month2.foo = cast('XY' as 'month2.foo'::regtype)

>  Is there a way for it ?

If the comparison values are always string literals, then you should just
drop the casts altogether, ie

  WHERE month2.foo = 'XY'

In this sort of situation the literal's type is preferentially resolved as
being the same as whatever it's being compared to.

            regards, tom lane

> -----Original Message-----
> From: Tom Lane [mailto:tgl@sss.pgh.pa.us]
> Sent: Mittwoch, 8. Juli 2015 15:44
> To: Marc Mamin
> Cc: pgsql-general@postgresql.org
> Subject: Re: [GENERAL] Problem with ALTER TYPE, Indexes and cast
>
> Marc Mamin <M.Mamin@intershop.de> writes:
> > Now I have generated queries that include cast information in order
> to ensure that the indexes get used.
>
> > e.g.:  WHERE month1.foo = cast('XY' as character(2))
>
> > with mixed type, this should become something like:
>
> >  SELECT ... FROM month1
> >  WHERE month1.foo = cast('XY' as character(2))  UNION ALL  SELECT...
> > FROM month2  WHERE month2.foo = cast('XY' as varchar)
>
> >  which is quite complicated to resolve in our "query builder
> framework"
>
>  >  There seems to be no way to have dynamic casting, something like:
>
> >   WHERE month2.foo = cast('XY' as 'month2.foo'::regtype)
>
> >  Is there a way for it ?
>
> If the comparison values are always string literals, then you should
> just drop the casts altogether, ie
>
>   WHERE month2.foo = 'XY'
>
> In this sort of situation the literal's type is preferentially resolved
> as being the same as whatever it's being compared to.

I had to dig a bit to find out why I was using the cast.
My issue is that I first clean the literal at some places with text returning functions.
The index won't get used when comparing to text:

    create temp table idtest (c character(8));
    insert into idtest select cast(s as character(8)) from generate_series(1,39999)s;
    create index idtest_c on idtest(c);
    analyze  idtest;

    explain analyze select * from idtest where c = substring (trim('1234567890abc') for 8)

    Seq Scan on idtest  (cost=0.00..816.99 rows=200 width=9) (actual time=20.302..20.302 rows=0 loops=1)
      Filter: ((c)::text = '12345678'::text)


I can easily get rid of the cast while preprocessing the literal before injecting it in the query though.

regards,
Marc Mamin




>             regards, tom lane