Re: CTIDs invalidations and dropping columns. - Mailing list pgsql-hackers

On Tuesday 11 July 2006 00:35, Martijn van Oosterhout wrote:
> On Mon, Jul 10, 2006 at 11:47:23PM +0300, Tzahi Fadida wrote:
> > Hi,
> > First, i use CTIDs to immensely speed up my function which is inherently
> > slow because of the problem itself.
> >
> > I have a question about CTID invalidation when you open a read only
> > cursor using SPI. Why does it at all happens? Why is it so important to
> > invalidate a ctid of a read only query (for example when using indices,
> > casting,etc...)?
>
> You're talking about "invalidation" as if it's something someone
> deliberately does. That's incorrect. The t_ctid field is filled in if
> and only if the tuple is exactly the on disk tuple. Otherwise it's a
> new tuple, which by definition does not have a ctid (it doesn't exist
> on disk).

As i understand rowids, i.e ctids, are supposed to allow for fast access to
the tables. I don't see the rational, for example, when casting some
attributes, to blank the ctid. So it is not exactly the same, but it still
came from the same tuple. What will happen if for read only SPI queries
it will not be blank?

>
> > Specifically, i encountered something unexpected. i created a table:
> > (a2 int4, a0 int4) then i did alter table add column a5 int4, then update
> > set a5=a0, update set a0=a0+1, alter table drop column a0.
> >
> > Now that i run a simple select * from SPI cursor query on this table and
> > look at the  t_data->t_ctid i see that the ctids are invalidated for some
> > unknown reason?
> > previously before the alter table it was ok.
>
> This doesn't make any sense. What is invalidated? Is it blank or what?
> I think you're going to have to provide some example code.

blank.

i am attaching a code. it is not supposed to run to completion but to print to
screen using elog.
i used this sql to declare the function but you'll need to alter it where it
says fdfuncs:
CREATE OR REPLACE FUNCTION ctest(text) RETURNS SETOF RECORD
AS 'fdfuncs','ctest'
LANGUAGE C STABLE STRICT;

anyway, run these commands:
create table ctest (a2 int4, a0 int4);
insert into ctest values (10,11);
insert into ctest values (12,13);
then run the function.
it should print 1 in posid and 0 in hi and lo.

Now run the following commands:
alter table ctest add column a5 int4;
alter table ctest DROP COLUMN a0;

and run the function.
it should print 0 on all three fields which means the ctid is blank.

>
> What do you mean by "invalidation" anyway?
>
> > I highly prefer not to use CTID as an attribute since it is going to
> > greatly lower the performance since it is sitting on a bottleneck.
>
> You've measured this performance difference?

Yes, i played with this in the past. since i can pass over a relation
potentially hundreds or thousands of times, this can be a bottleneck.
Full disjunctions is a difficult problem that in order to speed up uses the
tuple set concept where a set of tuples are represented as a set of CTIDs.

>
> Have a nice day,

--
Regards,
��������Tzahi.
--
Tzahi Fadida
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