Re: I think that my data is saved correctly, but when printing again, other data appears

Hi,
Worked on something similar a few years back, have a look - it might give
you a few pointers :)
It's on similar lines to what you are looking at (I kept it updated up to
PG version 9.4/9.5)
https://github.com/cohenjo/pg_idx_advisor

My main focus was to add support for more index types: partial, functional,
CTE, composites etc...
(I also had to keep the original context to restore it - so I think you
will find a sample to what you are looking for... )

there's also https://github.com/HypoPG/hypopg which is actively maintained.
I think you will find code samples even up to the latest versions here...

Regards,
- Jony

On Sat, Oct 26, 2019 at 4:06 AM Yessica Brinkmann <
yessica(dot)brinkmann(at)gmail(dot)com> wrote:

> Good evening, sorry for the delay in answering. I have a part-time job and
> I was at it.
> I understand what you tell me about the "data" value is just a pointer
> into the tupdesc associated with the SPI result, and that disappears the
> moment I do SPI_finish ().
> What I do not understand well is how to use CurrentMemoryContext and
> MemoryContextStrdup, since there are not many examples of using them on the
> Internet (most are only definitions) and it is the first time I have heard
> of this, although I already understand this part now of contexts, because I
> was reading on the subject.
> Could you please give me an example of use?
> Best Regards,
> Yessica Brinkmann
>
> El vie., 25 oct. 2019 a las 12:24, Yessica Brinkmann (<
> yessica(dot)brinkmann(at)gmail(dot)com>) escribió:
>
>> Thank you so much for your answer. I will be testing the indicated and
>> then I give you return.
>> Best regards,
>>
>> Yessica Brinkmann
>>
>>
>> -------- Mensaje original --------
>> Asunto: Re: I think that my data is saved correctly, but when printing
>> again, other data appears
>> De: Tom Lane
>> Para: Yessica Brinkmann
>> CC: pgsql-general(at)lists(dot)postgresql(dot)org
>>
>>
>> Yessica Brinkmann writes:
>> > I have a function called get_columnnames, which runs on a Postgresql
>> server
>> > and returns the names of the columns of a table in a Postgresql
>> database.
>> > In the first foreach, in the statement: idxcd-> varattnames [cont] =
>> data;
>> > column names are saved. Those column names are saved correctly. I could
>> > verify this by printing some values.
>> > But later, in the following foreach, when I reprint the values of the
>> names
>> > of the columns that had been saved in the idxcd-> varattnames [cont] =
>> data
>> > statement; They appear to me with errors.
>>
>> Yeah, because the "data" value is just a pointer into the tupdesc
>> associated with the SPI result, and that disappears the moment you
>> do SPI_finish(). You'd need to do something to copy the strings
>> into a longer-lived context. A plain pstrdup() won't suffice
>> because you're in a short-lived SPI context already inside that
>> loop; but you could save CurrentMemoryContext before starting up
>> SPI and then use MemoryContextStrdup.
>>
>> regards, tom lane
>>
>>