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

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@lists.postgresql.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

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.brinkmann@gmail.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@lists.postgresql.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

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.brinkmann@gmail.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@lists.postgresql.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

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.brinkmann@gmail.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.brinkmann@gmail.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@lists.postgresql.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

On 10/27/19 8:01 PM, Yessica Brinkmann wrote:

Thank you so much for the answers.
By telling me this: "MemoryContextStrdup enables you to create a copy of
a string in an explicitly specified memory context." I better understood
the function of MemoryContextStrdup.
And thank you very much to Mr. Jony Cohen for giving me the reference of
his work. I really was already researching about his work, and I
included it in the state of the art of my thesis, but I didn't look at
the source code.
I will be looking at the source code for a help, and especially in this
case to see for the moment, the maintenance of the context for its
subsequent restoration.

For better understanding of how Postgres manages memory, you might want
to also read this:

https://github.com/postgres/postgres/blob/master/src/backend/utils/mmgr/README

and possibly browse through this:

https://github.com/postgres/postgres/blob/master/src/backend/utils/mmgr/mcxt.c

HTH,

Joe
--
Crunchy Data - http://crunchydata.com
PostgreSQL Support for Secure Enterprises
Consulting, Training, & Open Source Development

Thank you very much for the answer.
Best regards,
Yessica Brinkmann

El lun., 28 oct. 2019 a las 8:03, Joe Conway (<mail@joeconway.com>)
escribió:

On 10/27/19 8:01 PM, Yessica Brinkmann wrote:

Thank you so much for the answers.
By telling me this: "MemoryContextStrdup enables you to create a copy of
a string in an explicitly specified memory context." I better understood
the function of MemoryContextStrdup.
And thank you very much to Mr. Jony Cohen for giving me the reference of
his work. I really was already researching about his work, and I
included it in the state of the art of my thesis, but I didn't look at
the source code.
I will be looking at the source code for a help, and especially in this
case to see for the moment, the maintenance of the context for its
subsequent restoration.

For better understanding of how Postgres manages memory, you might want
to also read this:

https://github.com/postgres/postgres/blob/master/src/backend/utils/mmgr/README

and possibly browse through this:

https://github.com/postgres/postgres/blob/master/src/backend/utils/mmgr/mcxt.c

HTH,

Joe
--
Crunchy Data - http://crunchydata.com
PostgreSQL Support for Secure Enterprises
Consulting, Training, & Open Source Development

On 11/25/19 4:38 PM, Yessica Brinkmann wrote:

Well, as I was told that I should save the
CurrentMemoryContext before starting the SPI, Let's say it would be
before doing SPI_connect (). Is this correct? And then I must use
MemoryContextStrdup. As you told me the MemoryContextStrdup It is
used to create a copy of a string in a specific memory context. Well,
where in the source code should I use MemoryContextStrdup? After
doing the SPI_connect () or where? I would use it from
MemoryContextStrdup to copy the data variable as I understand it, But
in what context would I have to create the copy? In a new context or
what would the theme be like? Should I use AllocSetContextCreate to
create the new context or what would the theme be like? And if I have
to create the new context with AllocSetContextCreate, where in the
source code will I have to create it? After doing SPI_connect () or
where? The truth is that I also read the source code of
https://github.com/cohenjo/pg_idx_advisor but I don't see that The
MemoryContextStrdup that they told me to use has been used there.
Sorry for the inconvenience and see the same thing again. But as I
indicated, I also read the explanations they told me to read (which
were explanations about the memory contexts in Postgresql mainly) but
there were no examples of source code. And since there is nothing on
the Internet of examples that will help me to use, I am asking
again. And the truth is that I didn't find examples of this in the
Postgres source code, just definitions, That is the source code
where MemoryContextStrdup is defined. It may be very easy for you and
you will see it very clearly, but for me it really is not, and there
are no examples of use on the Internet. I really searched a lot and
found nothing. I would greatly appreciate a help please.

Sorry but I am not going to try to address that wall of text ;-)
But here is some general information about how that stuff works:
---------------------------------
1. The most common pattern is something like this:

MemoryContext oldcontext;

oldcontext = MemoryContextSwitchTo(<some_memory_context>);

/* do stuff that allocates memory
* using PostgreSQL allocation functions
* e.g. palloc, pstrdup, other exported
* backend functions, etc
*/

MemoryContextSwitchTo(oldcontext);

2. MemoryContextStrdup() is similar to the above, except in that case
you do not need MemoryContextSwitchTo(). It directly allocates into
the specified memory context without all the switching back and
forth. If you are simply copying one string and need it in a context
other than what is current, it is more convenient. But either method
could be used.

3. When you run SPI_connect() the memory context is switched
transparently for you to a special SPI memory context. When you run
SPI_finish() the original memory context (the one in effect before
SPI_connect) is restored.

4. Depending on what you are trying to do, use method #1 or method #2 if
needed, including while doing SPI related things (in between
SPI_connect and SPI_finish)

Joe

--
Crunchy Data - http://crunchydata.com
PostgreSQL Support for Secure Enterprises
Consulting, Training, & Open Source Development