Обсуждение: Number of updated rows with LibPQ

Hi,

Is there a way to programmatically now how many rows an UPDATE did update?
I've read about [PQcmdTuples][1], but surely I shouldn't have to parse
that string, no?
For selects, I have [PQnTuples][2], but what to do on INSERT, UPDATE, DELETE?
Parse the result of PQcmdTuples myself??? If so, what's the 0 in the
INSERT below?
Is the output of PQcmdTuples "stable", i.e. "official"? Not locale dependent?

Thanks, --DD

[1]: https://www.postgresql.org/docs/14/libpq-exec.html#LIBPQ-PQCMDTUPLES
[2]: https://www.postgresql.org/docs/14/libpq-exec.html#LIBPQ-PQNTUPLES

postgres=# create table foo (v int);
CREATE TABLE
postgres=# insert into foo values (1), (2), (3);
INSERT 0 3
postgres=# update foo set v = 2*v where v = 2;
UPDATE 1
postgres=# delete from foo where v = 3;
DELETE 1
postgres=# select v from foo where v > 1;
...
(1 row)
postgres=# delete from foo;
DELETE 2
postgres=# drop table foo;
DROP TABLE
postgres=#

On Wed, 2022-10-05 at 16:38 +0200, Dominique Devienne wrote:
> Hi,
> 
> Is there a way to programmatically now how many rows an UPDATE did update?
> I've read about [PQcmdTuples][1], but surely I shouldn't have to parse
> that string, no?
> For selects, I have [PQnTuples][2], but what to do on INSERT, UPDATE, DELETE?
> Parse the result of PQcmdTuples myself??? If so, what's the 0 in the
> INSERT below?
> Is the output of PQcmdTuples "stable", i.e. "official"? Not locale dependent?
> 
> Thanks, --DD
> 
> [1]: https://www.postgresql.org/docs/14/libpq-exec.html#LIBPQ-PQCMDTUPLES
> [2]: https://www.postgresql.org/docs/14/libpq-exec.html#LIBPQ-PQNTUPLES
> 
> postgres=# create table foo (v int);
> CREATE TABLE
> postgres=# insert into foo values (1), (2), (3);
> INSERT 0 3
> postgres=# update foo set v = 2*v where v = 2;
> UPDATE 1
> postgres=# delete from foo where v = 3;
> DELETE 1
> postgres=# select v from foo where v > 1;
> ...
> (1 row)
> postgres=# delete from foo;
> DELETE 2
> postgres=# drop table foo;
> DROP TABLE
> postgres=#

Yes, you have to use PQcmdTuples(), and you have to convert the string to an integer.

But don't worry: the result will *not* be "INSERT 0 5", it will be just "5", so
you won't have to parse anything.

Yours,
Laurenz Albe
-- 
Cybertec | https://www.cybertec-postgresql.com

Laurenz Albe <laurenz.albe@cybertec.at> writes:
> On Wed, 2022-10-05 at 16:38 +0200, Dominique Devienne wrote:
>> For selects, I have [PQnTuples][2], but what to do on INSERT, UPDATE, DELETE?
>> Parse the result of PQcmdTuples myself??? If so, what's the 0 in the
>> INSERT below?

> Yes, you have to use PQcmdTuples(), and you have to convert the string to an integer.
> But don't worry: the result will *not* be "INSERT 0 5", it will be just "5", so
> you won't have to parse anything.

Yeah, just applying atoi() or atol() to the result should be enough.

            regards, tom lane

On Wed, Oct 5, 2022 at 8:17 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Laurenz Albe <laurenz.albe@cybertec.at> writes:
> > On Wed, 2022-10-05 at 16:38 +0200, Dominique Devienne wrote:
> > Yes, you have to use PQcmdTuples(), and you have to convert the string to an integer.
> > But don't worry: the result will *not* be "INSERT 0 5", it will be just "5"

Thanks. What's the leading 0 though, then?
I guessed it might be the number of rows returned, but it isn't, see below:

postgres=# create table foo (id serial primary key, v int);
CREATE TABLE
postgres=# insert into foo (v) values (1), (2) returning id;
 id
----
  1
  2
(2 rows)

INSERT 0 2

> Yeah, just applying atoi() or atol() to the result should be enough.

Thanks too. Since I'm in C++, I used <charconv> instead, and
discovered it can be empty something, not 0.
I guess atoi() would have hidden that distinction, and worked anyway
(returning 0).

In the same topic, I've noticed an INSERT returns PGRES_COMMAND_OK,
while an INSERT-RETURNING returns PGRES_TUPLES_OK. So there's no way
to use the status to distinguish a SELECT from anything else? A RETURNING clause
makes any statement supporting it an hybrid of a command and a query,
but then how
does one know the exact "kind" of the statement?

E.g. So how does psql show INSERT in either cases? By parsing the SQL
itself, client-side?
Or is there a libpq API on PGresult that would allow to get the type
of statement the result is from?

Thanks, --DD

On Fri, 2022-10-14 at 13:52 +0200, Dominique Devienne wrote:
> On Wed, Oct 5, 2022 at 8:17 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
> > Laurenz Albe <laurenz.albe@cybertec.at> writes:
> > > On Wed, 2022-10-05 at 16:38 +0200, Dominique Devienne wrote:
> > > Yes, you have to use PQcmdTuples(), and you have to convert the string to an integer.
> > > But don't worry: the result will *not* be "INSERT 0 5", it will be just "5"
> 
> Thanks. What's the leading 0 though, then?
> I guessed it might be the number of rows returned, but it isn't, see below:
> 
> postgres=# create table foo (id serial primary key, v int);
> CREATE TABLE
> postgres=# insert into foo (v) values (1), (2) returning id;
>  id
> ----
>   1
>   2
> (2 rows)
> 
> INSERT 0 2

That 0 is the OID of the newly inserted tuple.
Since there are no more tables WITH OIDS, the number is always 0, but
is left in the output for compatibility reasons.

> > Yeah, just applying atoi() or atol() to the result should be enough.
> 
> Thanks too. Since I'm in C++, I used <charconv> instead, and
> discovered it can be empty something, not 0.
> I guess atoi() would have hidden that distinction, and worked anyway
> (returning 0).
> 
> In the same topic, I've noticed an INSERT returns PGRES_COMMAND_OK,
> while an INSERT-RETURNING returns PGRES_TUPLES_OK. So there's no way
> to use the status to distinguish a SELECT from anything else? A RETURNING clause
> makes any statement supporting it an hybrid of a command and a query,
> but then how
> does one know the exact "kind" of the statement?
> 
> E.g. So how does psql show INSERT in either cases? By parsing the SQL
> itself, client-side?
> Or is there a libpq API on PGresult that would allow to get the type
> of statement the result is from?

The command tag is not what you look at.

You simply check the result from an INSERT statement.  If it is PGRES_TUPLES_OK,
it must have been INSERT ... RETRUNING.

Yours,
Laurenz Albe
-- 
Cybertec | https://www.cybertec-postgresql.com

    Laurenz Albe wrote:

> > Or is there a libpq API on PGresult that would allow to get the type
> > of statement the result is from?
>
> The command tag is not what you look at.

Yet that's what psql does. from PrintQueryResult():

    /* if it's INSERT/UPDATE/DELETE RETURNING, also print status */
    if (last || pset.show_all_results)
    {
        cmdstatus = PQcmdStatus(result);
        if (strncmp(cmdstatus, "INSERT", 6) == 0 ||
            strncmp(cmdstatus, "UPDATE", 6) == 0 ||
            strncmp(cmdstatus, "DELETE", 6) == 0)
            PrintQueryStatus(result, printStatusFout);
    }



> You simply check the result from an INSERT statement.  If it is
> PGRES_TUPLES_OK, it must have been INSERT ... RETRUNING.

Or a SELECT, or an UPDATE RETURNING, or a DELETE RETURNING.
The code still need to look at PQcmdStatus() to learn which kind it is,
if it needs to know this.


Best regards,
--
Daniel Vérité
https://postgresql.verite.pro/
Twitter: @DanielVerite

On Fri, Oct 14, 2022 at 4:39 PM Daniel Verite <daniel@manitou-mail.org> wrote:
> Or a SELECT, or an UPDATE RETURNING, or a DELETE RETURNING.
> The code still need to look at PQcmdStatus() to learn which kind it is,

Thanks Daniel. This is exactly what I needed. --DD

PS: Now I wonder whether PQcmdTuples() points to the same buffer as
PQcmdStatus(), except with an offset...