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From: Tim Fors <tim4stheenchanter@gmail.com>
Date: Wed, 3 Dec 2025 13:02:40 -0500
Message-ID: 
 <CAOjiQ0C7NEV9ZBFxbM6KgXnzquaUY3MrN1x8vz_tv7uXac0UwQ@mail.gmail.com>
Subject: Client-server communication for FETCH
To: pgsql-performance@lists.postgresql.org
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Hello community,

I have a simple C program using libpq V18 to FETCH one row at a time (see
below). Does the server send multiple rows to the client where they are
cached for satisfying the FETCH statements, or does it just send one row at
a time since that's all that each FETCH statement is asking for? If it's
sending multiple rows at a time, is there some way that I can observe this
e.g. some kind of trace info that would show it? Or where in the libpq
source would I look to see how the client is retrieving rows from the
server?

I have done extensive searching to try and find the definitive answer to
this. The searches indicate that libpq supports the concept of a
client-side cursor, where it has a cache of rows sent by the server and
uses that cache to perform each FETCH, but I'd like to be able to verify
whether this is true or not.

Thanks,

Tim

  res = PQexec(conn, "DECLARE cur CURSOR FOR select * from table1");
  if (PQresultStatus(res) != PGRES_COMMAND_OK)
  {
    printf("DECLARE CURSOR failed: %s", PQerrorMessage(conn));
    PQclear(res);
    exit(1);
  }
  PQclear(res);

  res = PQexec(conn, "FETCH in cur");
  if (PQresultStatus(res) != PGRES_TUPLES_OK)
  {
    printf("FETCH failed: %s", PQerrorMessage(conn));
    PQclear(res);
    exit(1);
  }
  PQclear(res);

  res = PQexec(conn, "FETCH in cur");
  if (PQresultStatus(res) != PGRES_TUPLES_OK)
  {
    printf("FETCH failed: %s", PQerrorMessage(conn));
    PQclear(res);
    exit(1);
  }

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<div dir=3D"ltr">Hello community,<div><br></div><div>I have a simple C prog=
ram using libpq V18 to FETCH one row at a time (see below). Does the server=
 send multiple rows to the client where they are cached for satisfying the =
FETCH statements, or does it just send one row at a time since that&#39;s a=
ll that each FETCH statement is asking for? If it&#39;s sending multiple ro=
ws at a time, is there some way that I can observe this e.g. some kind of t=
race info that would show=C2=A0it? Or where in the libpq source would I loo=
k to see how the client is retrieving rows from the server?</div><div><br><=
/div><div>I have done extensive=C2=A0searching to try and find the definiti=
ve answer=C2=A0to this. The searches indicate that libpq supports the conce=
pt of a client-side cursor, where it has a cache of rows sent by the server=
 and uses that cache to perform each FETCH, but I&#39;d like to be able to =
verify whether this is true or not.=C2=A0</div><div><br></div><div>Thanks,<=
/div><div><br></div><div>Tim</div><div><br></div><div>=C2=A0 res =3D PQexec=
(conn, &quot;DECLARE cur CURSOR FOR select * from table1&quot;);<br>=C2=A0 =
if (PQresultStatus(res) !=3D PGRES_COMMAND_OK)<br>=C2=A0 {<br>=C2=A0 =C2=A0=
 printf(&quot;DECLARE CURSOR failed: %s&quot;, PQerrorMessage(conn));<br>=
=C2=A0 =C2=A0 PQclear(res);<br>=C2=A0 =C2=A0 exit(1);<br>=C2=A0 }<br>=C2=A0=
 PQclear(res);<br><br>=C2=A0 res =3D PQexec(conn, &quot;FETCH in cur&quot;)=
;<br>=C2=A0 if (PQresultStatus(res) !=3D PGRES_TUPLES_OK)<br>=C2=A0 {<br>=
=C2=A0 =C2=A0 printf(&quot;FETCH failed: %s&quot;, PQerrorMessage(conn));<b=
r>=C2=A0 =C2=A0 PQclear(res);<br>=C2=A0 =C2=A0 exit(1);<br>=C2=A0 }<br>=C2=
=A0 PQclear(res);<br><br>=C2=A0 res =3D PQexec(conn, &quot;FETCH in cur&quo=
t;);<br>=C2=A0 if (PQresultStatus(res) !=3D PGRES_TUPLES_OK)<br>=C2=A0 {<br=
>=C2=A0 =C2=A0 printf(&quot;FETCH failed: %s&quot;, PQerrorMessage(conn));<=
br>=C2=A0 =C2=A0 PQclear(res);<br>=C2=A0 =C2=A0 exit(1);<br>=C2=A0 }<br></d=
iv></div>

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