prepare in a do loop

On 2/15/21 8:18 AM, Marc Millas wrote:

Hi,

in psql, with a postgres 12.5 db on a centos 7 intel:
I do create a function named randname() returning a varchar, and a table
matable with a column prenom varchar(50). then
postgres=# prepare moninsert(varchar) as
postgres-# insert into matable(prenoms) values($1);
PREPARE

I test it:
postgres=# execute moninsert(randname());
INSERT 0 1

up to now, everything fine. then:
do $$ begin for counter in 1..1000000 loop execute
moninsert(randname());end loop;end;$$;
ERREUR:  la fonction moninsert(character varying) n'existe pas
LIGNE 1 : SELECT moninsert(randname())

someone can explain ?

EXECUTE in plpgsql means something different:

https://www.postgresql.org/docs/12/plpgsql-statements.html#PLPGSQL-STATEMENTS-EXECUTING-DYN

thanks

(its a french db, so error message in french :-)

Marc MILLAS
Senior Architect
+33607850334
www.mokadb.com <http://www.mokadb.com&gt;

--
Adrian Klaver
adrian.klaver@aklaver.com

On Mon, Feb 15, 2021 at 9:19 AM Marc Millas <marc.millas@mokadb.com> wrote:

postgres=# prepare moninsert(varchar) as

do $$ begin for counter in 1..1000000 loop execute
moninsert(randname());end loop;end;$$;
ERREUR: la fonction moninsert(character varying) n'existe pas
someone can explain ?

From the pl/pgsql docs:

"The PL/pgSQL EXECUTE statement is not related to the EXECUTE SQL statement
supported by the PostgreSQL server. The server's EXECUTE statement cannot
be used directly within PL/pgSQL functions (and is not needed)."

https://www.postgresql.org/docs/current/plpgsql-statements.html#PLPGSQL-STATEMENTS-EXECUTING-DYN

David J.

Marc Millas <marc.millas@mokadb.com> writes:

in psql, with a postgres 12.5 db on a centos 7 intel:
I do create a function named randname() returning a varchar, and a table
matable with a column prenom varchar(50). then
postgres=# prepare moninsert(varchar) as
postgres-# insert into matable(prenoms) values($1);
PREPARE

I test it:
postgres=# execute moninsert(randname());
INSERT 0 1

up to now, everything fine. then:
do $$ begin for counter in 1..1000000 loop execute
moninsert(randname());end loop;end;$$;
ERREUR: la fonction moninsert(character varying) n'existe pas
LIGNE 1 : SELECT moninsert(randname())

someone can explain ?

EXECUTE in plpgsql has nothing whatever to do with the SQL EXECUTE
command. See the respective documentation.

You don't actually need to use SQL PREPARE/EXECUTE in plpgsql.
If you just write "insert into ..." as a command in a
plpgsql function, it's automatically prepared behind the scenes.
Indeed, one of the common uses for plpgsql's EXECUTE is to stop
a prepared plan from being used when you don't want that ... so
far from being the same thing, they're more nearly opposites.
Perhaps a different name should have been chosen, but we're
stuck now.

regards, tom lane

Hi Tom,

I do read the doc, and understand the caching behaviour of plpgsql.
if in psql I write begin;execute moninsert(randname()); execute
moninsert(randname());end;
it does work. And if I put this (begin execute end) inside a do loop it
doesnt anymore.
ok the begin execute end is ""pure"" SQL, and the same thing within a do
loop is plpgsql
so
postgres=# create function testexec()returns void as $$
postgres$# execute moninsert(randname());
postgres$# end;
postgres$# $$ language plpgsql;
ERREUR: erreur de syntaxe sur ou près de « execute »
LIGNE 2 : execute moninsert(randname());
fine, quite coherent.
then
postgres=# create function testexec()returns void as $$
execute moninsert(randname());
end;
$$ language sql;
CREATE FUNCTION
as SQL, legal syntax.. ok
but
postgres=# select testexec();
ERREUR: COMMIT n'est pas autorisé dans une fonction SQL
CONTEXTE : fonction SQL « testexec » lors du lancement
a bit more difficult to understand, as such.(where is the commit ??)
so.. the prepare//execute thing can only be used in embedded SQL (as not in
any plpg, nor in sql functions.
The doc states :
The SQL standard includes a PREPARE statement, but it is only for use in
embedded SQL. This version of the PREPARE statement also uses a somewhat
different syntax.
??? where is the difference for the prepare context thing (I dont mean the
different syntax part) ??

thanks for clarification

Marc MILLAS
Senior Architect
+33607850334
www.mokadb.com

On Mon, Feb 15, 2021 at 5:27 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:

On 2/15/21 9:55 AM, Marc Millas wrote:

Hi Tom,

I do read the doc, and understand the caching behaviour of plpgsql.

This is not about plpgsql caching. It is about EXECUTE in plpgsql
meaning something different then the PREPARE/EXECUTE combination in SQL.
You are trying to run EXECUTE moninsert(randname()) in plpgsql where
moninsert was a PREPARE statement. In plpgsql EXECUTE is something
different so it does not recognize moninsert(randname()) as a prepared
statement and fails.

if in psql I write begin;execute moninsert(randname()); execute
moninsert(randname());end;
it does work.  And if I put this (begin execute end) inside a do loop it
doesnt anymore.
ok the begin execute end is ""pure"" SQL, and the same thing within a do
loop is plpgsql
so
postgres=# create function testexec()returns void as $$
postgres$# execute moninsert(randname());
postgres$# end;
postgres$# $$ language plpgsql;
ERREUR:  erreur de syntaxe sur ou près de « execute »
LIGNE 2 : execute moninsert(randname());
fine, quite coherent.
then

--
Adrian Klaver
adrian.klaver@aklaver.com