plperl: invoking other subroutines

The official plperl documentation shows you a way to use a subref to invoke code shared across different plperl functions via the special global hash %_SHARED. While this is a good approach, it only works for code attached to the hash, that is a kind of closure (e.g., a dispatch table), and requires each time an initialization of the %_SHARED hash since plperl interpreters does not share nothing across sections. The other way, always working, is to execute a query to perform the SELECT that will invoke the function. As an example:
CREATE OR REPLACE FUNCTION plperl_trampoline( fun_name text )
   my ( $fun_name ) = @_;
   return undef if ( ! $fun_name );
   elog( DEBUG, "Calling [$fun_name]" );
   my $result_set = spi_exec_query( "SELECT $fun_name() AS result;" ); 
   return $result_set->{ rows }[ 0 ]->{ result };
LANGUAGE plperl;
so that you can simply do:
> select plperl_trampoline( 'now' );

 2018-05-04 13:09:17.11772+02
The problem of this solution should be clear: it can work only for a set of functions with the same prototype. In fact, while it could be simple to work around the argument passing (thank to some magic with Perl arrays), the return type and, most notably, its arity makes the approach not easily universal.

Another introspective approach could have been to use pg_proc.prosrc to translate the Perl code to an anonymous function on the fly, and put it into the %_SHARED global hash. However, this requires special care about arguments too, and makes it less than trivial to handle the function protytpe.

An example of using %_SHARED to get sequence values

Once common issue when dealing with stored procedures is to get new values from sequences. While this is really trivial in plpgsql, and reduces to a single call to nextval(), it is not so simple in plperl where an spi_exec_query() has to be issued. It is however possible to use the %_SHARED hash to add an handler for the same query:
CREATE OR REPLACE FUNCTION plperl_add_sequence_handler( s text )

   my ( $sequence ) = @_;
   return 0 if ( ! $sequence );
   my $query = sprintf "SELECT nextval( '%s' )", $sequence;

   elog( DEBUG, "Query [$query]" );
   $_SHARED{ $sequence } = sub {
      return spi_exec_query( $query )->{ rows }[ 0 ]->{ nextval };

LANGUAGE plperl;
The above function sets up an handler with the name of the sequence itself, and each time its code is executed a query is issued against the database to get the nextval(). Therefore, it is quite simple to set-up a sequence value in a session and do a retrieval:
> SELECT plperl_add_sequence_handler( 'persona_pk_seq' );
-- and later

> DO language plperl $$
   elog( INFO, $_SHARED{ persona_pk_seq }->() );
In the above plperl code the coderef is invoked via a reference in the %_SHARED hash, in particular: $_SHARED{ persona_pk_seq }->() so that is is easier to get a sequence value in a Perl-way.

The article plperl: invoking other subroutines has been posted by Luca Ferrari on May 4, 2018