Global Values in PL/Perl

PostgreSQL 9.6.3 Documentation
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# 43.4. Global Values in PL/Perl

You can use the global hash `%_SHARED` to store data, including code references, between function calls for the lifetime of the current session.

Here is a simple example for shared data:

    CREATE OR REPLACE FUNCTION set_var(name text, val text) RETURNS text AS $$
        if ($_SHARED{$_[0]} = $_[1]) {
            return 'ok';
        } else {
            return "cannot set shared variable $_[0] to $_[1]";
        }
    $$ LANGUAGE plperl;

    CREATE OR REPLACE FUNCTION get_var(name text) RETURNS text AS $$
        return $_SHARED{$_[0]};
    $$ LANGUAGE plperl;

    SELECT set_var('sample', 'Hello, PL/Perl!  How''s tricks?');
    SELECT get_var('sample');

Here is a slightly more complicated example using a code reference:

    CREATE OR REPLACE FUNCTION myfuncs() RETURNS void AS $$
        $_SHARED{myquote} = sub {
            my $arg = shift;
            $arg =~ s/(['\\])/\\$1/g;
            return "'$arg'";
        };
    $$ LANGUAGE plperl;

    SELECT myfuncs(); /* initializes the function */

    /* Set up a function that uses the quote function */

    CREATE OR REPLACE FUNCTION use_quote(TEXT) RETURNS text AS $$
        my $text_to_quote = shift;
        my $qfunc = $_SHARED{myquote};
        return &$qfunc($text_to_quote);
    $$ LANGUAGE plperl;

(You could have replaced the above with the one-liner `return $_SHARED{myquote}->($_[0]);` at the expense of readability.)

For security reasons, PL/Perl executes functions called by any one SQL role in a separate Perl interpreter for that role. This prevents accidental or malicious interference by one user with the behavior of another user's PL/Perl functions. Each such interpreter has its own value of the `%_SHARED` variable and other global state. Thus, two PL/Perl functions will share the same value of `%_SHARED` if and only if they are executed by the same SQL role. In an application wherein a single session executes code under multiple SQL roles (via `SECURITY DEFINER` functions, use of `SET ROLE`, etc) you may need to take explicit steps to ensure that PL/Perl functions can share data via `%_SHARED`. To do that, make sure that functions that should communicate are owned by the same user, and mark them `SECURITY DEFINER`. You must of course take care that such functions can't be used to do anything unintended.

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