45.8. PL/Perl Under the Hood

45.8.1. Configuration

45.8.2. Limitations and Missing Features

45.8.1. Configuration

This section lists configuration parameters that affect PL/Perl.

plperl.on_init (string) : Specifies Perl code to be executed when a Perl interpreter is first initialized, before it is specialized for use by plperl or plperlu. The SPI functions are not available when this code is executed. If the code fails with an error it will abort the initialization of the interpreter and propagate out to the calling query, causing the current transaction or subtransaction to be aborted.

The Perl code is limited to a single string. Longer code can be placed
into a module and loaded by the `on_init` string.
Examples:

```programlisting
plperl.on_init = 'require "plperlinit.pl"'
plperl.on_init = 'use lib "/my/app"; use MyApp::PgInit;'
```

Any modules loaded by `plperl.on_init`, either directly or
indirectly, will be available for use by `plperl`. This may
create a security risk. To see what modules have been loaded you can use:

```programlisting
DO 'elog(WARNING, join ", ", sort keys %INC)' LANGUAGE plperl;
```

Initialization will happen in the postmaster if the `plperl` library is
included in [shared_preload_libraries](runtime-config-client.md#GUC-SHARED-PRELOAD-LIBRARIES), in which
case extra consideration should be given to the risk of destabilizing
the postmaster. The principal reason for making use of this feature
is that Perl modules loaded by `plperl.on_init` need be
loaded only at postmaster start, and will be instantly available
without loading overhead in individual database sessions. However,
keep in mind that the overhead is avoided only for the first Perl
interpreter used by a database session — either PL/PerlU, or
PL/Perl for the first SQL role that calls a PL/Perl function. Any
additional Perl interpreters created in a database session will have
to execute `plperl.on_init` afresh. Also, on Windows there
will be no savings whatsoever from preloading, since the Perl
interpreter created in the postmaster process does not propagate to
child processes.

This parameter can only be set in the `postgresql.conf` file or on the server command line.

plperl.on_plperl_init (string) plperl.on_plperlu_init (string) : These parameters specify Perl code to be executed when a Perl interpreter is specialized for plperl or plperlu respectively. This will happen when a PL/Perl or PL/PerlU function is first executed in a database session, or when an additional interpreter has to be created because the other language is called or a PL/Perl function is called by a new SQL role. This follows any initialization done by plperl.on_init. The SPI functions are not available when this code is executed. The Perl code in plperl.on_plperl_init is executed after “locking down” the interpreter, and thus it can only perform trusted operations.

If the code fails with an error it will abort the initialization and
propagate out to the calling query, causing the current transaction or
subtransaction to be aborted. Any actions already done within Perl
won't be undone; however, that interpreter won't be used again.
If the language is used again the initialization will be attempted
again within a fresh Perl interpreter.

Only superusers can change these settings. Although these settings
can be changed within a session, such changes will not affect Perl
interpreters that have already been used to execute functions.

plperl.use_strict (boolean) : When set true subsequent compilations of PL/Perl functions will have the strict pragma enabled. This parameter does not affect functions already compiled in the current session.

45.8.2. Limitations and Missing Features

The following features are currently missing from PL/Perl, but they would make welcome contributions.

• PL/Perl functions cannot call each other directly.

• SPI is not yet fully implemented.

• If you are fetching very large data sets using spi_exec_query, you should be aware that these will all go into memory. You can avoid this by using spi_query/spi_fetchrow as illustrated earlier.

  A similar problem occurs if a set-returning function passes a large set of rows back to PostgreSQL via return. You can avoid this problem too by instead using return_next for each row returned, as shown previously.

• When a session ends normally, not due to a fatal error, any END blocks that have been defined are executed. Currently no other actions are performed. Specifically, file handles are not automatically flushed and objects are not automatically destroyed.