Monitoring the Postgres query parser
It's not possible to save query plans "for all future sessions". Query plans are only ever saved for the current session. And only reused under a number of favorable conditions.

Plans for ad-hoc SQL queries are not saved at all. All queries inside PL/pgSQL functions are treated like prepared statements. And there are more steps than just the query planning.

It's important to note that there have been substantial changes since Postgres 9.1, where generic plans were more generally saved and reused for statements inside PL/pgSQL. This has become a lot smarter with pg 9.2.

The manual (bold emphasis mine):


  Prepared statements only last for the duration of the current database session.


That's not saying anything about query plans, yet:


  When the PREPARE statement is executed, the specified statement is
  parsed, analyzed, and rewritten. When an EXECUTE command is
  subsequently issued, the prepared statement is planned and executed.


And further down:


  If a prepared statement is executed enough times, the server may
  eventually decide to save and re-use a generic plan rather than
  re-planning each time. This will occur immediately if the prepared
  statement has no parameters; otherwise it occurs only if the generic
  plan appears to be not much more expensive than a plan that depends on
  specific parameter values. Typically, a generic plan will be selected
  only if the query's performance is estimated to be fairly insensitive
  to the specific parameter values supplied.


How to tell whether a generic plan is used?


  To examine the query plan PostgreSQL is using for a prepared
  statement, use EXPLAIN. If a generic plan is in use, it will contain
  parameter symbols $n, while a custom plan will have the current actual
  parameter values substituted into it.


A saved plan is always a generic plan, but not vice versa.
Where are prepared statements saved?


  You can see all prepared statements available in the session by
  querying the pg_prepared_statements system view.


Query plans for SQL statements in PL/pgSQL

Those are not normally visible in EXPLAIN output. You can install the additional module auto-explain to write those plans to the log, too. Details here:


Postgres query plan of a UDF invocation written in pgpsql


For details about Plan Caching in PL/pgSQL functions, best read the manual directly.