SPI_prepare creates and returns a prepared
   statement for the specified command, but doesn't execute the command.
   The prepared statement can later be executed repeatedly using
   SPI_execute_plan.
  
   When the same or a similar command is to be executed repeatedly, it
   is generally advantageous to perform parse analysis only once, and
   might furthermore be advantageous to re-use an execution plan for the
   command.
   SPI_prepare converts a command string into a
   prepared statement that encapsulates the results of parse analysis.
   The prepared statement also provides a place for caching an execution plan
   if it is found that generating a custom plan for each execution is not
   helpful.
  
   A prepared command can be generalized by writing parameters
   ($1, $2, etc.) in place of what would be
   constants in a normal command.  The actual values of the parameters
   are then specified when SPI_execute_plan is called.
   This allows the prepared command to be used over a wider range of
   situations than would be possible without parameters.
  
   The statement returned by SPI_prepare can be used
   only in the current invocation of the procedure, since
   SPI_finish frees memory allocated for such a
   statement.  But the statement can be saved for longer using the functions
   SPI_keepplan or SPI_saveplan.
  
command string
number of input parameters ($1, $2, etc.)
pointer to an array containing the OIDs of the data types of the parameters
   SPI_prepare returns a non-null pointer to an
   SPIPlan, which is an opaque struct representing a prepared
   statement.  On error, NULL will be returned,
   and SPI_result will be set to one of the same
   error codes used by SPI_execute, except that
   it is set to SPI_ERROR_ARGUMENT if
   command is NULL, or if
   nargs is less than 0, or if nargs is
   greater than 0 and argtypes is NULL.
  
   If no parameters are defined, a generic plan will be created at the
   first use of SPI_execute_plan, and used for all
   subsequent executions as well.  If there are parameters, the first few uses
   of SPI_execute_plan will generate custom plans
   that are specific to the supplied parameter values.  After enough uses
   of the same prepared statement, SPI_execute_plan will
   build a generic plan, and if that is not too much more expensive than the
   custom plans, it will start using the generic plan instead of re-planning
   each time.  If this default behavior is unsuitable, you can alter it by
   passing the CURSOR_OPT_GENERIC_PLAN or
   CURSOR_OPT_CUSTOM_PLAN flag to
   SPI_prepare_cursor, to force use of generic or custom
   plans respectively.
  
Although the main point of a prepared statement is to avoid repeated parse analysis and planning of the statement, PostgreSQL will force re-analysis and re-planning of the statement before using it whenever database objects used in the statement have undergone definitional (DDL) changes since the previous use of the prepared statement. Also, if the value of search_path changes from one use to the next, the statement will be re-parsed using the new search_path. (This latter behavior is new as of PostgreSQL 9.3.) See PREPARE for more information about the behavior of prepared statements.
This function should only be called from a connected procedure.
SPIPlanPtr is declared as a pointer to an opaque struct type in spi.h. It is unwise to try to access its contents directly, as that makes your code much more likely to break in future revisions of PostgreSQL.
The name SPIPlanPtr is somewhat historical, since the data structure no longer necessarily contains an execution plan.