NAME

       SPI_prepare - prepare a statement, without executing it yet

SYNOPSIS

       SPIPlanPtr SPI_prepare(const char * command, int nargs, Oid * argtypes)

DESCRIPTION

       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 C
       function, 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.

ARGUMENTS

       const char * command
           command string

       int nargs
           number of input parameters ($1, $2, etc.)

       Oid * argtypes
           pointer to an array containing the OIDs of the data types of the parameters

RETURN VALUE

       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.

NOTES

       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(7) for more information about the behavior of prepared statements.

       This function should only be called from a connected C function.

       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.