Provided by: postgresql-doc-9.1_9.1.3-2_all
SPI_prepare - prepare a plan for a command, without executing it yet
SPIPlanPtr SPI_prepare(const char * command, int nargs, Oid * argtypes)
SPI_prepare creates and returns an execution plan for the specified command, but doesn't execute the command. This function should only be called from a connected procedure. When the same or a similar command is to be executed repeatedly, it might be advantageous to perform the planning only once. SPI_prepare converts a command string into an execution plan that can be executed repeatedly using SPI_execute_plan. 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 plan returned by SPI_prepare can be used only in the current invocation of the procedure, since SPI_finish frees memory allocated for a plan. But a plan can be saved for longer using the function SPI_saveplan.
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
SPI_prepare returns a non-null pointer to an execution plan. 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.
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. There is a disadvantage to using parameters: since the planner does not know the values that will be supplied for the parameters, it might make worse planning choices than it would make for a normal command with all constants visible.