When the same or similar query is to be executed repeatedly, it may
be advantageous to perform query planning only once.
SPI_prepare converts a query string into an execution
plan that can be passed repeatedly to SPI_execp.
A prepared query can be generalized by writing parameters ($1, $2, etc)
in place of what would be constants in a normal query. The values of
the parameters are then specified when SPI_execp
is called. This allows the prepared query to be used over a wider
range of situations than would be possible without parameters.
Note: However, there is a disadvantage: since the planner does not know the
values that will be supplied for the parameters, it may make worse
query planning choices than it would make for a simple query with
all constants visible.
If the query uses parameters, their number and data types must be
specified in the call to SPI_prepare.
The plan returned by SPI_prepare may be used only in current
invocation of the procedure since SPI_finish frees memory allocated for a plan.
But see SPI_saveplan to save a plan for longer.
If successful, a non-null pointer will be returned. Otherwise, you'll get
a NULL plan. In both cases SPI_result will be set like the value returned
by SPI_exec, except that it is set to
SPI_ERROR_ARGUMENT if query is NULL or nargs < 0 or nargs > 0 && argtypes