SPI_PREPARE(3) PostgreSQL 9.3.2 Documentation SPI_PREPARE(3)NAMESPI_prepare - prepare a statement, without executing it yet
SYNOPSIS
SPIPlanPtr SPI_prepare(const char * command, int nargs, Oid * argtypes)
DESCRIPTIONSPI_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.
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 VALUESPI_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 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.
PostgreSQL 9.3.2 2013 SPI_PREPARE(3)
