PostgreSQL 9.1.2 Documentation
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# SPI_prepare

## Name

SPI_prepare -- prepare a plan for a command, without executing it yet

## Synopsis

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

## Description

`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`.

## 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 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`.

## Notes

`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.

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