PostgreSQL 7.4.3 Documentation
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# SPI_prepare

## Name

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

## Synopsis

    void * 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 may 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_execp`.

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_execp` 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 non-null pointer to an execution plan. On error, `NULL` will be returned. In both cases, SPI_result will be set analogous to the value returned by `SPI_exec`, 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

There is a disadvantage to using parameters: since the planner does not know the values that will be supplied for the parameters, it may make worse planning choices than it would make for a normal command with all constants visible.

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