Postgres-XC 1.1 Documentation
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# SPI_prepare

## Name

SPI_prepare -- prepare a statement, without executing it yet

## Synopsis

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

## Description

`SPI_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 Value

`SPI_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.

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.

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