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PostgreSQL Reference Manual - Volume 2 - Programming Guide
by The PostgreSQL Global Development Group
Paperback (6"x9"), 408 pages
ISBN 0954612035
RRP £19.95 ($34.95)

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13.1.7 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_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

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.

ISBN 0954612035PostgreSQL Reference Manual - Volume 2 - Programming GuideSee the print edition