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><A
NAME="SPI-SPI-PREPARE"
></A
>SPI_prepare</H1
><DIV
CLASS="REFNAMEDIV"
><A
NAME="AEN50503"
></A
><H2
>Name</H2
>SPI_prepare -- prepare a plan for a command, without executing it yet</DIV
><A
NAME="AEN50506"
></A
><DIV
CLASS="REFSYNOPSISDIV"
><A
NAME="AEN50508"
></A
><H2
>Synopsis</H2
><PRE
CLASS="SYNOPSIS"
>SPIPlanPtr SPI_prepare(const char * <TT
CLASS="PARAMETER"
>command</TT
>, int <TT
CLASS="PARAMETER"
>nargs</TT
>, Oid * <TT
CLASS="PARAMETER"
>argtypes</TT
>)</PRE
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN50513"
></A
><H2
>Description</H2
><P
> <CODE
CLASS="FUNCTION"
>SPI_prepare</CODE
> 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.
</P
><P
> When the same or a similar command is to be executed repeatedly, it
might be advantageous to perform the planning only once.
<CODE
CLASS="FUNCTION"
>SPI_prepare</CODE
> converts a command string into an
execution plan that can be executed repeatedly using
<CODE
CLASS="FUNCTION"
>SPI_execute_plan</CODE
>.
</P
><P
> A prepared command can be generalized by writing parameters
(<TT
CLASS="LITERAL"
>$1</TT
>, <TT
CLASS="LITERAL"
>$2</TT
>, etc.) in place of what would be
constants in a normal command. The actual values of the parameters
are then specified when <CODE
CLASS="FUNCTION"
>SPI_execute_plan</CODE
> is called.
This allows the prepared command to be used over a wider range of
situations than would be possible without parameters.
</P
><P
> The plan returned by <CODE
CLASS="FUNCTION"
>SPI_prepare</CODE
> can be used
only in the current invocation of the procedure, since
<CODE
CLASS="FUNCTION"
>SPI_finish</CODE
> frees memory allocated for a plan.
But a plan can be saved for longer using the function
<CODE
CLASS="FUNCTION"
>SPI_saveplan</CODE
>.
</P
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN50528"
></A
><H2
>Arguments</H2
><P
></P
><DIV
CLASS="VARIABLELIST"
><DL
><DT
><TT
CLASS="LITERAL"
>const char * <TT
CLASS="PARAMETER"
>command</TT
></TT
></DT
><DD
><P
> command string
</P
></DD
><DT
><TT
CLASS="LITERAL"
>int <TT
CLASS="PARAMETER"
>nargs</TT
></TT
></DT
><DD
><P
> number of input parameters (<TT
CLASS="LITERAL"
>$1</TT
>, <TT
CLASS="LITERAL"
>$2</TT
>, etc.)
</P
></DD
><DT
><TT
CLASS="LITERAL"
>Oid * <TT
CLASS="PARAMETER"
>argtypes</TT
></TT
></DT
><DD
><P
> pointer to an array containing the <ACRONYM
CLASS="ACRONYM"
>OID</ACRONYM
>s of
the data types of the parameters
</P
></DD
></DL
></DIV
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN50552"
></A
><H2
>Return Value</H2
><P
> <CODE
CLASS="FUNCTION"
>SPI_prepare</CODE
> returns a non-null pointer to an
execution plan. On error, <TT
CLASS="SYMBOL"
>NULL</TT
> will be returned,
and <TT
CLASS="VARNAME"
>SPI_result</TT
> will be set to one of the same
error codes used by <CODE
CLASS="FUNCTION"
>SPI_execute</CODE
>, except that
it is set to <TT
CLASS="SYMBOL"
>SPI_ERROR_ARGUMENT</TT
> if
<TT
CLASS="PARAMETER"
>command</TT
> is <TT
CLASS="SYMBOL"
>NULL</TT
>, or if
<TT
CLASS="PARAMETER"
>nargs</TT
> is less than 0, or if <TT
CLASS="PARAMETER"
>nargs</TT
> is
greater than 0 and <TT
CLASS="PARAMETER"
>argtypes</TT
> is <TT
CLASS="SYMBOL"
>NULL</TT
>.
</P
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN50566"
></A
><H2
>Notes</H2
><P
> <TT
CLASS="TYPE"
>SPIPlanPtr</TT
> is declared as a pointer to an opaque struct type in
<TT
CLASS="FILENAME"
>spi.h</TT
>. It is unwise to try to access its contents
directly, as that makes your code much more likely to break in
future revisions of <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>.
</P
><P
> 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.
</P
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