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NAME="SPI-SPI-PREPARE"
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>SPI_prepare</H1
><DIV
CLASS="REFNAMEDIV"
><A
NAME="AEN60454"
></A
><H2
>Name</H2
>SPI_prepare -- prepare a statement, without executing it yet</DIV
><DIV
CLASS="REFSYNOPSISDIV"
><A
NAME="AEN60459"
></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="AEN60464"
></A
><H2
>Description</H2
><P
> <CODE
CLASS="FUNCTION"
>SPI_prepare</CODE
> 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
<CODE
CLASS="FUNCTION"
>SPI_execute_plan</CODE
>.
</P
><P
> 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.
<CODE
CLASS="FUNCTION"
>SPI_prepare</CODE
> 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.
</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 statement 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 such a
statement. But the statement can be saved for longer using the functions
<CODE
CLASS="FUNCTION"
>SPI_keepplan</CODE
> or <CODE
CLASS="FUNCTION"
>SPI_saveplan</CODE
>.
</P
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN60480"
></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="AEN60504"
></A
><H2
>Return Value</H2
><P
> <CODE
CLASS="FUNCTION"
>SPI_prepare</CODE
> returns a non-null pointer to an
<TT
CLASS="TYPE"
>SPIPlan</TT
>, which is an opaque struct representing a prepared
statement. 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="AEN60519"
></A
><H2
>Notes</H2
><P
> If no parameters are defined, a generic plan will be created at the
first use of <CODE
CLASS="FUNCTION"
>SPI_execute_plan</CODE
>, and used for all
subsequent executions as well. If there are parameters, the first few uses
of <CODE
CLASS="FUNCTION"
>SPI_execute_plan</CODE
> will generate custom plans
that are specific to the supplied parameter values. After enough uses
of the same prepared statement, <CODE
CLASS="FUNCTION"
>SPI_execute_plan</CODE
> 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 <TT
CLASS="LITERAL"
>CURSOR_OPT_GENERIC_PLAN</TT
> or
<TT
CLASS="LITERAL"
>CURSOR_OPT_CUSTOM_PLAN</TT
> flag to
<CODE
CLASS="FUNCTION"
>SPI_prepare_cursor</CODE
>, to force use of generic or custom
plans respectively.
</P
><P
> This function should only be called from a connected procedure.
</P
><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
> The name <TT
CLASS="TYPE"
>SPIPlanPtr</TT
> is somewhat historical, since the data
structure no longer necessarily contains an execution plan.
</P
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