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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&nbsp;--&nbsp;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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