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><A
NAME="PLTCL-ERROR-HANDLING"
>42.8. Error Handling in PL/Tcl</A
></H1
><P
>     Tcl code within or called from a PL/Tcl function can raise an error,
     either by executing some invalid operation or by generating an error
     using the Tcl <CODE
CLASS="FUNCTION"
>error</CODE
> command or
     PL/Tcl's <CODE
CLASS="FUNCTION"
>elog</CODE
> command.  Such errors can be caught
     within Tcl using the Tcl <CODE
CLASS="FUNCTION"
>catch</CODE
> command.  If they
     are not caught but are allowed to propagate out to the top level of
     execution of the PL/Tcl function, they turn into database errors.
    </P
><P
>     Conversely, database errors that occur within PL/Tcl's
     <CODE
CLASS="FUNCTION"
>spi_exec</CODE
>, <CODE
CLASS="FUNCTION"
>spi_prepare</CODE
>,
     and <CODE
CLASS="FUNCTION"
>spi_execp</CODE
> commands are reported as Tcl errors,
     so they are catchable by Tcl's <CODE
CLASS="FUNCTION"
>catch</CODE
> command.
     Again, if they propagate out to the top level without being caught,
     they turn back into database errors.
    </P
><P
>     Tcl provides an <TT
CLASS="VARNAME"
>errorCode</TT
> variable that can represent
     additional information about an error in a form that is easy for Tcl
     programs to interpret.  The contents are in Tcl list format, and the
     first word identifies the subsystem or library reporting the error;
     beyond that the contents are left to the individual subsystem or
     library.  For database errors reported by PL/Tcl commands, the first
     word is <TT
CLASS="LITERAL"
>POSTGRES</TT
>, the second word is the Postgres
     version number, and additional words are field name/value pairs
     providing detailed information about the error.
     Fields <TT
CLASS="VARNAME"
>SQLSTATE</TT
>, <TT
CLASS="VARNAME"
>condition</TT
>,
     and <TT
CLASS="VARNAME"
>message</TT
> are always supplied
     (the first two represent the error code and condition name as shown
     in <A
HREF="errcodes-appendix.html"
>Appendix A</A
>).
     Fields that may be present include
     <TT
CLASS="VARNAME"
>detail</TT
>, <TT
CLASS="VARNAME"
>hint</TT
>, <TT
CLASS="VARNAME"
>context</TT
>,
     <TT
CLASS="VARNAME"
>schema</TT
>, <TT
CLASS="VARNAME"
>table</TT
>, <TT
CLASS="VARNAME"
>column</TT
>,
     <TT
CLASS="VARNAME"
>datatype</TT
>, <TT
CLASS="VARNAME"
>constraint</TT
>,
     <TT
CLASS="VARNAME"
>statement</TT
>, <TT
CLASS="VARNAME"
>cursor_position</TT
>,
     <TT
CLASS="VARNAME"
>filename</TT
>, <TT
CLASS="VARNAME"
>lineno</TT
>, and
     <TT
CLASS="VARNAME"
>funcname</TT
>.
    </P
><P
>     A convenient way to work with PL/Tcl's <TT
CLASS="VARNAME"
>errorCode</TT
>
     information is to load it into an array, so that the field names become
     array subscripts.  Code for doing that might look like
</P><PRE
CLASS="PROGRAMLISTING"
>if {[catch { spi_exec $sql_command }]} {
    if {[lindex $::errorCode 0] == "POSTGRES"} {
        array set errorArray $::errorCode
        if {$errorArray(condition) == "undefined_table"} {
            # deal with missing table
        } else {
            # deal with some other type of SQL error
        }
    }
}</PRE
><P>
     (The double colons explicitly specify that <TT
CLASS="VARNAME"
>errorCode</TT
>
     is a global variable.)
    </P
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