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 error command or PL/Tcl's elog command. Such errors can be caught within Tcl using the Tcl catch command. If an error is not caught but is allowed to propagate out to the top level of execution of the PL/Tcl function, it is reported as a SQL error in the function's calling query.

Conversely, SQL errors that occur within PL/Tcl's spi_exec, spi_prepare, and spi_execp commands are reported as Tcl errors, so they are catchable by Tcl's catch command. (Each of these PL/Tcl commands runs its SQL operation in a subtransaction, which is rolled back on error, so that any partially-completed operation is automatically cleaned up.) Again, if an error propagates out to the top level without being caught, it turns back into a SQL error.

Tcl provides an errorCode 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 POSTGRES, the second word is the LightDB version number, and additional words are field name/value pairs providing detailed information about the error. Fields SQLSTATE, condition, and message are always supplied (the first two represent the error code and condition name as shown in Appendix A). Fields that may be present include detail, hint, context, schema, table, column, datatype, constraint, statement, cursor_position, filename, lineno, and funcname.

A convenient way to work with PL/Tcl's errorCode information is to load it into an array, so that the field names become array subscripts. Code for doing that might look like

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
        }
    }
}

(The double colons explicitly specify that errorCode is a global variable.)