42.8. Error Handling in PL/Tcl
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## 42.8. Error Handling in PL/Tcl #

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 an 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 an 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 PostgreSQL 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.)

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