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 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 부록 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.)