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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>44.8. Error Handling in PL/Tcl</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><link rel="prev" href="pltcl-event-trigger.html" title="44.7. Event Trigger Functions in PL/Tcl" /><link rel="next" href="pltcl-subtransactions.html" title="44.9. Explicit Subtransactions in PL/Tcl" /></head><body><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">44.8. Error Handling in PL/Tcl</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="pltcl-event-trigger.html" title="44.7. Event Trigger Functions in PL/Tcl">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="pltcl.html" title="Chapter 44. PL/Tcl - Tcl Procedural Language">Up</a></td><th width="60%" align="center">Chapter 44. PL/Tcl - Tcl Procedural Language</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 11.5 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="pltcl-subtransactions.html" title="44.9. Explicit Subtransactions in PL/Tcl">Next</a></td></tr></table><hr></hr></div><div class="sect1" id="PLTCL-ERROR-HANDLING"><div class="titlepage"><div><div><h2 class="title" style="clear: both">44.8. Error Handling in PL/Tcl</h2></div></div></div><a id="id-1.8.9.12.2" class="indexterm"></a><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 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.
</p><p>
Conversely, SQL 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.
(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.
</p><p>
Tcl provides an <code class="varname">errorCode</code> 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 <code class="literal">POSTGRES</code>, the second word is the PostgreSQL
version number, and additional words are field name/value pairs
providing detailed information about the error.
Fields <code class="varname">SQLSTATE</code>, <code class="varname">condition</code>,
and <code class="varname">message</code> are always supplied
(the first two represent the error code and condition name as shown
in <a class="xref" href="errcodes-appendix.html" title="Appendix A. PostgreSQL Error Codes">Appendix A</a>).
Fields that may be present include
<code class="varname">detail</code>, <code class="varname">hint</code>, <code class="varname">context</code>,
<code class="varname">schema</code>, <code class="varname">table</code>, <code class="varname">column</code>,
<code class="varname">datatype</code>, <code class="varname">constraint</code>,
<code class="varname">statement</code>, <code class="varname">cursor_position</code>,
<code class="varname">filename</code>, <code class="varname">lineno</code>, and
<code class="varname">funcname</code>.
</p><p>
A convenient way to work with PL/Tcl's <code class="varname">errorCode</code>
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 <code class="varname">errorCode</code>
is a global variable.)
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