Provide a function to emit DDL for CREATE EVENT TRIGGER

doc/src/sgml/func/func-info.sgml

@@ -3797,4 +3797,48 @@ acl      | {postgres=arwdDxtm/postgres,foo=r/postgres}
 
   </sect2>
 
+  <sect2 id="functions-info-retail">
+   <title>DDL Retail Functions</title>
+
+   <para>
+    <xref linkend="functions-info-retail-table"/> lists functions that
+    extract information about ....
+   </para>
+
+   <table id="functions-info-retail-table">
+    <title>DDL Retail Functions</title>
+    <tgroup cols="1">
+     <thead>
+      <row>
+       <entry role="func_table_entry"><para role="func_signature">
+        Function
+       </para>
+       <para>
+        Description
+       </para></entry>
+      </row>
+     </thead>
+
+     <tbody>
+      <row>
+       <entry role="func_table_entry"><para role="func_signature">
+        <indexterm>
+         <primary>pg_get_event_trigger_ddl</primary>
+        </indexterm>
+        <function>pg_get_event_trigger_ddl</function>
+        ( <parameter>event_trigger_name</parameter> <type>name</type> )
+        <returnvalue>text</returnvalue>
+       </para>
+       <para>
+        Reconstructs the underlying DDL statement for a specified event trigger.
+        It returns the complete <command>CREATE EVENT TRIGGER</command> statement. If
+        the event trigger does not exist, an error is raised.
+       </para></entry>
+      </row>
+     </tbody>
+    </tgroup>
+   </table>
+
+  </sect2>
+
   </sect1>

src/backend/utils/adt/ruleutils.c

@@ -29,6 +29,7 @@
 #include "catalog/pg_collation.h"
 #include "catalog/pg_constraint.h"
 #include "catalog/pg_depend.h"
+#include "catalog/pg_event_trigger.h"
 #include "catalog/pg_language.h"
 #include "catalog/pg_opclass.h"
 #include "catalog/pg_operator.h"
@@ -1162,6 +1163,124 @@ pg_get_triggerdef_worker(Oid trigid, bool pretty)
 	return buf.data;
 }
 
+/* ----------
+* pg_get_event_trigger_ddl - Get the DDL statement for an event trigger
+*  ----------
+*/
+Datum
+pg_get_event_trigger_ddl(PG_FUNCTION_ARGS)
+{
+	Name		evtName;
+	HeapTuple	evtTup;
+	HeapTuple	procTup;
+	Form_pg_event_trigger evtForm;
+	char		*evtevent;
+	Oid			evtfoid;
+	Datum		evttagsDatum;
+	bool		evttagsIsNull;
+	Form_pg_proc proc;
+	char		*proname;
+	char		prokind;
+	StringInfoData buf;
+
+	evtName = PG_GETARG_NAME(0);
+
+	/*
+	 * Check provided event trigger name exists
+	 */
+	evtTup = SearchSysCache1(EVENTTRIGGERNAME, NameGetDatum(evtName));
+
+	if (!HeapTupleIsValid(evtTup))
+		ereport(ERROR,
+			(errcode(ERRCODE_UNDEFINED_OBJECT),
+			 errmsg("event trigger \"%s\" does not exist", NameStr(*evtName))));
+
+	evtForm = (Form_pg_event_trigger) GETSTRUCT(evtTup);
+	evtevent = NameStr(evtForm->evtevent);
+	evtfoid = evtForm->evtfoid;
+
+	/*
+	 * Look up event trigger function.
+	 */
+	procTup = SearchSysCache1(PROCOID, ObjectIdGetDatum(evtfoid));
+
+	if (!HeapTupleIsValid(procTup))
+		elog(ERROR,
+			 "cache lookup failed for function %u", evtfoid);
+
+	proc = (Form_pg_proc) GETSTRUCT(procTup);
+	proname = NameStr(proc->proname);
+	prokind = proc->prokind;
+
+	Assert(prokind == PROKIND_FUNCTION || prokind == PROKIND_PROCEDURE);
+
+	/*
+	 * Assemble output.
+	 */
+	initStringInfo(&buf);
+
+	appendStringInfo(&buf, "CREATE EVENT TRIGGER %s ON %s",
+					quote_identifier(NameStr(*evtName)),
+					quote_identifier(evtevent));
+
+	/*
+	 * Generate WHEN clause, if any filter events were specified.
+	 *
+	 * Note that as of PostgreSQL 19, the only filter_variable supported
+	 * is "TAG", which is not stored in pg_event_trigger, so is hard-coded
+	 * in the generated output. This also means that by definition, the AND
+	 * clause supported by the CREATE EVENT TRIGGER grammar cannot be actually
+	 * be used in a valid statement, and thus will never be part of the
+	 * generated output.
+	 */
+	evttagsDatum = SysCacheGetAttr(EVENTTRIGGEROID, evtTup,
+							   Anum_pg_event_trigger_evttags,
+							   &evttagsIsNull);
+
+	if (!evttagsIsNull)
+	{
+		ArrayType  *arr = DatumGetArrayTypeP(evttagsDatum);
+		Datum	   *elems;
+		int			nelems;
+		int			i;
+
+		deconstruct_array_builtin(arr, TEXTOID, &elems, NULL, &nelems);
+
+		appendStringInfoString(&buf, " WHEN TAG IN (");
+
+		for (i = 0; i < nelems; ++i)
+		{
+			char	   *str = TextDatumGetCString(elems[i]);
+
+			if (i)
+				appendStringInfoChar(&buf, ',');
+
+			appendStringInfo(&buf, "'%s'", str);
+		}
+
+		appendStringInfoString(&buf, ")");
+	}
+
+	/*
+	 * Finally, add EXECUTE clause.
+	 */
+	appendStringInfo(&buf, " EXECUTE %s %s.%s()",
+					 prokind == PROKIND_FUNCTION ? "FUNCTION" : "PROCEDURE",
+					 quote_identifier(get_namespace_name(proc->pronamespace)),
+					 quote_identifier(proname));
+
+	/*
+	 * Terminate query
+	 */
+	appendStringInfoChar(&buf, ';');
+
+	/* Clean up */
+	ReleaseSysCache(evtTup);
+	ReleaseSysCache(procTup);
+	PG_RETURN_TEXT_P(string_to_text(buf.data));
+}
+
+
 /* ----------
  * pg_get_indexdef			- Get the definition of an index
  *

src/include/catalog/pg_proc.dat

@@ -8547,6 +8547,9 @@
 { oid => '2730', descr => 'trigger description with pretty-print option',
   proname => 'pg_get_triggerdef', provolatile => 's', prorettype => 'text',
   proargtypes => 'oid bool', prosrc => 'pg_get_triggerdef_ext' },
+{ oid => '9067', descr => 'get CREATE statement for event trigger',
+  proname => 'pg_get_event_trigger_ddl', proisstrict => 't',
+  prorettype => 'text', proargtypes => 'name', prosrc => 'pg_get_event_trigger_ddl' },
 
 # asynchronous notifications
 { oid => '3035',

src/test/regress/expected/event_trigger.out

@@ -84,6 +84,32 @@ create event trigger regress_event_trigger2 on ddl_command_start
    execute procedure test_event_trigger();
 -- OK
 comment on event trigger regress_event_trigger is 'test comment';
+-- these are all OK - checks the DDL output without a tag filter and with one
+SELECT pg_get_event_trigger_ddl('regress_event_trigger');
+                                           pg_get_event_trigger_ddl                                            
+---------------------------------------------------------------------------------------------------------------
+ CREATE EVENT TRIGGER regress_event_trigger ON ddl_command_start EXECUTE FUNCTION public.test_event_trigger();
+(1 row)
+
+SELECT pg_get_event_trigger_ddl('regress_event_trigger2');
+                                                                   pg_get_event_trigger_ddl                                                                    
+---------------------------------------------------------------------------------------------------------------------------------------------------------------
+ CREATE EVENT TRIGGER regress_event_trigger2 ON ddl_command_start WHEN TAG IN ('CREATE TABLE','CREATE FUNCTION') EXECUTE FUNCTION public.test_event_trigger();
+(1 row)
+
+-- will return NULL
+SELECT pg_get_event_trigger_ddl(NULL);
+ pg_get_event_trigger_ddl 
+--------------------------
+
+(1 row)
+
+-- should fail, no argument
+SELECT pg_get_event_trigger_ddl();
+ERROR:  function pg_get_event_trigger_ddl() does not exist
+LINE 1: SELECT pg_get_event_trigger_ddl();
+               ^
+DETAIL:  No function of that name accepts the given number of arguments.
 -- drop as non-superuser should fail
 create role regress_evt_user;
 set role regress_evt_user;

src/test/regress/sql/event_trigger.sql

@@ -85,6 +85,16 @@ create event trigger regress_event_trigger2 on ddl_command_start
 -- OK
 comment on event trigger regress_event_trigger is 'test comment';
 
+-- these are all OK - checks the DDL output without a tag filter and with one
+SELECT pg_get_event_trigger_ddl('regress_event_trigger');
+SELECT pg_get_event_trigger_ddl('regress_event_trigger2');
+
+-- will return NULL
+SELECT pg_get_event_trigger_ddl(NULL);
+
+-- should fail, no argument
+SELECT pg_get_event_trigger_ddl();
+
 -- drop as non-superuser should fail
 create role regress_evt_user;
 set role regress_evt_user;