Allow on-the-fly capture of DDL event details

This feature lets user code inspect and take action on DDL events.
Whenever a ddl_command_end event trigger is installed, DDL actions
executed are saved to a list which can be inspected during execution of
a function attached to ddl_command_end.

The set-returning function pg_event_trigger_ddl_commands can be used to
list actions so captured; it returns data about the type of command
executed, as well as the affected object.  This is sufficient for many
uses of this feature.  For the cases where it is not, we also provide a
"command" column of a new pseudo-type pg_ddl_command, which is a
pointer to a C structure that can be accessed by C code.  The struct
contains all the info necessary to completely inspect and even
reconstruct the executed command.

There is no actual deparse code here; that's expected to come later.
What we have is enough infrastructure that the deparsing can be done in
an external extension.  The intention is that we will add some deparsing
code in a later release, as an in-core extension.

A new test module is included.  It's probably insufficient as is, but it
should be sufficient as a starting point for a more complete and
future-proof approach.

Authors: Álvaro Herrera, with some help from Andres Freund, Ian Barwick,
Abhijit Menon-Sen.

Reviews by Andres Freund, Robert Haas, Amit Kapila, Michael Paquier,
Craig Ringer, David Steele.
Additional input from Chris Browne, Dimitri Fontaine, Stephen Frost,
Petr Jelínek, Tom Lane, Jim Nasby, Steven Singer, Pavel Stěhule.

Based on original work by Dimitri Fontaine, though I didn't use his
code.

Discussion:
  https://www.postgresql.org/message-id/m2txrsdzxa.fsf@2ndQuadrant.fr
  https://www.postgresql.org/message-id/20131108153322.GU5809@eldon.alvh.no-ip.org
  https://www.postgresql.org/message-id/20150215044814.GL3391@alvh.no-ip.org

1 files changed, 12 insertions, 0 deletions

diff --git a/src/backend/commands/schemacmds.c b/src/backend/commands/schemacmds.c
index c090ed220f8..5a7beff7d56 100644
--- a/src/backend/commands/schemacmds.c
+++ b/src/backend/commands/schemacmds.c
@@ -25,6 +25,7 @@
 #include "catalog/objectaccess.h"
 #include "catalog/pg_namespace.h"
 #include "commands/dbcommands.h"
+#include "commands/event_trigger.h"
 #include "commands/schemacmds.h"
 #include "miscadmin.h"
 #include "parser/parse_utilcmd.h"
@@ -52,6 +53,7 @@ CreateSchemaCommand(CreateSchemaStmt *stmt, const char *queryString)
 	Oid			saved_uid;
 	int			save_sec_context;
 	AclResult	aclresult;
+	ObjectAddress address;
 
 	GetUserIdAndSecContext(&saved_uid, &save_sec_context);
 
@@ -143,6 +145,16 @@ CreateSchemaCommand(CreateSchemaStmt *stmt, const char *queryString)
 	PushOverrideSearchPath(overridePath);
 
 	/*
+	 * Report the new schema to possibly interested event triggers.  Note we
+	 * must do this here and not in ProcessUtilitySlow because otherwise the
+	 * objects created below are reported before the schema, which would be
+	 * wrong.
+	 */
+	ObjectAddressSet(address, NamespaceRelationId, namespaceId);
+	EventTriggerCollectSimpleCommand(address, InvalidObjectAddress,
+									 (Node *) stmt);
+
+	/*
 	 * Examine the list of commands embedded in the CREATE SCHEMA command, and
 	 * reorganize them into a sequentially executable order with no forward
 	 * references.  Note that the result is still a list of raw parsetrees ---