Event Trigger for table_rewrite

Generate a table_rewrite event when ALTER TABLE
attempts to rewrite a table. Provide helper
functions to identify table and reason.

Intended use case is to help assess or to react
to schema changes that might hold exclusive locks
for long periods.

Dimitri Fontaine, triggering an edit by Simon Riggs

Reviewed in detail by Michael Paquier

doc/src/sgml/func.sgml

@@ -17607,15 +17607,23 @@ FOR EACH ROW EXECUTE PROCEDURE suppress_redundant_updates_trigger();
   <sect1 id="functions-event-triggers">
    <title>Event Trigger Functions</title>
 
-   <indexterm>
-     <primary>pg_event_trigger_dropped_objects</primary>
-   </indexterm>
+   <para>
+    <productname>PostgreSQL</> provides these helper functions
+    to retrieve information from event triggers.
+   </para>
 
    <para>
-    Currently <productname>PostgreSQL</> provides one built-in event trigger
-    helper function, <function>pg_event_trigger_dropped_objects</>.
+    For more information about event triggers,
+    see <xref linkend="event-triggers">.
    </para>
 
+  <sect2 id="pg-event-trigger-sql-drop-functions">
+   <title>Processing objects dropped by a DDL command.</title>
+
+   <indexterm>
+     <primary>pg_event_trigger_dropped_objects</primary>
+   </indexterm>
+
    <para>
     <function>pg_event_trigger_dropped_objects</> returns a list of all objects
     dropped by the command in whose <literal>sql_drop</> event it is called.
@@ -17709,11 +17717,72 @@ CREATE EVENT TRIGGER test_event_trigger_for_drops
    EXECUTE PROCEDURE test_event_trigger_for_drops();
 </programlisting>
     </para>
+  </sect2>
 
-     <para>
-       For more information about event triggers,
-       see <xref linkend="event-triggers">.
+  <sect2 id="pg-event-trigger-table-rewrite-functions">
+   <title>Handling a Table Rewrite Event</title>
+
+   <para>
+    The functions shown in
+    <xref linkend="functions-event-trigger-table-rewrite">
+    provide information about a table for which a
+    <literal>table_rewrite</> event has just been called.
+    If called in any other context, an error is raised.
+   </para>
+
+   <table id="functions-event-trigger-table-rewrite">
+    <title>Table Rewrite information</title>
+    <tgroup cols="3">
+     <thead>
+      <row><entry>Name</entry> <entry>Return Type</entry> <entry>Description</entry></row>
+     </thead>
+
+     <tbody>
+      <row>
+       <entry>
+        <indexterm><primary>pg_event_trigger_table_rewrite_oid</primary></indexterm>
+        <literal><function>pg_event_trigger_table_rewrite_oid()</function></literal>
+       </entry>
+       <entry><type>Oid</type></entry>
+       <entry>The Oid of the table about to be rewritten.</entry>
+      </row>
+
+      <row>
+       <entry>
+        <indexterm><primary>pg_event_trigger_table_rewrite_reason</primary></indexterm>
+        <literal><function>pg_event_trigger_table_rewrite_reason()</function></literal>
+       </entry>
+       <entry><type>int</type></entry>
+       <entry>
+        The reason code(s) explaining the reason for rewriting. The exact
+        meaning of the codes is release dependent.
+       </entry>
+      </row>
+     </tbody>
+    </tgroup>
+   </table>
+
+   <para>
+    The <function>pg_event_trigger_table_rewrite_oid</> function can be used
+    in an event trigger like this:
+<programlisting>
+CREATE FUNCTION test_event_trigger_table_rewrite_oid()
+ RETURNS event_trigger
+ LANGUAGE plpgsql AS
+$$
+BEGIN
+  RAISE NOTICE 'rewriting table % for reason %',
+                pg_event_trigger_table_rewrite_oid()::regclass,
+                pg_event_trigger_table_rewrite_reason();
+END;
+$$;
+
+CREATE EVENT TRIGGER test_table_rewrite_oid
+                  ON table_rewrite
+   EXECUTE PROCEDURE test_event_trigger_table_rewrite_oid();
+</programlisting>
     </para>
+  </sect2>
   </sect1>
 
 </chapter>

src/backend/commands/event_trigger.c

@@ -42,11 +42,16 @@
 #include "utils/syscache.h"
 #include "tcop/utility.h"
 
-
 typedef struct EventTriggerQueryState
 {
+	/* sql_drop */
 	slist_head	SQLDropList;
 	bool		in_sql_drop;
+
+	/* table_rewrite */
+	Oid			table_rewrite_oid;	/* InvalidOid, or set for table_rewrite event */
+	int			table_rewrite_reason;	/* AT_REWRITE reason */
+
 	MemoryContext cxt;
 	struct EventTriggerQueryState *previous;
 } EventTriggerQueryState;
@@ -119,11 +124,14 @@ static void AlterEventTriggerOwner_internal(Relation rel,
 								HeapTuple tup,
 								Oid newOwnerId);
 static event_trigger_command_tag_check_result check_ddl_tag(const char *tag);
+static event_trigger_command_tag_check_result check_table_rewrite_ddl_tag(
+	const char *tag);
 static void error_duplicate_filter_variable(const char *defname);
 static Datum filter_list_to_array(List *filterlist);
 static Oid insert_event_trigger_tuple(char *trigname, char *eventname,
 						   Oid evtOwner, Oid funcoid, List *tags);
 static void validate_ddl_tags(const char *filtervar, List *taglist);
+static void validate_table_rewrite_tags(const char *filtervar, List *taglist);
 static void EventTriggerInvoke(List *fn_oid_list, EventTriggerData *trigdata);
 
 /*
@@ -154,7 +162,8 @@ CreateEventTrigger(CreateEventTrigStmt *stmt)
 	/* Validate event name. */
 	if (strcmp(stmt->eventname, "ddl_command_start") != 0 &&
 		strcmp(stmt->eventname, "ddl_command_end") != 0 &&
-		strcmp(stmt->eventname, "sql_drop") != 0)
+		strcmp(stmt->eventname, "sql_drop") != 0 &&
+		strcmp(stmt->eventname, "table_rewrite") != 0)
 		ereport(ERROR,
 				(errcode(ERRCODE_SYNTAX_ERROR),
 				 errmsg("unrecognized event name \"%s\"",
@@ -183,6 +192,9 @@ CreateEventTrigger(CreateEventTrigStmt *stmt)
 		 strcmp(stmt->eventname, "sql_drop") == 0)
 		&& tags != NULL)
 		validate_ddl_tags("tag", tags);
+	else if (strcmp(stmt->eventname, "table_rewrite") == 0
+			 && tags != NULL)
+		validate_table_rewrite_tags("tag", tags);
 
 	/*
 	 * Give user a nice error message if an event trigger of the same name
@@ -280,6 +292,38 @@ check_ddl_tag(const char *tag)
 	return EVENT_TRIGGER_COMMAND_TAG_OK;
 }
 
+/*
+ * Validate DDL command tags for event table_rewrite.
+ */
+static void
+validate_table_rewrite_tags(const char *filtervar, List *taglist)
+{
+	ListCell   *lc;
+
+	foreach(lc, taglist)
+	{
+		const char *tag = strVal(lfirst(lc));
+		event_trigger_command_tag_check_result result;
+
+		result = check_table_rewrite_ddl_tag(tag);
+		if (result == EVENT_TRIGGER_COMMAND_TAG_NOT_SUPPORTED)
+			ereport(ERROR,
+					(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+			/* translator: %s represents an SQL statement name */
+					 errmsg("event triggers are not supported for %s",
+							tag)));
+	}
+}
+
+static event_trigger_command_tag_check_result
+check_table_rewrite_ddl_tag(const char *tag)
+{
+	if (pg_strcasecmp(tag, "ALTER TABLE") == 0)
+		return EVENT_TRIGGER_COMMAND_TAG_OK;
+
+	return EVENT_TRIGGER_COMMAND_TAG_NOT_SUPPORTED;
+}
+
 /*
  * Complain about a duplicate filter variable.
  */
@@ -641,8 +685,18 @@ EventTriggerCommonSetup(Node *parsetree,
 		const char *dbgtag;
 
 		dbgtag = CreateCommandTag(parsetree);
-		if (check_ddl_tag(dbgtag) != EVENT_TRIGGER_COMMAND_TAG_OK)
-			elog(ERROR, "unexpected command tag \"%s\"", dbgtag);
+		if (event == EVT_DDLCommandStart ||
+			event == EVT_DDLCommandEnd   ||
+			event == EVT_SQLDrop)
+		{
+			if (check_ddl_tag(dbgtag) != EVENT_TRIGGER_COMMAND_TAG_OK)
+				elog(ERROR, "unexpected command tag \"%s\"", dbgtag);
+		}
+		else if (event == EVT_TableRewrite)
+		{
+			if (check_table_rewrite_ddl_tag(dbgtag) != EVENT_TRIGGER_COMMAND_TAG_OK)
+				elog(ERROR, "unexpected command tag \"%s\"", dbgtag);
+		}
 	}
 #endif
 
@@ -838,6 +892,80 @@ EventTriggerSQLDrop(Node *parsetree)
 	list_free(runlist);
 }
 
+
+/*
+ * Fire table_rewrite triggers.
+ */
+void
+EventTriggerTableRewrite(Node *parsetree, Oid tableOid, int reason)
+{
+	List	   *runlist;
+	EventTriggerData trigdata;
+
+	elog(DEBUG1, "EventTriggerTableRewrite(%u)", tableOid);
+
+	/*
+	 * Event Triggers are completely disabled in standalone mode.  There are
+	 * (at least) two reasons for this:
+	 *
+	 * 1. A sufficiently broken event trigger might not only render the
+	 * database unusable, but prevent disabling itself to fix the situation.
+	 * In this scenario, restarting in standalone mode provides an escape
+	 * hatch.
+	 *
+	 * 2. BuildEventTriggerCache relies on systable_beginscan_ordered, and
+	 * therefore will malfunction if pg_event_trigger's indexes are damaged.
+	 * To allow recovery from a damaged index, we need some operating mode
+	 * wherein event triggers are disabled.  (Or we could implement
+	 * heapscan-and-sort logic for that case, but having disaster recovery
+	 * scenarios depend on code that's otherwise untested isn't appetizing.)
+	 */
+	if (!IsUnderPostmaster)
+		return;
+
+	runlist = EventTriggerCommonSetup(parsetree,
+									  EVT_TableRewrite,
+									  "table_rewrite",
+									  &trigdata);
+	if (runlist == NIL)
+		return;
+
+	/*
+	 * Make sure pg_event_trigger_table_rewrite_oid only works when running
+	 * these triggers. Use PG_TRY to ensure table_rewrite_oid is reset even
+	 * when one trigger fails. (This is perhaps not necessary, as the
+	 * currentState variable will be removed shortly by our caller, but it
+	 * seems better to play safe.)
+	 */
+	currentEventTriggerState->table_rewrite_oid = tableOid;
+	currentEventTriggerState->table_rewrite_reason = reason;
+
+	/* Run the triggers. */
+	PG_TRY();
+	{
+		EventTriggerInvoke(runlist, &trigdata);
+	}
+	PG_CATCH();
+	{
+		currentEventTriggerState->table_rewrite_oid = InvalidOid;
+		currentEventTriggerState->table_rewrite_reason = 0;
+		PG_RE_THROW();
+	}
+	PG_END_TRY();
+
+	currentEventTriggerState->table_rewrite_oid = InvalidOid;
+	currentEventTriggerState->table_rewrite_reason = 0;
+
+	/* Cleanup. */
+	list_free(runlist);
+
+	/*
+	 * Make sure anything the event triggers did will be visible to the main
+	 * command.
+	 */
+	CommandCounterIncrement();
+}
+
 /*
  * Invoke each event trigger in a list of event triggers.
  */
@@ -871,6 +999,8 @@ EventTriggerInvoke(List *fn_oid_list, EventTriggerData *trigdata)
 		FunctionCallInfoData fcinfo;
 		PgStat_FunctionCallUsage fcusage;
 
+		elog(DEBUG1, "EventTriggerInvoke %u", fnoid);
+
 		/*
 		 * We want each event trigger to be able to see the results of the
 		 * previous event trigger's action.  Caller is responsible for any
@@ -1026,8 +1156,9 @@ EventTriggerBeginCompleteQuery(void)
 	MemoryContext cxt;
 
 	/*
-	 * Currently, sql_drop events are the only reason to have event trigger
-	 * state at all; so if there are none, don't install one.
+	 * Currently, sql_drop and table_rewrite events are the only reason to
+	 * have event trigger state at all; so if there are none, don't install
+	 * one.
 	 */
 	if (!trackDroppedObjectsNeeded())
 		return false;
@@ -1041,6 +1172,7 @@ EventTriggerBeginCompleteQuery(void)
 	state->cxt = cxt;
 	slist_init(&(state->SQLDropList));
 	state->in_sql_drop = false;
+	state->table_rewrite_oid = InvalidOid;
 
 	state->previous = currentEventTriggerState;
 	currentEventTriggerState = state;
@@ -1080,8 +1212,9 @@ EventTriggerEndCompleteQuery(void)
 bool
 trackDroppedObjectsNeeded(void)
 {
-	/* true if any sql_drop event trigger exists */
-	return list_length(EventCacheLookup(EVT_SQLDrop)) > 0;
+	/* true if any sql_drop or table_rewrite event trigger exists */
+	return list_length(EventCacheLookup(EVT_SQLDrop)) > 0 ||
+		list_length(EventCacheLookup(EVT_TableRewrite)) > 0;
 }
 
 /*
@@ -1297,3 +1430,46 @@ pg_event_trigger_dropped_objects(PG_FUNCTION_ARGS)
 
 	return (Datum) 0;
 }
+
+/*
+ * pg_event_trigger_table_rewrite_oid
+ *
+ * Make the Oid of the table going to be rewritten available to the user
+ * function run by the Event Trigger.
+ */
+Datum
+pg_event_trigger_table_rewrite_oid(PG_FUNCTION_ARGS)
+{
+	/*
+	 * Protect this function from being called out of context
+	 */
+	if (!currentEventTriggerState ||
+		currentEventTriggerState->table_rewrite_oid == InvalidOid)
+		ereport(ERROR,
+				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+		 errmsg("%s can only be called in a table_rewrite event trigger function",
+				"pg_event_trigger_table_rewrite_oid()")));
+
+	PG_RETURN_OID(currentEventTriggerState->table_rewrite_oid);
+}
+
+/*
+ * pg_event_trigger_table_rewrite_reason
+ *
+ * Make the rewrite reason available to the user.
+ */
+Datum
+pg_event_trigger_table_rewrite_reason(PG_FUNCTION_ARGS)
+{
+	/*
+	 * Protect this function from being called out of context
+	 */
+	if (!currentEventTriggerState ||
+		currentEventTriggerState->table_rewrite_reason == 0)
+		ereport(ERROR,
+				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+		 errmsg("%s can only be called in a table_rewrite event trigger function",
+				"pg_event_trigger_table_rewrite_reason()")));
+
+	PG_RETURN_INT32(currentEventTriggerState->table_rewrite_reason);
+}

src/backend/commands/tablecmds.c

@@ -46,6 +46,7 @@
 #include "commands/cluster.h"
 #include "commands/comment.h"
 #include "commands/defrem.h"
+#include "commands/event_trigger.h"
 #include "commands/policy.h"
 #include "commands/sequence.h"
 #include "commands/tablecmds.h"
@@ -153,7 +154,7 @@ typedef struct AlteredTableInfo
 	List	   *constraints;	/* List of NewConstraint */
 	List	   *newvals;		/* List of NewColumnValue */
 	bool		new_notnull;	/* T if we added new NOT NULL constraints */
-	bool		rewrite;		/* T if a rewrite is forced */
+	int			rewrite;		/* Reason for forced rewrite, if any */
 	Oid			newTableSpace;	/* new tablespace; 0 means no change */
 	bool		chgPersistence; /* T if SET LOGGED/UNLOGGED is used */
 	char		newrelpersistence;		/* if above is true */
@@ -303,13 +304,15 @@ static void validateForeignKeyConstraint(char *conname,
 static void createForeignKeyTriggers(Relation rel, Oid refRelOid,
 						 Constraint *fkconstraint,
 						 Oid constraintOid, Oid indexOid);
-static void ATController(Relation rel, List *cmds, bool recurse, LOCKMODE lockmode);
+static void ATController(AlterTableStmt *parsetree,
+						 Relation rel, List *cmds, bool recurse, LOCKMODE lockmode);
 static void ATPrepCmd(List **wqueue, Relation rel, AlterTableCmd *cmd,
 		  bool recurse, bool recursing, LOCKMODE lockmode);
 static void ATRewriteCatalogs(List **wqueue, LOCKMODE lockmode);
 static void ATExecCmd(List **wqueue, AlteredTableInfo *tab, Relation rel,
 		  AlterTableCmd *cmd, LOCKMODE lockmode);
-static void ATRewriteTables(List **wqueue, LOCKMODE lockmode);
+static void ATRewriteTables(AlterTableStmt *parsetree,
+							List **wqueue, LOCKMODE lockmode);
 static void ATRewriteTable(AlteredTableInfo *tab, Oid OIDNewHeap, LOCKMODE lockmode);
 static AlteredTableInfo *ATGetQueueEntry(List **wqueue, Relation rel);
 static void ATSimplePermissions(Relation rel, int allowed_targets);
@@ -2724,7 +2727,8 @@ AlterTable(Oid relid, LOCKMODE lockmode, AlterTableStmt *stmt)
 
 	CheckTableNotInUse(rel, "ALTER TABLE");
 
-	ATController(rel, stmt->cmds, interpretInhOption(stmt->relation->inhOpt),
+	ATController(stmt,
+				 rel, stmt->cmds, interpretInhOption(stmt->relation->inhOpt),
 				 lockmode);
 }
 
@@ -2747,7 +2751,7 @@ AlterTableInternal(Oid relid, List *cmds, bool recurse)
 
 	rel = relation_open(relid, lockmode);
 
-	ATController(rel, cmds, recurse, lockmode);
+	ATController(NULL, rel, cmds, recurse, lockmode);
 }
 
 /*
@@ -3015,8 +3019,15 @@ AlterTableGetLockLevel(List *cmds)
 	return lockmode;
 }
 
+/*
+ * ATController provides top level control over the phases.
+ *
+ * parsetree is passed in to allow it to be passed to event triggers
+ * when requested.
+ */
 static void
-ATController(Relation rel, List *cmds, bool recurse, LOCKMODE lockmode)
+ATController(AlterTableStmt *parsetree,
+			 Relation rel, List *cmds, bool recurse, LOCKMODE lockmode)
 {
 	List	   *wqueue = NIL;
 	ListCell   *lcmd;
@@ -3036,7 +3047,7 @@ ATController(Relation rel, List *cmds, bool recurse, LOCKMODE lockmode)
 	ATRewriteCatalogs(&wqueue, lockmode);
 
 	/* Phase 3: scan/rewrite tables as needed */
-	ATRewriteTables(&wqueue, lockmode);
+	ATRewriteTables(parsetree, &wqueue, lockmode);
 }
 
 /*
@@ -3195,7 +3206,7 @@ ATPrepCmd(List **wqueue, Relation rel, AlterTableCmd *cmd,
 			/* force rewrite if necessary; see comment in ATRewriteTables */
 			if (tab->chgPersistence)
 			{
-				tab->rewrite = true;
+				tab->rewrite |= AT_REWRITE_ALTER_PERSISTENCE;
 				tab->newrelpersistence = RELPERSISTENCE_PERMANENT;
 			}
 			pass = AT_PASS_MISC;
@@ -3206,7 +3217,7 @@ ATPrepCmd(List **wqueue, Relation rel, AlterTableCmd *cmd,
 			/* force rewrite if necessary; see comment in ATRewriteTables */
 			if (tab->chgPersistence)
 			{
-				tab->rewrite = true;
+				tab->rewrite |= AT_REWRITE_ALTER_PERSISTENCE;
 				tab->newrelpersistence = RELPERSISTENCE_UNLOGGED;
 			}
 			pass = AT_PASS_MISC;
@@ -3607,7 +3618,7 @@ ATExecCmd(List **wqueue, AlteredTableInfo *tab, Relation rel,
  * ATRewriteTables: ALTER TABLE phase 3
  */
 static void
-ATRewriteTables(List **wqueue, LOCKMODE lockmode)
+ATRewriteTables(AlterTableStmt *parsetree, List **wqueue, LOCKMODE lockmode)
 {
 	ListCell   *ltab;
 
@@ -3634,7 +3645,7 @@ ATRewriteTables(List **wqueue, LOCKMODE lockmode)
 		 * constraints, so it's not necessary/appropriate to enforce them just
 		 * during ALTER.)
 		 */
-		if (tab->newvals != NIL || tab->rewrite)
+		if (tab->newvals != NIL || tab->rewrite > 0)
 		{
 			Relation	rel;
 
@@ -3655,7 +3666,7 @@ ATRewriteTables(List **wqueue, LOCKMODE lockmode)
 		 * and assigns a new relfilenode, we automatically create or drop an
 		 * init fork for the relation as appropriate.
 		 */
-		if (tab->rewrite)
+		if (tab->rewrite > 0)
 		{
 			/* Build a temporary relation and copy data */
 			Relation	OldHeap;
@@ -3709,6 +3720,21 @@ ATRewriteTables(List **wqueue, LOCKMODE lockmode)
 
 			heap_close(OldHeap, NoLock);
 
+			/*
+			 * Fire off an Event Trigger now, before actually rewriting the
+			 * table.
+			 *
+			 * We don't support Event Trigger for nested commands anywhere,
+			 * here included, and parsetree is given NULL when coming from
+			 * AlterTableInternal.
+			 *
+			 * And fire it only once.
+			 */
+			if (parsetree)
+				EventTriggerTableRewrite((Node *)parsetree,
+										 tab->relid,
+										 tab->rewrite);
+
 			/*
 			 * Create transient table that will receive the modified data.
 			 *
@@ -4002,7 +4028,7 @@ ATRewriteTable(AlteredTableInfo *tab, Oid OIDNewHeap, LOCKMODE lockmode)
 
 		while ((tuple = heap_getnext(scan, ForwardScanDirection)) != NULL)
 		{
-			if (tab->rewrite)
+			if (tab->rewrite > 0)
 			{
 				Oid			tupOid = InvalidOid;
 
@@ -4828,7 +4854,7 @@ ATExecAddColumn(List **wqueue, AlteredTableInfo *tab, Relation rel,
 			newval->expr = expression_planner(defval);
 
 			tab->newvals = lappend(tab->newvals, newval);
-			tab->rewrite = true;
+			tab->rewrite |= AT_REWRITE_DEFAULT_VAL;
 		}
 
 		/*
@@ -4845,7 +4871,7 @@ ATExecAddColumn(List **wqueue, AlteredTableInfo *tab, Relation rel,
 	 * table to fix that.
 	 */
 	if (isOid)
-		tab->rewrite = true;
+		tab->rewrite |= AT_REWRITE_ALTER_OID;
 
 	/*
 	 * Add needed dependency entries for the new column.
@@ -5657,7 +5683,7 @@ ATExecDropColumn(List **wqueue, Relation rel, const char *colName,
 		tab = ATGetQueueEntry(wqueue, rel);
 
 		/* Tell Phase 3 to physically remove the OID column */
-		tab->rewrite = true;
+		tab->rewrite |= AT_REWRITE_ALTER_OID;
 	}
 }
 
@@ -5683,7 +5709,7 @@ ATExecAddIndex(AlteredTableInfo *tab, Relation rel,
 	/* suppress schema rights check when rebuilding existing index */
 	check_rights = !is_rebuild;
 	/* skip index build if phase 3 will do it or we're reusing an old one */
-	skip_build = tab->rewrite || OidIsValid(stmt->oldNode);
+	skip_build = tab->rewrite > 0 || OidIsValid(stmt->oldNode);
 	/* suppress notices when rebuilding existing index */
 	quiet = is_rebuild;
 
@@ -7686,7 +7712,7 @@ ATPrepAlterColumnType(List **wqueue,
 
 		tab->newvals = lappend(tab->newvals, newval);
 		if (ATColumnChangeRequiresRewrite(transform, attnum))
-			tab->rewrite = true;
+			tab->rewrite |= AT_REWRITE_COLUMN_REWRITE;
 	}
 	else if (transform)
 		ereport(ERROR,
@@ -8431,7 +8457,7 @@ ATPostAlterTypeParse(Oid oldId, Oid oldRelId, Oid refRelId, char *cmd,
 								con->old_pktable_oid = refRelId;
 								/* rewriting neither side of a FK */
 								if (con->contype == CONSTR_FOREIGN &&
-									!rewrite && !tab->rewrite)
+									!rewrite && tab->rewrite == 0)
 									TryReuseForeignKey(oldId, con);
 								cmd->subtype = AT_ReAddConstraint;
 								tab->subcmds[AT_PASS_OLD_CONSTR] =

src/backend/utils/cache/evtcache.c

@@ -169,6 +169,8 @@ BuildEventTriggerCache(void)
 			event = EVT_DDLCommandEnd;
 		else if (strcmp(evtevent, "sql_drop") == 0)
 			event = EVT_SQLDrop;
+		else if (strcmp(evtevent, "table_rewrite") == 0)
+			event = EVT_TableRewrite;
 		else
 			continue;
 

src/include/catalog/pg_proc.h

@@ -5045,6 +5045,10 @@ DESCR("peek at binary changes from replication slot");
 /* event triggers */
 DATA(insert OID = 3566 (  pg_event_trigger_dropped_objects		PGNSP PGUID 12 10 100 0 0 f f f f t t s 0 0 2249 "" "{26,26,23,25,25,25,25}" "{o,o,o,o,o,o,o}" "{classid, objid, objsubid, object_type, schema_name, object_name, object_identity}" _null_ pg_event_trigger_dropped_objects _null_ _null_ _null_ ));
 DESCR("list objects dropped by the current command");
+DATA(insert OID = 4566 (  pg_event_trigger_table_rewrite_oid	PGNSP PGUID 12 1 0 0 0 f f f f t f s 0 0 26 "" "{26}" "{o}" "{oid}" _null_ pg_event_trigger_table_rewrite_oid _null_ _null_ _null_ ));
+DESCR("return Oid of the table getting rewritten");
+DATA(insert OID = 4567 (  pg_event_trigger_table_rewrite_reason PGNSP PGUID 12 1 0 0 0 f f f f t f s 0 0 23 "" _null_ _null_ _null_ _null_ pg_event_trigger_table_rewrite_reason _null_ _null_ _null_ ));
+DESCR("return reason code for table getting rewritten");
 
 /* generic transition functions for ordered-set aggregates */
 DATA(insert OID = 3970 ( ordered_set_transition			PGNSP PGUID 12 1 0 0 0 f f f f f f i 2 0 2281 "2281 2276" _null_ _null_ _null_ _null_ ordered_set_transition _null_ _null_ _null_ ));

src/include/commands/event_trigger.h

@@ -26,6 +26,11 @@ typedef struct EventTriggerData
 	const char *tag;			/* command tag */
 } EventTriggerData;
 
+#define AT_REWRITE_ALTER_PERSISTENCE	0x01
+#define AT_REWRITE_DEFAULT_VAL			0x02
+#define AT_REWRITE_COLUMN_REWRITE		0x04
+#define AT_REWRITE_ALTER_OID			0x08
+
 /*
  * EventTriggerData is the node type that is passed as fmgr "context" info
  * when a function is called by the event trigger manager.
@@ -46,6 +51,7 @@ extern bool EventTriggerSupportsObjectClass(ObjectClass objclass);
 extern void EventTriggerDDLCommandStart(Node *parsetree);
 extern void EventTriggerDDLCommandEnd(Node *parsetree);
 extern void EventTriggerSQLDrop(Node *parsetree);
+extern void EventTriggerTableRewrite(Node *parsetree, Oid tableOid, int reason);
 
 extern bool EventTriggerBeginCompleteQuery(void);
 extern void EventTriggerEndCompleteQuery(void);

src/include/utils/builtins.h

@@ -1200,6 +1200,8 @@ extern Datum unique_key_recheck(PG_FUNCTION_ARGS);
 
 /* commands/event_trigger.c */
 extern Datum pg_event_trigger_dropped_objects(PG_FUNCTION_ARGS);
+extern Datum pg_event_trigger_table_rewrite_oid(PG_FUNCTION_ARGS);
+extern Datum pg_event_trigger_table_rewrite_reason(PG_FUNCTION_ARGS);
 
 /* commands/extension.c */
 extern Datum pg_available_extensions(PG_FUNCTION_ARGS);

src/include/utils/evtcache.h

@@ -20,7 +20,8 @@ typedef enum
 {
 	EVT_DDLCommandStart,
 	EVT_DDLCommandEnd,
-	EVT_SQLDrop
+	EVT_SQLDrop,
+	EVT_TableRewrite
 } EventTriggerEvent;
 
 typedef struct

src/test/regress/expected/event_trigger.out

@@ -294,3 +294,41 @@ SELECT * FROM dropped_objects WHERE type = 'schema';
 DROP ROLE regression_bob;
 DROP EVENT TRIGGER regress_event_trigger_drop_objects;
 DROP EVENT TRIGGER undroppable;
+-- only allowed from within an event trigger function, should fail
+select pg_event_trigger_table_rewrite_oid();
+ERROR:  pg_event_trigger_table_rewrite_oid() can only be called in a table_rewrite event trigger function
+-- test Table Rewrite Event Trigger
+CREATE OR REPLACE FUNCTION test_evtrig_no_rewrite() RETURNS event_trigger
+LANGUAGE plpgsql AS $$
+BEGIN
+  RAISE EXCEPTION 'I''m sorry Sir, No Rewrite Allowed.';
+END;
+$$;
+create event trigger no_rewrite_allowed on table_rewrite
+  execute procedure test_evtrig_no_rewrite();
+create table rewriteme (id serial primary key, foo float);
+insert into rewriteme
+     select x * 1.001 from generate_series(1, 500) as t(x);
+alter table rewriteme alter column foo type numeric;
+ERROR:  I'm sorry Sir, No Rewrite Allowed.
+alter table rewriteme add column baz int default 0;
+ERROR:  I'm sorry Sir, No Rewrite Allowed.
+-- test with more than one reason to rewrite a single table
+CREATE OR REPLACE FUNCTION test_evtrig_no_rewrite() RETURNS event_trigger
+LANGUAGE plpgsql AS $$
+BEGIN
+  RAISE NOTICE 'Table ''%'' is being rewritten (reason = %)',
+               pg_event_trigger_table_rewrite_oid()::regclass,
+               pg_event_trigger_table_rewrite_reason();
+END;
+$$;
+alter table rewriteme
+ add column onemore int default 0,
+ add column another int default -1,
+ alter column foo type numeric(10,4);
+NOTICE:  Table 'rewriteme' is being rewritten (reason = 6)
+-- shouldn't trigger a table_rewrite event
+alter table rewriteme alter column foo type numeric(12,4);
+drop table rewriteme;
+drop event trigger no_rewrite_allowed;
+drop function test_evtrig_no_rewrite();

src/test/regress/parallel_schedule

@@ -92,8 +92,6 @@ test: alter_generic misc psql async
 
 # rules cannot run concurrently with any test that creates a view
 test: rules
-# event triggers cannot run concurrently with any test that runs DDL
-test: event_trigger
 
 # ----------
 # Another group of parallel tests
@@ -106,5 +104,8 @@ test: select_views portals_p2 foreign_key cluster dependency guc bitmapops combo
 # ----------
 test: plancache limit plpgsql copy2 temp domain rangefuncs prepare without_oid conversion truncate alter_table sequence polymorphism rowtypes returning largeobject with xml
 
+# event triggers cannot run concurrently with any test that runs DDL
+test: event_trigger
+
 # run stats by itself because its delay may be insufficient under heavy load
 test: stats

src/test/regress/serial_schedule

@@ -111,7 +111,6 @@ test: misc
 test: psql
 test: async
 test: rules
-test: event_trigger
 test: select_views
 test: portals_p2
 test: foreign_key
@@ -150,4 +149,5 @@ test: returning
 test: largeobject
 test: with
 test: xml
+test: event_trigger
 test: stats

src/test/regress/sql/event_trigger.sql

@@ -206,3 +206,45 @@ DROP ROLE regression_bob;
 
 DROP EVENT TRIGGER regress_event_trigger_drop_objects;
 DROP EVENT TRIGGER undroppable;
+
+-- only allowed from within an event trigger function, should fail
+select pg_event_trigger_table_rewrite_oid();
+
+-- test Table Rewrite Event Trigger
+CREATE OR REPLACE FUNCTION test_evtrig_no_rewrite() RETURNS event_trigger
+LANGUAGE plpgsql AS $$
+BEGIN
+  RAISE EXCEPTION 'I''m sorry Sir, No Rewrite Allowed.';
+END;
+$$;
+
+create event trigger no_rewrite_allowed on table_rewrite
+  execute procedure test_evtrig_no_rewrite();
+
+create table rewriteme (id serial primary key, foo float);
+insert into rewriteme
+     select x * 1.001 from generate_series(1, 500) as t(x);
+alter table rewriteme alter column foo type numeric;
+alter table rewriteme add column baz int default 0;
+
+-- test with more than one reason to rewrite a single table
+CREATE OR REPLACE FUNCTION test_evtrig_no_rewrite() RETURNS event_trigger
+LANGUAGE plpgsql AS $$
+BEGIN
+  RAISE NOTICE 'Table ''%'' is being rewritten (reason = %)',
+               pg_event_trigger_table_rewrite_oid()::regclass,
+               pg_event_trigger_table_rewrite_reason();
+END;
+$$;
+
+alter table rewriteme
+ add column onemore int default 0,
+ add column another int default -1,
+ alter column foo type numeric(10,4);
+
+-- shouldn't trigger a table_rewrite event
+alter table rewriteme alter column foo type numeric(12,4);
+
+drop table rewriteme;
+drop event trigger no_rewrite_allowed;
+drop function test_evtrig_no_rewrite();