Add DROP INDEX CONCURRENTLY [IF EXISTS], uses ShareUpdateExclusiveLock

14 files changed, 337 insertions, 26 deletions

diff --git a/doc/src/sgml/ref/drop_index.sgml b/doc/src/sgml/ref/drop_index.sgml
index 7177ef2d81..343f7aca00 100644
--- a/doc/src/sgml/ref/drop_index.sgml
+++ b/doc/src/sgml/ref/drop_index.sgml
@@ -21,7 +21,7 @@ PostgreSQL documentation
 
  <refsynopsisdiv>
 <synopsis>
-DROP INDEX [ IF EXISTS ] <replaceable class="PARAMETER">name</replaceable> [, ...] [ CASCADE | RESTRICT ]
+DROP INDEX [ CONCURRENTLY ] [ IF EXISTS ] <replaceable class="PARAMETER">name</replaceable> [, ...] [ CASCADE | RESTRICT ]
 </synopsis>
  </refsynopsisdiv>
 
@@ -50,6 +50,29 @@ DROP INDEX [ IF EXISTS ] <replaceable class="PARAMETER">name</replaceable> [, ..
    </varlistentry>
 
    <varlistentry>
+    <term><literal>CONCURRENTLY</literal></term>
+    <listitem>
+     <para>
+      When this option is used, <productname>PostgreSQL</> will drop the
+      index without taking any locks that prevent concurrent selects, inserts,
+      updates, or deletes on the table; whereas a standard index drop
+      waits for a lock that locks out everything on the table until it's done.
+      Concurrent drop index is a two stage process. First, we mark the index
+      both invalid and not ready then commit the change. Next we wait until
+      there are no users locking the table who can see the index.
+     </para>
+     <para>
+      There are several caveats to be aware of when using this option.
+      Only one index name can be specified if the <literal>CONCURRENTLY</literal>
+      parameter is specified.  Regular <command>DROP INDEX</> command can be
+      performed within a transaction block, but
+      <command>DROP INDEX CONCURRENTLY</> cannot.
+      The CASCADE option is not supported when dropping an index concurrently.
+     </para>
+    </listitem>
+   </varlistentry>
+
+   <varlistentry>
     <term><replaceable class="PARAMETER">name</replaceable></term>
     <listitem>
      <para>
diff --git a/src/backend/catalog/dependency.c b/src/backend/catalog/dependency.c
index fed724c51c..db6769cb90 100644
--- a/src/backend/catalog/dependency.c
+++ b/src/backend/catalog/dependency.c
@@ -174,8 +174,8 @@ static void reportDependentObjects(const ObjectAddresses *targetObjects,
 					   const ObjectAddress *origObject);
 static void deleteOneObject(const ObjectAddress *object,
 							Relation depRel, int32 flags);
-static void doDeletion(const ObjectAddress *object);
-static void AcquireDeletionLock(const ObjectAddress *object);
+static void doDeletion(const ObjectAddress *object, int flags);
+static void AcquireDeletionLock(const ObjectAddress *object, int flags);
 static void ReleaseDeletionLock(const ObjectAddress *object);
 static bool find_expr_references_walker(Node *node,
 							find_expr_references_context *context);
@@ -233,7 +233,7 @@ performDeletion(const ObjectAddress *object,
 	 * Acquire deletion lock on the target object.	(Ideally the caller has
 	 * done this already, but many places are sloppy about it.)
 	 */
-	AcquireDeletionLock(object);
+	AcquireDeletionLock(object, 0);
 
 	/*
 	 * Construct a list of objects to delete (ie, the given object plus
@@ -317,7 +317,7 @@ performMultipleDeletions(const ObjectAddresses *objects,
 		 * Acquire deletion lock on each target object.  (Ideally the caller
 		 * has done this already, but many places are sloppy about it.)
 		 */
-		AcquireDeletionLock(thisobj);
+		AcquireDeletionLock(thisobj, flags);
 
 		findDependentObjects(thisobj,
 							 DEPFLAG_ORIGINAL,
@@ -351,7 +351,11 @@ performMultipleDeletions(const ObjectAddresses *objects,
 	/* And clean up */
 	free_object_addresses(targetObjects);
 
-	heap_close(depRel, RowExclusiveLock);
+	/*
+	 * We closed depRel earlier in deleteOneObject if doing a drop concurrently
+	 */
+	if ((flags & PERFORM_DELETION_CONCURRENTLY) != PERFORM_DELETION_CONCURRENTLY)
+		heap_close(depRel, RowExclusiveLock);
 }
 
 /*
@@ -381,7 +385,7 @@ deleteWhatDependsOn(const ObjectAddress *object,
 	 * Acquire deletion lock on the target object.	(Ideally the caller has
 	 * done this already, but many places are sloppy about it.)
 	 */
-	AcquireDeletionLock(object);
+	AcquireDeletionLock(object, 0);
 
 	/*
 	 * Construct a list of objects to delete (ie, the given object plus
@@ -631,7 +635,7 @@ findDependentObjects(const ObjectAddress *object,
 				 * deletion of the owning object.)
 				 */
 				ReleaseDeletionLock(object);
-				AcquireDeletionLock(&otherObject);
+				AcquireDeletionLock(&otherObject, 0);
 
 				/*
 				 * The owning object might have been deleted while we waited
@@ -726,7 +730,7 @@ findDependentObjects(const ObjectAddress *object,
 		/*
 		 * Must lock the dependent object before recursing to it.
 		 */
-		AcquireDeletionLock(&otherObject);
+		AcquireDeletionLock(&otherObject, 0);
 
 		/*
 		 * The dependent object might have been deleted while we waited to
@@ -1045,9 +1049,16 @@ deleteOneObject(const ObjectAddress *object, Relation depRel, int flags)
 									 object->objectSubId);
 
 	/*
+	 * Close depRel if we are doing a drop concurrently because it
+	 * commits the transaction, so we don't want dangling references.
+	 */
+	if ((flags & PERFORM_DELETION_CONCURRENTLY) == PERFORM_DELETION_CONCURRENTLY)
+		heap_close(depRel, RowExclusiveLock);
+
+	/*
 	 * Now delete the object itself, in an object-type-dependent way.
 	 */
-	doDeletion(object);
+	doDeletion(object, flags);
 
 	/*
 	 * Delete any comments or security labels associated with this object.
@@ -1072,7 +1083,7 @@ deleteOneObject(const ObjectAddress *object, Relation depRel, int flags)
  * doDeletion: actually delete a single object
  */
 static void
-doDeletion(const ObjectAddress *object)
+doDeletion(const ObjectAddress *object, int flags)
 {
 	switch (getObjectClass(object))
 	{
@@ -1082,8 +1093,11 @@ doDeletion(const ObjectAddress *object)
 
 				if (relKind == RELKIND_INDEX)
 				{
+					bool concurrent = ((flags & PERFORM_DELETION_CONCURRENTLY)
+													== PERFORM_DELETION_CONCURRENTLY);
+
 					Assert(object->objectSubId == 0);
-					index_drop(object->objectId);
+					index_drop(object->objectId, concurrent);
 				}
 				else
 				{
@@ -1219,10 +1233,15 @@ doDeletion(const ObjectAddress *object)
  * shared-across-databases object, so we have no need for LockSharedObject.
  */
 static void
-AcquireDeletionLock(const ObjectAddress *object)
+AcquireDeletionLock(const ObjectAddress *object, int flags)
 {
 	if (object->classId == RelationRelationId)
-		LockRelationOid(object->objectId, AccessExclusiveLock);
+	{
+		if ((flags & PERFORM_DELETION_CONCURRENTLY) == PERFORM_DELETION_CONCURRENTLY)
+			LockRelationOid(object->objectId, ShareUpdateExclusiveLock);
+		else
+			LockRelationOid(object->objectId, AccessExclusiveLock);
+	}
 	else
 		/* assume we should lock the whole object not a sub-object */
 		LockDatabaseObject(object->classId, object->objectId, 0,
diff --git a/src/backend/catalog/index.c b/src/backend/catalog/index.c
index bfbe642535..5fae488c74 100644
--- a/src/backend/catalog/index.c
+++ b/src/backend/catalog/index.c
@@ -1282,7 +1282,7 @@ index_constraint_create(Relation heapRelation,
  * else associated dependencies won't be cleaned up.
  */
 void
-index_drop(Oid indexId)
+index_drop(Oid indexId, bool concurrent)
 {
 	Oid			heapId;
 	Relation	userHeapRelation;
@@ -1290,6 +1290,12 @@ index_drop(Oid indexId)
 	Relation	indexRelation;
 	HeapTuple	tuple;
 	bool		hasexprs;
+	LockRelId	heaprelid,
+				indexrelid;
+	LOCKTAG		heaplocktag,
+				indexlocktag;
+	VirtualTransactionId *old_lockholders;
+	Form_pg_index indexForm;
 
 	/*
 	 * To drop an index safely, we must grab exclusive lock on its parent
@@ -1302,17 +1308,129 @@ index_drop(Oid indexId)
 	 * that will make them update their index lists.
 	 */
 	heapId = IndexGetRelation(indexId, false);
-	userHeapRelation = heap_open(heapId, AccessExclusiveLock);
-
-	userIndexRelation = index_open(indexId, AccessExclusiveLock);
+	if (concurrent)
+	{
+		userHeapRelation = heap_open(heapId, ShareUpdateExclusiveLock);
+		userIndexRelation = index_open(indexId, ShareUpdateExclusiveLock);
+	}
+	else
+	{
+		userHeapRelation = heap_open(heapId, AccessExclusiveLock);
+		userIndexRelation = index_open(indexId, AccessExclusiveLock);
+	}
 
 	/*
-	 * There can no longer be anyone *else* touching the index, but we might
-	 * still have open queries using it in our own session.
+	 * We might still have open queries using it in our own session.
 	 */
 	CheckTableNotInUse(userIndexRelation, "DROP INDEX");
 
 	/*
+	 * Drop Index concurrently is similar in many ways to creating an
+	 * index concurrently, so some actions are similar to DefineIndex()
+	 */
+	if (concurrent)
+	{
+		/*
+		 * Mark index invalid by updating its pg_index entry
+		 *
+		 * Don't Assert(indexForm->indisvalid) because we may be trying to
+		 * clear up after an error when trying to create an index which left
+		 * the index invalid
+		 */
+		indexRelation = heap_open(IndexRelationId, RowExclusiveLock);
+
+		tuple = SearchSysCacheCopy1(INDEXRELID,
+										 ObjectIdGetDatum(indexId));
+		if (!HeapTupleIsValid(tuple))
+			elog(ERROR, "cache lookup failed for index %u", indexId);
+		indexForm = (Form_pg_index) GETSTRUCT(tuple);
+
+		indexForm->indisvalid = false;	/* make unusable for queries */
+		indexForm->indisready = false;	/* make invisible to changes */
+
+		simple_heap_update(indexRelation, &tuple->t_self, tuple);
+		CatalogUpdateIndexes(indexRelation, tuple);
+
+		heap_close(indexRelation, RowExclusiveLock);
+
+		/*
+		 * Invalidate the relcache for the table, so that after this
+		 * transaction we will refresh the index list. Forgetting just the
+		 * index is not enough.
+		 */
+		CacheInvalidateRelcache(userHeapRelation);
+
+		/* save lockrelid and locktag for below, then close but keep locks */
+		heaprelid = userHeapRelation->rd_lockInfo.lockRelId;
+		SET_LOCKTAG_RELATION(heaplocktag, heaprelid.dbId, heaprelid.relId);
+		heap_close(userHeapRelation, NoLock);
+
+		indexrelid = userIndexRelation->rd_lockInfo.lockRelId;
+		SET_LOCKTAG_RELATION(indexlocktag, indexrelid.dbId, indexrelid.relId);
+		index_close(userIndexRelation, NoLock);
+
+		/*
+		 * For a concurrent drop, it's important to make the catalog entries
+		 * visible to other transactions before we drop the index. The index
+		 * will be marked not indisvalid, so that no one else tries to either
+		 * insert into it or use it for queries.
+		 *
+		 * We must commit our current transaction so that the index update becomes
+		 * visible; then start another.  Note that all the data structures we just
+		 * built are lost in the commit.  The only data we keep past here are the
+		 * relation IDs.
+		 *
+		 * Before committing, get a session-level lock on the table, to ensure
+		 * that neither it nor the index can be dropped before we finish. This
+		 * cannot block, even if someone else is waiting for access, because we
+		 * already have the same lock within our transaction.
+		 */
+		LockRelationIdForSession(&heaprelid, ShareUpdateExclusiveLock);
+		LockRelationIdForSession(&indexrelid, ShareUpdateExclusiveLock);
+
+		PopActiveSnapshot();
+		CommitTransactionCommand();
+		StartTransactionCommand();
+
+		/*
+		 * Now we must wait until no running transaction could have the table open
+		 * with the old list of indexes.  To do this, inquire which xacts
+		 * currently would conflict with AccessExclusiveLock on the table -- ie,
+		 * which ones have a lock of any kind on the table.	Then wait for each of
+		 * these xacts to commit or abort.	Note we do not need to worry about
+		 * xacts that open the table for writing after this point; they will see
+		 * the index as invalid when they open the relation.
+		 *
+		 * Note: the reason we use actual lock acquisition here, rather than just
+		 * checking the ProcArray and sleeping, is that deadlock is possible if
+		 * one of the transactions in question is blocked trying to acquire an
+		 * exclusive lock on our table.  The lock code will detect deadlock and
+		 * error out properly.
+		 *
+		 * Note: GetLockConflicts() never reports our own xid, hence we need not
+		 * check for that.	Also, prepared xacts are not reported, which is fine
+		 * since they certainly aren't going to do anything more.
+		 */
+		old_lockholders = GetLockConflicts(&heaplocktag, AccessExclusiveLock);
+
+		while (VirtualTransactionIdIsValid(*old_lockholders))
+		{
+			VirtualXactLock(*old_lockholders, true);
+			old_lockholders++;
+		}
+
+		/*
+		 * Re-open relations to allow us to complete our actions.
+		 *
+		 * At this point, nothing should be accessing the index, but lets
+		 * leave nothing to chance and grab AccessExclusiveLock on the index
+		 * before the physical deletion.
+		 */
+		userHeapRelation = heap_open(heapId, ShareUpdateExclusiveLock);
+		userIndexRelation = index_open(indexId, AccessExclusiveLock);
+	}
+
+	/*
 	 * All predicate locks on the index are about to be made invalid. Promote
 	 * them to relation locks on the heap.
 	 */
@@ -1378,6 +1496,15 @@ index_drop(Oid indexId)
 	 * Close owning rel, but keep lock
 	 */
 	heap_close(userHeapRelation, NoLock);
+
+	/*
+	 * Release the session locks before we go.
+	 */
+	if (concurrent)
+	{
+		UnlockRelationIdForSession(&heaprelid, ShareUpdateExclusiveLock);
+		UnlockRelationIdForSession(&indexrelid, ShareUpdateExclusiveLock);
+	}
 }
 
 /* ----------------------------------------------------------------
diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c
index 9853686fe9..a35e338cc8 100644
--- a/src/backend/commands/tablecmds.c
+++ b/src/backend/commands/tablecmds.c
@@ -239,6 +239,7 @@ struct DropRelationCallbackState
 {
 	char	relkind;
 	Oid		heapOid;
+	bool	concurrent;
 };
 
 /* Alter table target-type flags for ATSimplePermissions */
@@ -738,6 +739,21 @@ RemoveRelations(DropStmt *drop)
 	ObjectAddresses *objects;
 	char		relkind;
 	ListCell   *cell;
+	int			flags = 0;
+	LOCKMODE	lockmode = AccessExclusiveLock;
+
+	if (drop->concurrent)
+	{
+		lockmode = ShareUpdateExclusiveLock;
+		if (list_length(drop->objects) > 1)
+			ereport(ERROR,
+					(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+					 errmsg("DROP INDEX CONCURRENTLY does not support dropping multiple objects")));
+		if (drop->behavior == DROP_CASCADE)
+			ereport(ERROR,
+					(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+					 errmsg("DROP INDEX CONCURRENTLY does not support CASCADE")));
+	}
 
 	/*
 	 * First we identify all the relations, then we delete them in a single
@@ -800,7 +816,8 @@ RemoveRelations(DropStmt *drop)
 		/* Look up the appropriate relation using namespace search. */
 		state.relkind = relkind;
 		state.heapOid = InvalidOid;
-		relOid = RangeVarGetRelidExtended(rel, AccessExclusiveLock, true,
+		state.concurrent = drop->concurrent;
+		relOid = RangeVarGetRelidExtended(rel, lockmode, true,
 										  false,
 										  RangeVarCallbackForDropRelation,
 										  (void *) &state);
@@ -820,7 +837,20 @@ RemoveRelations(DropStmt *drop)
 		add_exact_object_address(&obj, objects);
 	}
 
-	performMultipleDeletions(objects, drop->behavior, 0);
+	/*
+	 * Set options and check further requirements for concurrent drop
+	 */
+	if (drop->concurrent)
+	{
+		/*
+		 * Confirm that concurrent behaviour is restricted in grammar.
+		 */
+		Assert(drop->removeType == OBJECT_INDEX);
+
+		flags |= PERFORM_DELETION_CONCURRENTLY;
+	}
+
+	performMultipleDeletions(objects, drop->behavior, flags);
 
 	free_object_addresses(objects);
 }
@@ -837,9 +867,12 @@ RangeVarCallbackForDropRelation(const RangeVar *rel, Oid relOid, Oid oldRelOid,
 	struct DropRelationCallbackState *state;
 	char		relkind;
 	Form_pg_class classform;
+	LOCKMODE	heap_lockmode;
 
 	state = (struct DropRelationCallbackState *) arg;
 	relkind = state->relkind;
+	heap_lockmode = state->concurrent ?
+		ShareUpdateExclusiveLock : AccessExclusiveLock;
 
 	/*
 	 * If we previously locked some other index's heap, and the name we're
@@ -848,7 +881,7 @@ RangeVarCallbackForDropRelation(const RangeVar *rel, Oid relOid, Oid oldRelOid,
 	 */
 	if (relOid != oldRelOid && OidIsValid(state->heapOid))
 	{
-		UnlockRelationOid(state->heapOid, AccessExclusiveLock);
+		UnlockRelationOid(state->heapOid, heap_lockmode);
 		state->heapOid = InvalidOid;
 	}
 
@@ -889,7 +922,7 @@ RangeVarCallbackForDropRelation(const RangeVar *rel, Oid relOid, Oid oldRelOid,
 	{
 		state->heapOid = IndexGetRelation(relOid, true);
 		if (OidIsValid(state->heapOid))
-			LockRelationOid(state->heapOid, AccessExclusiveLock);
+			LockRelationOid(state->heapOid, heap_lockmode);
 	}
 }
 
diff --git a/src/backend/nodes/copyfuncs.c b/src/backend/nodes/copyfuncs.c
index 33ee62f40d..f864af5d66 100644
--- a/src/backend/nodes/copyfuncs.c
+++ b/src/backend/nodes/copyfuncs.c
@@ -2751,6 +2751,7 @@ _copyDropStmt(const DropStmt *from)
 	COPY_SCALAR_FIELD(removeType);
 	COPY_SCALAR_FIELD(behavior);
 	COPY_SCALAR_FIELD(missing_ok);
+	COPY_SCALAR_FIELD(concurrent);
 
 	return newnode;
 }
diff --git a/src/backend/nodes/equalfuncs.c b/src/backend/nodes/equalfuncs.c
index b749e9bbe3..956421020c 100644
--- a/src/backend/nodes/equalfuncs.c
+++ b/src/backend/nodes/equalfuncs.c
@@ -1189,6 +1189,7 @@ _equalDropStmt(const DropStmt *a, const DropStmt *b)
 	COMPARE_SCALAR_FIELD(removeType);
 	COMPARE_SCALAR_FIELD(behavior);
 	COMPARE_SCALAR_FIELD(missing_ok);
+	COMPARE_SCALAR_FIELD(concurrent);
 
 	return true;
 }
diff --git a/src/backend/parser/gram.y b/src/backend/parser/gram.y
index f2151ef498..ae1658a4ff 100644
--- a/src/backend/parser/gram.y
+++ b/src/backend/parser/gram.y
@@ -3276,6 +3276,7 @@ DropPLangStmt:
 					n->arguments = NIL;
 					n->behavior = $5;
 					n->missing_ok = false;
+					n->concurrent = false;
 					$$ = (Node *)n;
 				}
 			| DROP opt_procedural LANGUAGE IF_P EXISTS ColId_or_Sconst opt_drop_behavior
@@ -3285,6 +3286,7 @@ DropPLangStmt:
 					n->objects = list_make1(list_make1(makeString($6)));
 					n->behavior = $7;
 					n->missing_ok = true;
+					n->concurrent = false;
 					$$ = (Node *)n;
 				}
 		;
@@ -3680,6 +3682,7 @@ DropFdwStmt: DROP FOREIGN DATA_P WRAPPER name opt_drop_behavior
 					n->arguments = NIL;
 					n->missing_ok = false;
 					n->behavior = $6;
+					n->concurrent = false;
 					$$ = (Node *) n;
 				}
 				|  DROP FOREIGN DATA_P WRAPPER IF_P EXISTS name opt_drop_behavior
@@ -3690,6 +3693,7 @@ DropFdwStmt: DROP FOREIGN DATA_P WRAPPER name opt_drop_behavior
 					n->arguments = NIL;
 					n->missing_ok = true;
 					n->behavior = $8;
+					n->concurrent = false;
 					$$ = (Node *) n;
 				}
 		;
@@ -3840,6 +3844,7 @@ DropForeignServerStmt: DROP SERVER name opt_drop_behavior
 					n->arguments = NIL;
 					n->missing_ok = false;
 					n->behavior = $4;
+					n->concurrent = false;
 					$$ = (Node *) n;
 				}
 				|  DROP SERVER IF_P EXISTS name opt_drop_behavior
@@ -3850,6 +3855,7 @@ DropForeignServerStmt: DROP SERVER name opt_drop_behavior
 					n->arguments = NIL;
 					n->missing_ok = true;
 					n->behavior = $6;
+					n->concurrent = false;
 					$$ = (Node *) n;
 				}
 		;
@@ -4237,6 +4243,7 @@ DropTrigStmt:
 					n->arguments = NIL;
 					n->behavior = $6;
 					n->missing_ok = false;
+					n->concurrent = false;
 					$$ = (Node *) n;
 				}
 			| DROP TRIGGER IF_P EXISTS name ON any_name opt_drop_behavior
@@ -4247,6 +4254,7 @@ DropTrigStmt:
 					n->arguments = NIL;
 					n->behavior = $8;
 					n->missing_ok = true;
+					n->concurrent = false;
 					$$ = (Node *) n;
 				}
 		;
@@ -4707,6 +4715,7 @@ DropOpClassStmt:
 					n->removeType = OBJECT_OPCLASS;
 					n->behavior = $7;
 					n->missing_ok = false;
+					n->concurrent = false;
 					$$ = (Node *) n;
 				}
 			| DROP OPERATOR CLASS IF_P EXISTS any_name USING access_method opt_drop_behavior
@@ -4717,6 +4726,7 @@ DropOpClassStmt:
 					n->removeType = OBJECT_OPCLASS;
 					n->behavior = $9;
 					n->missing_ok = true;
+					n->concurrent = false;
 					$$ = (Node *) n;
 				}
 		;
@@ -4730,6 +4740,7 @@ DropOpFamilyStmt:
 					n->removeType = OBJECT_OPFAMILY;
 					n->behavior = $7;
 					n->missing_ok = false;
+					n->concurrent = false;
 					$$ = (Node *) n;
 				}
 			| DROP OPERATOR FAMILY IF_P EXISTS any_name USING access_method opt_drop_behavior
@@ -4740,6 +4751,7 @@ DropOpFamilyStmt:
 					n->removeType = OBJECT_OPFAMILY;
 					n->behavior = $9;
 					n->missing_ok = true;
+					n->concurrent = false;
 					$$ = (Node *) n;
 				}
 		;
@@ -4790,6 +4802,7 @@ DropStmt:	DROP drop_type IF_P EXISTS any_name_list opt_drop_behavior
 					n->objects = $5;
 					n->arguments = NIL;
 					n->behavior = $6;
+					n->concurrent = false;
 					$$ = (Node *)n;
 				}
 			| DROP drop_type any_name_list opt_drop_behavior
@@ -4800,6 +4813,29 @@ DropStmt:	DROP drop_type IF_P EXISTS any_name_list opt_drop_behavior
 					n->objects = $3;
 					n->arguments = NIL;
 					n->behavior = $4;
+					n->concurrent = false;
+					$$ = (Node *)n;
+				}
+			| DROP INDEX CONCURRENTLY any_name_list opt_drop_behavior
+				{
+					DropStmt *n = makeNode(DropStmt);
+					n->removeType = OBJECT_INDEX;
+					n->missing_ok = FALSE;
+					n->objects = $4;
+					n->arguments = NIL;
+					n->behavior = $5;
+					n->concurrent = true;
+					$$ = (Node *)n;
+				}
+			| DROP INDEX CONCURRENTLY IF_P EXISTS any_name_list opt_drop_behavior
+				{
+					DropStmt *n = makeNode(DropStmt);
+					n->removeType = OBJECT_INDEX;
+					n->missing_ok = FALSE;
+					n->objects = $6;
+					n->arguments = NIL;
+					n->behavior = $7;
+					n->concurrent = true;
 					$$ = (Node *)n;
 				}
 		;
@@ -6246,6 +6282,7 @@ RemoveFuncStmt:
 					n->arguments = list_make1(extractArgTypes($4));
 					n->behavior = $5;
 					n->missing_ok = false;
+					n->concurrent = false;
 					$$ = (Node *)n;
 				}
 			| DROP FUNCTION IF_P EXISTS func_name func_args opt_drop_behavior
@@ -6256,6 +6293,7 @@ RemoveFuncStmt:
 					n->arguments = list_make1(extractArgTypes($6));
 					n->behavior = $7;
 					n->missing_ok = true;
+					n->concurrent = false;
 					$$ = (Node *)n;
 				}
 		;
@@ -6269,6 +6307,7 @@ RemoveAggrStmt:
 					n->arguments = list_make1($4);
 					n->behavior = $5;
 					n->missing_ok = false;
+					n->concurrent = false;
 					$$ = (Node *)n;
 				}
 			| DROP AGGREGATE IF_P EXISTS func_name aggr_args opt_drop_behavior
@@ -6279,6 +6318,7 @@ RemoveAggrStmt:
 					n->arguments = list_make1($6);
 					n->behavior = $7;
 					n->missing_ok = true;
+					n->concurrent = false;
 					$$ = (Node *)n;
 				}
 		;
@@ -6292,6 +6332,7 @@ RemoveOperStmt:
 					n->arguments = list_make1($4);
 					n->behavior = $5;
 					n->missing_ok = false;
+					n->concurrent = false;
 					$$ = (Node *)n;
 				}
 			| DROP OPERATOR IF_P EXISTS any_operator oper_argtypes opt_drop_behavior
@@ -6302,6 +6343,7 @@ RemoveOperStmt:
 					n->arguments = list_make1($6);
 					n->behavior = $7;
 					n->missing_ok = true;
+					n->concurrent = false;
 					$$ = (Node *)n;
 				}
 		;
@@ -6418,6 +6460,7 @@ DropCastStmt: DROP CAST opt_if_exists '(' Typename AS Typename ')' opt_drop_beha
 					n->arguments = list_make1(list_make1($7));
 					n->behavior = $9;
 					n->missing_ok = $3;
+					n->concurrent = false;
 					$$ = (Node *)n;
 				}
 		;
@@ -7339,6 +7382,7 @@ DropRuleStmt:
 					n->arguments = NIL;
 					n->behavior = $6;
 					n->missing_ok = false;
+					n->concurrent = false;
 					$$ = (Node *) n;
 				}
 			| DROP RULE IF_P EXISTS name ON any_name opt_drop_behavior
@@ -7349,6 +7393,7 @@ DropRuleStmt:
 					n->arguments = NIL;
 					n->behavior = $8;
 					n->missing_ok = true;
+					n->concurrent = false;
 					$$ = (Node *) n;
 				}
 		;
diff --git a/src/backend/tcop/utility.c b/src/backend/tcop/utility.c
index ea2a6c6a08..89f78f2e82 100644
--- a/src/backend/tcop/utility.c
+++ b/src/backend/tcop/utility.c
@@ -631,10 +631,15 @@ standard_ProcessUtility(Node *parsetree,
 		case T_DropStmt:
 			switch (((DropStmt *) parsetree)->removeType)
 			{
+				case OBJECT_INDEX:
+					if (((DropStmt *) parsetree)->concurrent)
+						PreventTransactionChain(isTopLevel,
+											"DROP INDEX CONCURRENTLY");
+					/* fall through */
+
 				case OBJECT_TABLE:
 				case OBJECT_SEQUENCE:
 				case OBJECT_VIEW:
-				case OBJECT_INDEX:
 				case OBJECT_FOREIGN_TABLE:
 					RemoveRelations((DropStmt *) parsetree);
 					break;
diff --git a/src/backend/utils/cache/relcache.c b/src/backend/utils/cache/relcache.c
index a59950e45a..9cadb3f21a 100644
--- a/src/backend/utils/cache/relcache.c
+++ b/src/backend/utils/cache/relcache.c
@@ -3355,6 +3355,12 @@ RelationGetIndexList(Relation relation)
 		oidvector  *indclass;
 		bool		isnull;
 
+		/*
+		 * Ignore any indexes that are currently being dropped
+		 */
+		if (!index->indisvalid && !index->indisready)
+			continue;
+
 		/* Add index's OID to result list in the proper order */
 		result = insert_ordered_oid(result, index->indexrelid);
 
diff --git a/src/include/catalog/dependency.h b/src/include/catalog/dependency.h
index 28e68c5ab5..f0eb564ebd 100644
--- a/src/include/catalog/dependency.h
+++ b/src/include/catalog/dependency.h
@@ -153,6 +153,7 @@ typedef enum ObjectClass
 /* in dependency.c */
 
 #define PERFORM_DELETION_INTERNAL			0x0001
+#define PERFORM_DELETION_CONCURRENTLY		0x0002
 
 extern void performDeletion(const ObjectAddress *object,
 				DropBehavior behavior, int flags);
diff --git a/src/include/catalog/index.h b/src/include/catalog/index.h
index c7f1dd285c..3f73a6c58c 100644
--- a/src/include/catalog/index.h
+++ b/src/include/catalog/index.h
@@ -63,7 +63,7 @@ extern void index_constraint_create(Relation heapRelation,
 						bool update_pgindex,
 						bool allow_system_table_mods);
 
-extern void index_drop(Oid indexId);
+extern void index_drop(Oid indexId, bool concurrent);
 
 extern IndexInfo *BuildIndexInfo(Relation index);
 
diff --git a/src/include/nodes/parsenodes.h b/src/include/nodes/parsenodes.h
index bc9b6bd774..aaa950db26 100644
--- a/src/include/nodes/parsenodes.h
+++ b/src/include/nodes/parsenodes.h
@@ -1909,6 +1909,7 @@ typedef struct DropStmt
 	ObjectType	removeType;		/* object type */
 	DropBehavior behavior;		/* RESTRICT or CASCADE behavior */
 	bool		missing_ok;		/* skip error if object is missing? */
+	bool		concurrent;		/* drop index concurrently? */
 } DropStmt;
 
 /* ----------------------
diff --git a/src/test/regress/expected/create_index.out b/src/test/regress/expected/create_index.out
index b7497b047f..36609c5bbf 100644
--- a/src/test/regress/expected/create_index.out
+++ b/src/test/regress/expected/create_index.out
@@ -2316,6 +2316,34 @@ Indexes:
     "concur_index5" btree (f2) WHERE f1 = 'x'::text
     "std_index" btree (f2)
 
+--
+-- Try some concurrent index drops
+--
+DROP INDEX CONCURRENTLY "concur_index2";				-- works
+DROP INDEX CONCURRENTLY IF EXISTS "concur_index2";		-- notice
+ERROR:  index "concur_index2" does not exist
+-- failures
+DROP INDEX CONCURRENTLY "concur_index2", "concur_index3";
+ERROR:  DROP INDEX CONCURRENTLY does not support dropping multiple objects
+BEGIN;
+DROP INDEX CONCURRENTLY "concur_index5";
+ERROR:  DROP INDEX CONCURRENTLY cannot run inside a transaction block
+ROLLBACK;
+-- successes
+DROP INDEX CONCURRENTLY IF EXISTS "concur_index3";
+DROP INDEX CONCURRENTLY "concur_index4";
+DROP INDEX CONCURRENTLY "concur_index5";
+DROP INDEX CONCURRENTLY "concur_index1";
+DROP INDEX CONCURRENTLY "concur_heap_expr_idx";
+\d concur_heap
+Table "public.concur_heap"
+ Column | Type | Modifiers 
+--------+------+-----------
+ f1     | text | 
+ f2     | text | 
+Indexes:
+    "std_index" btree (f2)
+
 DROP TABLE concur_heap;
 --
 -- Test ADD CONSTRAINT USING INDEX
diff --git a/src/test/regress/sql/create_index.sql b/src/test/regress/sql/create_index.sql
index 57f52612df..deb1989726 100644
--- a/src/test/regress/sql/create_index.sql
+++ b/src/test/regress/sql/create_index.sql
@@ -727,6 +727,27 @@ COMMIT;
 
 \d concur_heap
 
+--
+-- Try some concurrent index drops
+--
+DROP INDEX CONCURRENTLY "concur_index2";				-- works
+DROP INDEX CONCURRENTLY IF EXISTS "concur_index2";		-- notice
+
+-- failures
+DROP INDEX CONCURRENTLY "concur_index2", "concur_index3";
+BEGIN;
+DROP INDEX CONCURRENTLY "concur_index5";
+ROLLBACK;
+
+-- successes
+DROP INDEX CONCURRENTLY IF EXISTS "concur_index3";
+DROP INDEX CONCURRENTLY "concur_index4";
+DROP INDEX CONCURRENTLY "concur_index5";
+DROP INDEX CONCURRENTLY "concur_index1";
+DROP INDEX CONCURRENTLY "concur_heap_expr_idx";
+
+\d concur_heap
+
 DROP TABLE concur_heap;
 
 --