Preserve clustered index after rewrites with ALTER TABLE

A table rewritten by ALTER TABLE would lose tracking of an index usable
for CLUSTER.  This setting is tracked by pg_index.indisclustered and is
controlled by ALTER TABLE, so some extra work was needed to restore it
properly.  Note that ALTER TABLE only marks the index that can be used
for clustering, and does not do the actual operation.

Author: Amit Langote, Justin Pryzby
Reviewed-by: Ibrar Ahmed, Michael Paquier
Discussion: https://postgr.es/m/20200202161718.GI13621@telsasoft.com
Backpatch-through: 9.5

2 files changed, 69 insertions, 0 deletions

diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c
index c23e6af7c14..6162fb018c7 100644
--- a/src/backend/commands/tablecmds.c
+++ b/src/backend/commands/tablecmds.c
@@ -177,6 +177,7 @@ typedef struct AlteredTableInfo
 	List	   *changedIndexOids;	/* OIDs of indexes to rebuild */
 	List	   *changedIndexDefs;	/* string definitions of same */
 	char	   *replicaIdentityIndex;	/* index to reset as REPLICA IDENTITY */
+	char	   *clusterOnIndex;	/* index to use for CLUSTER */
 } AlteredTableInfo;
 
 /* Struct describing one new constraint to check in Phase 3 scan */
@@ -11582,6 +11583,21 @@ RememberReplicaIdentityForRebuilding(Oid indoid, AlteredTableInfo *tab)
 }
 
 /*
+ * Subroutine for ATExecAlterColumnType: remember any clustered index.
+ */
+static void
+RememberClusterOnForRebuilding(Oid indoid, AlteredTableInfo *tab)
+{
+	if (!get_index_isclustered(indoid))
+		return;
+
+	if (tab->clusterOnIndex)
+		elog(ERROR, "relation %u has multiple clustered indexes", tab->relid);
+
+	tab->clusterOnIndex = get_rel_name(indoid);
+}
+
+/*
  * Subroutine for ATExecAlterColumnType: remember that a constraint needs
  * to be rebuilt (which we might already know).
  */
@@ -11606,9 +11622,18 @@ RememberConstraintForRebuilding(Oid conoid, AlteredTableInfo *tab)
 		tab->changedConstraintDefs = lappend(tab->changedConstraintDefs,
 											 defstring);
 
+		/*
+		 * For the index of a constraint, if any, remember if it is used for
+		 * the table's replica identity or if it is a clustered index, so that
+		 * ATPostAlterTypeCleanup() can queue up commands necessary to restore
+		 * those properties.
+		 */
 		indoid = get_constraint_index(conoid);
 		if (OidIsValid(indoid))
+		{
 			RememberReplicaIdentityForRebuilding(indoid, tab);
+			RememberClusterOnForRebuilding(indoid, tab);
+		}
 	}
 }
 
@@ -11652,7 +11677,13 @@ RememberIndexForRebuilding(Oid indoid, AlteredTableInfo *tab)
 			tab->changedIndexDefs = lappend(tab->changedIndexDefs,
 											defstring);
 
+			/*
+			 * Remember if this index is used for the table's replica identity
+			 * or if it is a clustered index, so that ATPostAlterTypeCleanup()
+			 * can queue up commands necessary to restore those properties.
+			 */
 			RememberReplicaIdentityForRebuilding(indoid, tab);
+			RememberClusterOnForRebuilding(indoid, tab);
 		}
 	}
 }
@@ -11780,6 +11811,21 @@ ATPostAlterTypeCleanup(List **wqueue, AlteredTableInfo *tab, LOCKMODE lockmode)
 	}
 
 	/*
+	 * Queue up command to restore marking of index used for cluster.
+	 */
+	if (tab->clusterOnIndex)
+	{
+		AlterTableCmd *cmd = makeNode(AlterTableCmd);
+
+		cmd->subtype = AT_ClusterOn;
+		cmd->name = tab->clusterOnIndex;
+
+		/* do it after indexes and constraints */
+		tab->subcmds[AT_PASS_OLD_CONSTR] =
+			lappend(tab->subcmds[AT_PASS_OLD_CONSTR], cmd);
+	}
+
+	/*
 	 * It should be okay to use DROP_RESTRICT here, since nothing else should
 	 * be depending on these objects.
 	 */
diff --git a/src/backend/utils/cache/lsyscache.c b/src/backend/utils/cache/lsyscache.c
index 0a6db0d478e..a7d63f107f2 100644
--- a/src/backend/utils/cache/lsyscache.c
+++ b/src/backend/utils/cache/lsyscache.c
@@ -3334,3 +3334,26 @@ get_index_isvalid(Oid index_oid)
 
 	return isvalid;
 }
+
+/*
+ * get_index_isclustered
+ *
+ *		Given the index OID, return pg_index.indisclustered.
+ */
+bool
+get_index_isclustered(Oid index_oid)
+{
+	bool		isclustered;
+	HeapTuple	tuple;
+	Form_pg_index rd_index;
+
+	tuple = SearchSysCache1(INDEXRELID, ObjectIdGetDatum(index_oid));
+	if (!HeapTupleIsValid(tuple))
+		elog(ERROR, "cache lookup failed for index %u", index_oid);
+
+	rd_index = (Form_pg_index) GETSTRUCT(tuple);
+	isclustered = rd_index->indisclustered;
+	ReleaseSysCache(tuple);
+
+	return isclustered;
+}