Set all variable-length fields of pg_attribute to null on column drop

petere · petere · commit 3e2e0d5ad7fc · 2023-12-22T21:44:55.000+01:00
When a column is dropped, the fields attacl, attoptions, and attfdwoptions were kept unchanged. This is probably harmless, but it seems wasteful, and leaves potentially dangling data lying around (for example, attacl could contain references to users that are later also dropped). Change this to set those fields to null when a column is marked as dropped. Reviewed-by: Alvaro Herrera <alvherre@alvh.no-ip.org> Discussion: https://www.postgresql.org/message-id/flat/249d819d-1763-4580-8110-0bf91a0f08b7@eisentraut.org
diff --git a/src/backend/catalog/heap.c b/src/backend/catalog/heap.c
@@ -1647,6 +1647,9 @@ RemoveAttributeById(Oid relid, AttrNumber attnum)
 	HeapTuple	tuple;
 	Form_pg_attribute attStruct;
 	char		newattname[NAMEDATALEN];
+	Datum		valuesAtt[Natts_pg_attribute] = {0};
+	bool		nullsAtt[Natts_pg_attribute] = {0};
+	bool		replacesAtt[Natts_pg_attribute] = {0};
 
 	/*
 	 * Grab an exclusive lock on the target table, which we will NOT release
@@ -1695,24 +1698,24 @@ RemoveAttributeById(Oid relid, AttrNumber attnum)
 			 "........pg.dropped.%d........", attnum);
 	namestrcpy(&(attStruct->attname), newattname);
 
-	/* clear the missing value if any */
-	if (attStruct->atthasmissing)
-	{
-		Datum		valuesAtt[Natts_pg_attribute] = {0};
-		bool		nullsAtt[Natts_pg_attribute] = {0};
-		bool		replacesAtt[Natts_pg_attribute] = {0};
-
-		/* update the tuple - set atthasmissing and attmissingval */
-		valuesAtt[Anum_pg_attribute_atthasmissing - 1] =
-			BoolGetDatum(false);
-		replacesAtt[Anum_pg_attribute_atthasmissing - 1] = true;
-		valuesAtt[Anum_pg_attribute_attmissingval - 1] = (Datum) 0;
-		nullsAtt[Anum_pg_attribute_attmissingval - 1] = true;
-		replacesAtt[Anum_pg_attribute_attmissingval - 1] = true;
-
-		tuple = heap_modify_tuple(tuple, RelationGetDescr(attr_rel),
-								  valuesAtt, nullsAtt, replacesAtt);
-	}
+	/* Clear the missing value */
+	attStruct->atthasmissing = false;
+	nullsAtt[Anum_pg_attribute_attmissingval - 1] = true;
+	replacesAtt[Anum_pg_attribute_attmissingval - 1] = true;
+
+	/*
+	 * Clear the other variable-length fields.  This saves some space in
+	 * pg_attribute and removes no longer useful information.
+	 */
+	nullsAtt[Anum_pg_attribute_attacl - 1] = true;
+	replacesAtt[Anum_pg_attribute_attacl - 1] = true;
+	nullsAtt[Anum_pg_attribute_attoptions - 1] = true;
+	replacesAtt[Anum_pg_attribute_attoptions - 1] = true;
+	nullsAtt[Anum_pg_attribute_attfdwoptions - 1] = true;
+	replacesAtt[Anum_pg_attribute_attfdwoptions - 1] = true;
+
+	tuple = heap_modify_tuple(tuple, RelationGetDescr(attr_rel),
+							  valuesAtt, nullsAtt, replacesAtt);
 
 	CatalogTupleUpdate(attr_rel, &tuple->t_self, tuple);
 
