Hi

I send the appropriate patch for bug #2073. This fix disallows to change the 
default sequence.
I ran the regression test and passed. The bug details are given below. I am awaiting to answer for any further clarifications.


===================================================================
Bug reference:      2073
Logged by:          Aaron Dummer
Email address:      aaron ( at ) dummer ( dot ) info
PostgreSQL version: 8.0.3
Operating system:   Debian Linux
Description:        Can't drop sequence when created via SERIAL column
Details:
If I create a table named foo with a column named bar, column type SERIAL,
it auto-generates a sequence named foo_bar_seq.  Now if I manually create a
new sequence called custom_seq, and change the default value of foo.bar to
reference the new sequence, I still can't delete the old sequence
(foo_bar_seq).

In other words, from a user's point of view, the foo table is no longer
dependent on the foo_bar_seq, yet the system still sees it as dependent.

I brought this topic up on the #postgresql IRC channel and the behavior was
confirmed by AndrewSN, scampbell_, and mastermind.

Right.  We have this TODO item:

        * %Disallow changing default expression of a SERIAL column?

which would prevent you from changing the default expression for a
SERIAL column.  So the answer is, don't do that, and in the future, we
might prevent it.

--
Bruce Momjian
==================================================================

*** ./src/backend/catalog/dependency.c.orig	Wed Apr 26 10:54:40 2006
--- ./src/backend/catalog/dependency.c	Wed Apr 26 12:09:01 2006
***************
*** 1931,1933 ****
--- 1931,2019 ----
  
  	ReleaseSysCache(relTup);
  }
+ 
+ /* Recursively travel and search for the default sequence. Finally detach it */
+ 
+ void performSequenceDefaultDeletion(const ObjectAddress *object,
+ 					DropBehavior behavior, int deleteFlag)
+ {        
+         
+         ScanKeyData key[3];
+         int                     nkeys;
+         SysScanDesc scan;
+         HeapTuple       tup;
+         ObjectAddress otherObject;            
+ 	  Relation	depRel;
+ 	
+ 	  depRel = heap_open(DependRelationId, RowExclusiveLock);
+ 
+         ScanKeyInit(&key[0],
+                                 Anum_pg_depend_classid,
+                                 BTEqualStrategyNumber, F_OIDEQ,
+                                 ObjectIdGetDatum(object->classId));
+         ScanKeyInit(&key[1],
+                                 Anum_pg_depend_objid,
+                                 BTEqualStrategyNumber, F_OIDEQ,
+                                 ObjectIdGetDatum(object->objectId));
+         if (object->objectSubId != 0)
+         {
+                ScanKeyInit(&key[2],
+                                         Anum_pg_depend_objsubid,
+                                         BTEqualStrategyNumber, F_INT4EQ,
+                                         Int32GetDatum(object->objectSubId));
+                 nkeys = 3;
+         }
+         else
+                 nkeys = 2;
+ 
+         scan = systable_beginscan(depRel, DependDependerIndexId, true,
+                                                           SnapshotNow, nkeys, key);
+ 
+         while (HeapTupleIsValid(tup = systable_getnext(scan)))
+         {
+ 		
+                 Form_pg_depend foundDep = (Form_pg_depend) GETSTRUCT(tup);
+ 
+                 otherObject.classId = foundDep->refclassid;
+                 otherObject.objectId = foundDep->refobjid;
+                 otherObject.objectSubId = foundDep->refobjsubid;
+ 
+ 		  /* Detach the default sequence from the relation */
+ 		  if(deleteFlag == 1)	
+ 		  {	
+                 	simple_heap_delete(depRel, &tup->t_self);	
+ 			break;
+ 		  }
+ 
+                 switch (foundDep->deptype)
+                 {
+                         case DEPENDENCY_NORMAL:                        
+ 			{
+ 
+ 				if(getObjectClass(&otherObject) == OCLASS_CLASS)
+ 				{
+ 					/* Dont allow to change the default sequence */
+ 					if(deleteFlag == 2)	
+ 					{ 
+ 						systable_endscan(scan);
+ 				                heap_close(depRel, RowExclusiveLock);
+                                         	elog(ERROR, "%s is a SERIAL sequence. Can't alter the relation", getObjectDescription(&otherObject));
+ 	                                        return;
+ 					}
+ 					else /* Detach the default sequence from the relation */
+ 					{
+ 						performSequenceDefaultDeletion(&otherObject, behavior, 1);
+ 						systable_endscan(scan);
+ 						heap_close(depRel, RowExclusiveLock);
+         	                                return;					
+ 					}
+ 				}
+ 			}
+                 		
+         	}
+ 	}
+ 
+         systable_endscan(scan);
+ 	heap_close(depRel, RowExclusiveLock);  	
+ 
+ }
*** ./src/backend/catalog/heap.c.orig	Wed Apr 26 10:59:18 2006
--- ./src/backend/catalog/heap.c	Wed Apr 26 12:03:49 2006
***************
*** 2136,2138 ****
--- 2136,2185 ----
  
  	return result;
  }
+ 
+ 
+ /* Detach the default sequence and the relation */
+ 
+ void 
+ RemoveSequenceDefault(Oid relid, AttrNumber attnum,
+ 				  DropBehavior behavior, bool flag)
+ {
+ 	Relation	attrdef_rel;
+ 	ScanKeyData scankeys[2];
+ 	SysScanDesc scan;
+ 	HeapTuple	tuple;
+ 
+ 	attrdef_rel = heap_open(AttrDefaultRelationId, RowExclusiveLock);
+ 
+ 	ScanKeyInit(&scankeys[0],
+ 				Anum_pg_attrdef_adrelid,
+ 				BTEqualStrategyNumber, F_OIDEQ,
+ 				ObjectIdGetDatum(relid));
+ 	ScanKeyInit(&scankeys[1],
+ 				Anum_pg_attrdef_adnum,
+ 				BTEqualStrategyNumber, F_INT2EQ,
+ 				Int16GetDatum(attnum));
+ 
+ 	scan = systable_beginscan(attrdef_rel, AttrDefaultIndexId, true,
+ 							  SnapshotNow, 2, scankeys);
+ 
+ 	/* There should be at most one matching tuple, but we loop anyway */
+ 	while (HeapTupleIsValid(tuple = systable_getnext(scan)))
+ 	{
+ 		ObjectAddress object;
+ 
+ 		object.classId = AttrDefaultRelationId;
+ 		object.objectId = HeapTupleGetOid(tuple);
+ 		object.objectSubId = 0;
+ 
+ 		if(flag == true) /* Detach the sequence */
+   			performSequenceDefaultDeletion(&object, behavior, 0);
+ 		else	/* Don't allow to change the default sequence */
+ 			performSequenceDefaultDeletion(&object, behavior, 2);
+ 
+ 	}
+ 
+ 	systable_endscan(scan);
+ 	heap_close(attrdef_rel, RowExclusiveLock);
+ 
+ }
*** ./src/backend/commands/tablecmds.c.orig	Wed Apr 26 11:00:14 2006
--- ./src/backend/commands/tablecmds.c	Wed Apr 26 11:57:43 2006
***************
*** 3362,3367 ****
--- 3362,3372 ----
  	 * safety, but at present we do not expect anything to depend on the
  	 * default.
  	 */
+ 	if (newDefault)
+ 		RemoveSequenceDefault(RelationGetRelid(rel), attnum, DROP_RESTRICT, false);
+ 	else 
+ 		RemoveSequenceDefault(RelationGetRelid(rel), attnum, DROP_RESTRICT, true);		
+ 
  	RemoveAttrDefault(RelationGetRelid(rel), attnum, DROP_RESTRICT, false);
  
  	if (newDefault)
*** ./src/include/catalog/dependency.h.orig	Wed Apr 26 11:05:28 2006
--- ./src/include/catalog/dependency.h	Wed Apr 26 11:06:13 2006
***************
*** 207,210 ****
--- 207,213 ----
  
  extern void shdepReassignOwned(List *relids, Oid newrole);
  
+ extern void performSequenceDefaultDeletion(const ObjectAddress *object,
+ 					DropBehavior behavior, int deleteFlag);
+ 
  #endif   /* DEPENDENCY_H */
*** ./src/include/catalog/heap.h.orig	Wed Apr 26 11:04:59 2006
--- ./src/include/catalog/heap.h	Wed Apr 26 11:57:56 2006
***************
*** 97,100 ****
--- 97,103 ----
  
  extern void CheckAttributeType(const char *attname, Oid atttypid);
  
+ extern void RemoveSequenceDefault(Oid relid, AttrNumber attnum,
+ 				  DropBehavior behavior, bool flag);
+ 
  #endif   /* HEAP_H */
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