diff --git a/src/backend/commands/functioncmds.c b/src/backend/commands/functioncmds.c
new file mode 100644
index 470db57..06dbe28
*** a/src/backend/commands/functioncmds.c
--- b/src/backend/commands/functioncmds.c
***************
*** 41,46 ****
--- 41,47 ----
  #include "catalog/objectaccess.h"
  #include "catalog/pg_aggregate.h"
  #include "catalog/pg_cast.h"
+ #include "catalog/pg_depend.h"
  #include "catalog/pg_language.h"
  #include "catalog/pg_namespace.h"
  #include "catalog/pg_proc.h"
*************** interpret_AS_clause(Oid languageOid, con
*** 805,810 ****
--- 806,875 ----
  }
  
  
+ /*
+  * Checks if there is an index that depends on the function.
+  */
+ static bool
+ has_dependent_index(Oid funcoid)
+ {
+ 	bool result = false;
+ 	Relation depRel, relRel;
+ 	ScanKeyData key[2];
+ 	SysScanDesc depScan;
+ 	HeapTuple depTup, relTup;
+ 
+ 	/*
+ 	 * We scan pg_depend to find dependent indexes.
+ 	 */
+ 	depRel = heap_open(DependRelationId, AccessShareLock);
+ 
+ 	ScanKeyInit(&key[0],
+ 				Anum_pg_depend_refclassid,
+ 				BTEqualStrategyNumber, F_OIDEQ,
+ 				ObjectIdGetDatum(ProcedureRelationId));
+ 	ScanKeyInit(&key[1],
+ 				Anum_pg_depend_refobjid,
+ 				BTEqualStrategyNumber, F_OIDEQ,
+ 				ObjectIdGetDatum(funcoid));
+ 
+ 	depScan = systable_beginscan(depRel, DependReferenceIndexId, true,
+ 								NULL, 2, key);
+ 
+ 	while (HeapTupleIsValid(depTup = systable_getnext(depScan)))
+ 	{
+ 		Form_pg_depend pg_depend = (Form_pg_depend)GETSTRUCT(depTup);
+ 
+ 		if (pg_depend->classid == RelationRelationId &&
+ 			pg_depend->deptype == DEPENDENCY_NORMAL)
+ 		{
+ 			Form_pg_class pg_class;
+ 
+ 			/* check if the dependent relation is an index */
+ 			relRel = heap_open(RelationRelationId, AccessShareLock);
+ 
+ 			relTup = SearchSysCache1(RELOID, ObjectIdGetDatum(pg_depend->objid));
+ 			if (!HeapTupleIsValid(relTup))
+ 				elog(ERROR, "cache lookup failed for relation %u", pg_depend->objid);
+ 
+ 			pg_class = (Form_pg_class)GETSTRUCT(relTup);
+ 
+ 			if (pg_class->relkind == RELKIND_INDEX)
+ 				result = true;
+ 
+ 			ReleaseSysCache(relTup);
+ 
+ 			relation_close(relRel, AccessShareLock);
+ 		}
+ 	}
+ 
+ 	systable_endscan(depScan);
+ 
+ 	relation_close(depRel, AccessShareLock);
+ 
+ 	return result;
+ }
+ 
+ 
  
  /*
   * CreateFunction
*************** CreateFunction(CreateFunctionStmt *stmt,
*** 929,934 ****
--- 994,1027 ----
  									  &variadicArgType,
  									  &requiredResultType);
  
+ 	/*
+ 	 * Replacing an existing function is not allowed if there is an index
+ 	 * that depends on the function.  Changing such a function would corrupt
+ 	 * the index.
+ 	 */
+ 	if (stmt->replace)
+ 	{
+ 		HeapTuple tup;
+ 		Oid funcoid = InvalidOid;
+ 
+ 		/* find the function if it exists */
+ 		tup = SearchSysCache3(PROCNAMEARGSNSP,
+ 							PointerGetDatum(funcname),
+ 							PointerGetDatum(parameterTypes),
+ 							ObjectIdGetDatum(namespaceId));
+ 
+ 		if (HeapTupleIsValid(tup))
+ 			funcoid = HeapTupleGetOid(tup);
+ 
+ 		ReleaseSysCache(tup);
+ 
+ 		if (funcoid != InvalidOid && has_dependent_index(funcoid))
+ 			ereport(ERROR,
+ 					(errcode(ERRCODE_DEPENDENT_OBJECTS_STILL_EXIST),
+ 				 	errmsg("cannot replace a function that is used to define an index"),
+ 					errhint("Drop all indexes that depend on the function before modifying it.")));
+ 	}
+ 
  	if (stmt->returnType)
  	{
  		/* explicit RETURNS clause */
*************** AlterFunction(AlterFunctionStmt *stmt)
*** 1126,1131 ****
--- 1219,1234 ----
  			elog(ERROR, "option \"%s\" not recognized", defel->defname);
  	}
  
+ 	/*
+ 	 * Forbid changing the strictness of functions used in index definitions.
+ 	 * That could lead to behaviour changes, thus corrupting the indexes.
+ 	 */
+ 	if (has_dependent_index(funcOid) && strict_item)
+ 		ereport(ERROR,
+ 				(errcode(ERRCODE_DEPENDENT_OBJECTS_STILL_EXIST),
+ 			 	errmsg("cannot change strictness of a function that is used to define an index"),
+ 				errhint("Drop all indexes that depend on the function before modifying it.")));
+ 
  	if (volatility_item)
  		procForm->provolatile = interpret_func_volatility(volatility_item);
  	if (strict_item)