What exactly does "No Join Predicate" mean in SQL Server?

MSDN "Missing Join Predicate Event Class" says it "indicates that a query is being executed that has no join predicate".

But unfortunately it does not seem to be as easy as that.

For example, very simple situation:

create table #temp1(i int);create table #temp2(i int);Select * from #temp1, #temp2 option (recompile);

There is no data in the tables, there is no warning either, although it obviously has no join predicate.

If I take a look at documentation for SQL Server 2005 (the same link, just other server version), there is an extra sentence: "This event is produced only if both sides of the join return more than one row." This would make perfect sense in the previous situation. There is no data, so both sides returns 0 rows and no warning. Insert rows, get warning. Ok, cool.

But for the next confusing situation, I insert the same values in both tables:

Insert into #temp1 (i) values (1)Insert into #temp1 (i) values (1)Insert into #temp2 (i) values (1)Insert into #temp2 (i) values (1)

And I get:

-- no warning:Select * from #temp1 t1     inner join #temp2 t2 on t1.i = t2.i option (recompile)-- has warning:Select * from #temp1 t1     inner join (select 1 i union all select 1) t2 on t1.i = t2.i option (recompile)

Why is this so?

Note: some scripts I used to detect these bad queries on my server.

1. of course, execution plan of procedures

2. used default server trace to find warnings

   Declare @trace nvarchar(500);Select @trace = cast(value as nvarchar(500))From sys.fn_trace_getinfo(Null)Where traceid = 1 and property = 2;Select t.StartTime, te.name, *From sys.fn_trace_gettable(@trace, 1) t    Inner join sys.trace_events te on t.EventClass = te.trace_event_id    where EventClass = 80order by t.StartTime desc

3. execution plan cache, to find those plans with warnings (like this one)

   WITH XMLNAMESPACES (default 'http://schemas.microsoft.com/sqlserver/2004/07/showplan')SELECT    Cast('<?SQL '+ st.text +' ?>' as xml) sql_text,    pl.query_plan,    ps.execution_count,    ps.last_execution_time,    ps.last_elapsed_time,    ps.last_logical_reads,    ps.last_logical_writesFROM sys.dm_exec_query_stats ps with (NOLOCK)    Cross Apply sys.dm_exec_sql_text(ps.sql_handle) st    Cross Apply sys.dm_exec_query_plan(ps.plan_handle) plWHERE pl.query_plan.value('(//Warnings/@NoJoinPredicate)[1]', 'bit') = 1Order By last_execution_time descOPTION (RECOMPILE);