SQL select field causing major performance problem - sql

Question

I have a stored procedure that joins in numerous tables and selects fields from them. One of the tables being a temporary table.
SELECT
a.Field1,
a.Field2,
b.Field3,
b.Field4,
c.Field5
FROM table1 a
LEFT JOIN #table2 b ON a.Field1 = b.Field1
INNER JOIN table3 c ON a.Field1 = c.Field1
The above takes 10+ minutes, however if I comment out the two b fields from the select while leaving the join in place it runs in just seconds.
I have pulled this out of procedure to simplify and same behavior. Also the execution plans are almost identical.
Any help is appreciated.

Answer 1

How many rows are in the temp table, and is "Field2" in the temp table a primary key?
If you're not selecting any rows from the right table of a left join, and the join is to the primary key (or possibly a unique key), and you reference no columns from the right table, SQL Server can avoid having to access the temp table at all (since the presence or absence of a joining row has no impact on the final result):
Example. Table setup:
create table T1 (
ID int not null primary key,
Col1 varchar(10) not null
)
go
insert into T1 (ID,Col1)
select 1,'a' union all
select 2,'b' union all
select 3,'c'
go
create table #t2 (
ID int not null primary key,
Col2 varchar(10) not null
)
go
insert into #t2 (ID,Col2)
select 1,'d' union all
select 2,'e' union all
select 4,'f'
go
create table #t3 (
ID int not null,
Col3 varchar(10) not null
)
go
insert into #t3 (ID,Col3)
select 1,'d' union all
select 2,'e' union all
select 1,'f'
And the queries:
select T1.ID,T1.Col1 from T1 left join #t2 t2 on T1.ID = t2.ID
select T1.ID,T1.Col1,t2.Col2 from T1 left join #t2 t2 on T1.ID = t2.ID
select T1.ID,T1.Col1 from T1 left join #t3 t3 on T1.ID = t3.ID
select T1.ID,T1.Col1,t3.Col2 from T1 left join #t2 t3 on T1.ID = t3.ID
In all but the first query, the join happens as expected. But because the presence or absence of rows in #t2 can't affect the final result for the first query, it avoids performing the join entirely.
But if it's not something like that (and I'd expect it to be an obvious difference in the query plans)< I#m a bit stumped.

Answer 2

Have you tried inverting the joins? (although you are missing a join condition for table c in the sample query)
SELECT
a.Field1,
a.Field2,
b.Field3,
b.Field4,
c.Field5
FROM table1 a
INNER JOIN table3 c
LEFT JOIN #table2 b ON a.Field1 = b.Field1

Answer 3

I would try adding an index with included columns to #table2 and see if it helps:
CREATE NONCLUSTERED INDEX IX_table2
ON #table2 (Field1)
INCLUDE (Field3, Field4);

Answer 4

How about running the query in two parts. Make the first part as restrictive as possible and then only outer join on the filtered set.
SELECT a.Field1,
a.Field2,
b.Field3,
c.Field5
INTO #t
FROM table1 a
INNER JOIN table3 c ON a.Field1 = c.Field1
SELECT t.Field1,
t.field2,
b.field3,
b.field4,
t.field5
FROM #t t
LEFT OUTER JOIN #table2 b ON t.Field1 = b.Field1

Answer 5

select * into #temp from table1
select * into #temp1 from table2
select * into #temp2 from table3
SELECT
a.Field1,
a.Field2,
b.Field3,
b.Field4,
c.Field5
FROM #temp a
LEFT JOIN #temp1 b ON a.Field1 = b.Field1
INNER JOIN #temp2 c ON a.Field1 = c.Field1
if(Object_Id('TempDB..#temp') Is Not Null)
Begin
Drop table #temp
End
if(Object_Id('TempDB..#temp1') Is Not Null)
Begin
Drop table #temp1
End
if(Object_Id('TempDB..#temp2') Is Not Null)
Begin
Drop table #temp2
End