Multi-table joins & tempdb growth / query performance

Hi
When I run a LARGE query (20 tables, some have 4 million rows), it
takes hours to run and fills up 35GB on tempdb.
Could someone tell me, when joining these tables, does SQL server join
the entire table into tempdb or only the columns that are needed, i.e.
:
* returned in output
* used as part of the join
* used in a later join
I'm trying to reduce the time AND the impact on tempdb space, and I
think I can do this by doing the following:
Say (simple example) TableA joins to TableB which joins to TableC
I want to return the fields: A.1, B.1, C.1
I have to join A to B on A.2 = B.2
I have to join B to C on B.3 = C.2
Table B has 10 columns (B.4, B.5, B.6, B.7, etc...)
Does SQL merge (join) into tempdb the following after the first join:
A.1, B.1, B.3 ' Or does it merge & store the entire table?
If it does the entire table, I think I can speed things up & save time
by doing:
SELECT B.1, B.2, B.3
INTO TableB_temp
FROM TableB
Then join the query on field 2 & 3, but now tempdb can ONLY put a max
of 3 fields from TableB_Temp into storage during query processing...
saving space & hopefully (read/write) time'
Or does SQL do this automatically?
Thanks
Sean"Sean" <plugwalsh@.yahoo.com> wrote in message
news:3698af3c.0405250724.523e4c7a@.posting.google.com...
> Hi
> When I run a LARGE query (20 tables, some have 4 million rows), it
> takes hours to run and fills up 35GB on tempdb.
> Could someone tell me, when joining these tables, does SQL server join
> the entire table into tempdb or only the columns that are needed, i.e.
> :
> * returned in output
> * used as part of the join
> * used in a later join
> I'm trying to reduce the time AND the impact on tempdb space, and I
> think I can do this by doing the following:
> Say (simple example) TableA joins to TableB which joins to TableC
> I want to return the fields: A.1, B.1, C.1
> I have to join A to B on A.2 = B.2
> I have to join B to C on B.3 = C.2
> Table B has 10 columns (B.4, B.5, B.6, B.7, etc...)
> Does SQL merge (join) into tempdb the following after the first join:
> A.1, B.1, B.3 ' Or does it merge & store the entire table?
> If it does the entire table, I think I can speed things up & save time
> by doing:
> SELECT B.1, B.2, B.3
> INTO TableB_temp
> FROM TableB
> Then join the query on field 2 & 3, but now tempdb can ONLY put a max
> of 3 fields from TableB_Temp into storage during query processing...
> saving space & hopefully (read/write) time'
> Or does SQL do this automatically?
A merge join requires that the columns that it is joining them on are both
sorted. If it is using the tempdb then it is having to sort them into order
first, then merging them. It would speed things up if you indexed the
columns (the Primary Key will already be indexed, but not necessarily the
Foreign Key). As an index is already sorted, then the mergedb should not be
required, and the whole thing should run much faster
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