Hash join

In some cases when I see the query execution plan I found that Hash
join or Merge join or sort operation is taking about 50% of execution
time.Can I do something to improve it? What is the cause of this high
proportion of execution time taken by this operation?
Regards
amish wrote:
> In some cases when I see the query execution plan I found that Hash
> join or Merge join or sort operation is taking about 50% of execution
> time.Can I do something to improve it? What is the cause of this high
> proportion of execution time taken by this operation?
>
> Regards
Merge joins are generally very quick. They are used when you have sorted
intermediate result sets that must be combined. If they are not
presorted, then a sort operation is visible and this will slow down
thing significantly. Imagine combining two sorted result sets. You start
scanning both of them, top down, looking at key values, and creating a
new result set from the simultaneous scan of both. Easy.
A hash join is more involved and is generally used when a result sets
needs to be joined, but there no keys available to relate them. Hash
values are created from the keys and these values are used for scanning.
A more involved process.
To prevent seeing these types of joins (for most queries) make sure your
joins have index support and the join clauses are SARGable.
Where TABLE1.ID = TABLE2.ID -- Needs indexes that contains the ID column
as the first column on both tables
Where LEFT(TABLE1.ID, 2) = LEFT(TABLE2.ID, 2) -- Not SARGable. Index
will not help
Where ISNULL(TABLE1.ID, 0) = TABLE2.ID -- Not SARGable
Post your tables, ddl, indexes, and query for more help.
David Gugick
Quest Software
www.imceda.com
www.quest.com
|||I guess the real question is: are you satisfied with the query's
performance?
I mean, what do you care which part of the query execution is
responsible for which estimated portion of the execution? SQL-Server
will try to execute the query as fast as possible, and hash joins and
merge joins are tools that are used in the process.
Gert-Jan
amish wrote:
> In some cases when I see the query execution plan I found that Hash
> join or Merge join or sort operation is taking about 50% of execution
> time.Can I do something to improve it? What is the cause of this high
> proportion of execution time taken by this operation?
> Regards