February 1, 2013 at 3:34 am
Hi all experts,
I am trying to read execution plan. I have a simple select query with no where, but a group by clause. If i move my mouse over this Hash Match there is something know as Build Residual. I search on internet that what it is but to no help.
Could you guys tell me what is this Build Residual doing in the Hash Match
February 1, 2013 at 4:39 am
Basically, a hash builds a table with a key and then uses a probe function to join together two streams of data. Then, if the key in that table is inadequate for all the filtering, an additional filter, a residual, is used to eliminate data.
You're seeing a hash aggregate (I'm guessing based on what you've posted) but it's still going to work the same way. The hash table is being built to group your data, but the key alone is not enough so it's also running a residual. To see the complete set of information, don't use the tool tip, use the properties.
For more info on how to read execution plans, see my book in the link below.
"The credit belongs to the man who is actually in the arena, whose face is marred by dust and sweat and blood"
- Theodore Roosevelt
Author of:
SQL Server Execution Plans
SQL Server Query Performance Tuning
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