six I imagine that in very first question you've shared variable concerning outer query and subquery, so For each row in most important query, subquery executes but in 2nd one particular the subquery only executes just one time which way functionality enhanced.
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We call the upper from the joined relations (In this instance the sequential scan on the) the outer relation
to stay away from any confusion, It can be superior to specify column names explicitly using the column aliases. look at this informative article for more SQL JOIN guidelines.
You surely want to have indexes click here on attributeID on both of those the characteristics and expressions table. If you don't at the moment have those indexes in place, I think you'll see a huge speedup.
Now I might choose to ask, is this behaviour ANSI standard mandated or SQL Server-certain? If you are not certain, I would open up an issue on this!
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( We see the majority of the data has null values, which suggests the many null searchSessionId rows will hash to the same worth resulting in all of these visiting the exact same partition submit the shuffling phase)
JerSchneidJerSchneid 5,87744 gold badges3535 silver badges3737 bronze badges five I guess I could filter with OR disjuncts on attributeNames+AttributeValues on this question to hurry it up but the issue is the fact that I would must dynamically append the OR disjuncts 'lead to I need smt like WHERE (attributeName = 'X' AND attributeValue = 'Y') OR (attributeName = 'Z' AND attributeValue = 'W') .
choose * with exactly where circumstances will use clustered index by default so it may well not use optimal other indexes.
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knowing the 3 join methods - nested loop join, hash join, and merge join - is very important for any person who wants to be familiar with execution options and tune queries. The art of query tuning can not be conveyed in an individual post, but I hope I could collect some applicable data right here.
begin to think about the execution designs to see the differences in how the SQl Server will interpret them. You can also use Profiler to truly run the queries several periods and obtain the differnce.