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# Exercise 7: Query Optimization

Module by: Nguyen Kim Anh. E-mail the author

## Exercise 1

Consider 3 relations r(ABC), s(CDEFG), t(EHK) và 2 relational algebra expressions

• π BK ( σ A = a D = d ( σ s . E = t . E ( ( r s ) × t ) ) ) π BK ( σ A = a D = d ( σ s . E = t . E ( ( r s ) × t ) ) ) size 12{π rSub { size 8{ ital "BK"} } $$σ rSub { size 8{A=a and D=d} } \( σ rSub { size 8{s "." E=t "." E} } \( \( r*s$$ times t \) \) \) } {}
• π BK ( π BC ( σ A = a r ) π C ( σ D = d s ) π EK ( t ) ) π BK ( π BC ( σ A = a r ) π C ( σ D = d s ) π EK ( t ) ) size 12{π rSub { size 8{ ital "BK"} } $$π rSub { size 8{ ital "BC"} } \( σ rSub { size 8{A=a} } r$$ *π rSub { size 8{C} } $$σ rSub { size 8{D=d} } s$$ *π rSub { size 8{ ital "EK"} } $$t$$ \) } {}
2. Using the transformation rules and heuristic algebraic optimization algorithm, optimize the first expression.

## Exercise 2

Consider the following query:

FROM EMPLOYEE E , DEPARTMENT D

WHERE D.Name = ‘Research’ AND D.DeptId = E.DeptId;

1. Draw at two query trees that can represent this query.
2. Draw the initial query tree for the query and show how the query tree is optimized

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#### Definition of a lens

##### Lenses

A lens is a custom view of the content in the repository. You can think of it as a fancy kind of list that will let you see content through the eyes of organizations and people you trust.

##### What is in a lens?

Lens makers point to materials (modules and collections), creating a guide that includes their own comments and descriptive tags about the content.

##### Who can create a lens?

Any individual member, a community, or a respected organization.

##### What are tags?

Tags are descriptors added by lens makers to help label content, attaching a vocabulary that is meaningful in the context of the lens.

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