What is query equivalence in DBMS?

What is query equivalence in DBMS?

Twitter Facebook LinkedIn Buffer Reddit Tumblr. As we saw above, any two relational expressions are said to be equivalent, if both the expression generate same set of records. When two expressions are equivalent we can use them interchangeably.

How do you find the equivalence relation?

To prove an equivalence relation, you must show reflexivity, symmetry, and transitivity, so using our example above, we can say:

1. Reflexivity: Since a – a = 0 and 0 is an integer, this shows that (a, a) is in the relation; thus, proving R is reflexive.
2. Symmetry: If a – b is an integer, then b – a is also an integer.

What is query evaluation plan?

A query evaluation plan (Oracle calls it an “execution plan”) is a program for an abstract machine (interpreter) inside the DBMS. It is produced by the query optimizer. Another name is “access plan” (the DBMS has to decide how to access the rows, e.g. whether to use an index).

What is query in DBMS?

A query is a request for data or information from a database table or combination of tables. One of several different query languages may be used to perform a range of simple to complex database queries. SQL, the most well-known and widely-used query language, is familiar to most database administrators (DBAs).

What is query evaluation plan in DBMS?

What are different techniques used in query optimization?

There are two methods of query optimization.

• Cost based Optimization (Physical) This is based on the cost of the query. The query can use different paths based on indexes, constraints, sorting methods etc.
• Heuristic Optimization (Logical) This method is also known as rule based optimization.

What is equivalence relation with example?

Equivalence relations are often used to group together objects that are similar, or “equiv- alent”, in some sense. Example: The relation “is equal to”, denoted “=”, is an equivalence relation on the set of real numbers since for any x, y, z ∈ R: 1. (Reflexivity) x = x, 2.

What is an equivalence relation in sets?

Definition. An equivalence relation on a set S, is a relation on S which is reflexive, symmetric and transitive. Examples: Let S = ℤ and define R = {(x,y) | x and y have the same parity} i.e., x and y are either both even or both odd. The parity relation is an equivalence relation.

How are relational expressions written under the equivalence rule?

Actually, relational expressions are written in this form as a part of equivalence relation. Above query is written for better understanding. Going forward, let us try to understand the equivalence rule in terms of relational expression. Relational expression for above query can be written as below:

How to calculate the number of equivalence relations?

The corresponding equivalence relationships are those where one element is related only to itself, and the others are all related to each other. There are clearly 4 ways to choose that distinguished element. There are ( 4 2) / 2 = 6 / 2 = 3 ways.

How to optimize a query in relational algebra?

To analyze equivalent expression, listed are a set of equivalence rules. These generate equivalent expressions for a query written in relational algebra. To optimize a query, we must convert the query into its equivalent form as long as an equivalence rule is satisfied.

When to use the equivalence rule in DBMS?

Equivalence Rule in DBMS As we saw above, any two relational expressions are said to be equivalent, if both the expression generate same set of records. When two expressions are equivalent we can use them interchangeably. i.e.; we can use either of the expression whichever gives better performance.