Query Optimisation

Dominant cost in query processing is secondary storage access. The fewer blocks accessed, the faster the database queries can be. Many tuples will fit into a single block, requiring the query to: find the block, find the tuple, edit the tuple, and place it back in the block. This is inefficient if we are doing many different queries accessing different blocks.

Types of Index

Primary Index

Data file sequentially ordered by ordering key field, indexing field is built on the ordering key field. Guaranteed to have unique value for each tuple.

Clustering Index

Data file sequentially ordered on non-key field, indexing field built on same non-key field. Can be more than one tuple corresponding to a value in the indexing field.

Primary / Clustered Indices

Affect the order the data is stored in a file.

Secondary Indices

Give a lookup table to the file.

Index Restrictions

Table can have 1 primary index OR 1 clustering index.
- Most frequently looked up value is often the best choice
- Some DBMS' assume PK is primary index, as it is usually used to refer to rows

Exercise

What is a Query Tree? A query tree is a visual model used to represent a logical model of database queries. Each leaf node represents a relation. Each internal node is a different query function ex. selection, projection, product, etc.
Write a relational algebra expression for the following query

SELECT lecName, schedule 
FROM lecturer, module, enroll, student 
WHERE lastName=“Burns” 
AND firstName=“Edward” 
AND module.moduleNumber=enrol.moduleNumber 
AND lecturer.lecID=module.lectID 
AND student.stuID=enrol.stuID;

pi lecName, schedule ( sigma lastName = burns ( sigma firstName = edwards ( sigma module.moduleNumber=enrol.moduleNumber ( sigma (lecturer.ledID=module.lectID (lecturer x module ) x enrol ) x student ) ) )

Draw a Query Tree for this SQL query:

Why can we not use SQL for query optimisation? Using relational algebra is easier for us to visualise, and less abstracted than SQL, allowing us to optimise the query flow.
List the heuristics that optimisers use to reduce optimisation cost.

Begin with initial query tree for SQL
Move SELECT operations down the tree
Apply more restrictive SELECT operations first ( eg. equalities before range queries )
Replace Cartesian products followed by selection with theta joins ( eg. sigma(f) ( RxS ) -> R theta(f) S )
Move PROJECT operations down the query tree ( add project operations as inputs to theta joins ).

Draw a near optimal query tree for the following SQL query, and write a relational algebra expression for this tree.

SELECT sailors.name 
FROM sailors, reservations 
WHERE reservations.sID=sailors.ID 
AND reservations.bID=100 
AND sailors.rating=7;

Workshop

Count number of tuples in the following relations:

Products
Suppliers

How many suppliers does each product have? Many suppliers to many products

Write SQL queries which count the number of tuples in each of the following algebraic statements.

The relation created by products x suppliers
The relation created by Products theta Products.SupplierID=Suppliers.SupplierID Suppliers

For each of the following write a description of the data it will retrieve and execute a single SQL statement which retrieves this data from the database

This will return the product name of all products shipped from Manchester.
This will return the same as the first, but be more efficient.
This will return the city of all employees that live in the same city as a customer.

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