1. 1. Query Processing
2. Translating SQL queries into RA
3. Evaluation Plan
4. Query Execution
5. Query Optimization
6. Translation Rules
7. Cost Estimations

2. 1. Query Processing
1. Query Processing
▪ Aim of query processing
- to find information in one or more databases,
- and deliver it to the user quickly and efficiently.
- to choose the most cost effective.
▪ Translation of queries
- into expressions that can be used at physical level of file system.
- Includes query optimization and query evaluation.

3. 1. Query Processing
1. Query Processing
▪ Typical steps when processing a high-level query (e.g. SQL query)
Query tree
internal representation
of the query
Execution strategy
how to retrieve
results of query

4. 2. Translating SQL queries into RA
2. Translating SQL queries into RA
▪ Translate query into its internal form.
- This is then translated into Relational Algebra(RA).
- The parser checks syntax, verifies relations.
▪ A RA expression may have many equivalent expressions.
▪ Example
Σbalance<2500(πbalance(account))
Is equivalent to
Πbalance(σbalance<2500(account))
Each relational algebra operation can be evaluated using one of
several different algorithms. Correspondingly, a relational-algebra
expression can be evaluated in many ways.

5. 3. Evaluation Plan
3. Evaluation Plan
▪ Annotated expression specifying detailed evaluation strategy.
▪ Example
Use an index on balance to find accounts with balance < 2500,
Or perform complete relation scan and discard accounts with balance ≥ 2500.
Initial canonical query tree
Book (access#, title)
Member (ticket#, name)
Loan(loanedbook,loanedto)
Select member.name
rom book, loan, member
where book.title = "dracula"
and member.ticket# = loan.loanedto
and loan.loanedbook = book.access#

6. 4. Query Execution
4. Query Execution
For each operation (join, select, project, aggregation …)
- Typical algorithms (e.g. Binary search for simple selection)
- Specific or not to storage structure and access paths
Book (access#, title)
Member (ticket#, name)
Loan(loanedbook,loanedto)
Select member.name
From book, loan, member
where book.title = "dracula"
and member.ticket# = loan.loanedto
and loan.loanedbook = book.access#

7. 4. Query Execution
4. Query Execution