1. Query Processing

2. Query Processing
Query processing refers to the range of activities involved in extracting data from a database.
Basic steps in query processing
Parsing and translation
Optimization
Evaluation

3. Steps in Query Processing

4. Parsing and translation
Translate the query into its internal form. This is then translated into relational algebra.
Parser checks syntax, verifies relations
Evaluation
The query execution engine takes a query – evaluation plan, executes that plan and returns the answers to the query.

5. Optimization
Optimizer will choose a lowest cost evaluation plan amoung all.
Cost is estimated using statistical information from the database catalog.
Eg: number of tuples in each relation, size of tuples etc.
A relation algebra expression may have many equivalent expressions.
balance<2500 ( balance (account)) is equivalent to
balance ( balance<2500 (account))  
Each relation algebra operation can be evaluated using one of several different algorithms. Correspondingly a relational – algebra expressions can be evaluated in many ways. Annotated expression specifying detailed evaluation strategy is called an evaluation plan.
Eg: can use an index on balance to find accounts with balance <2500

6. Catalog information for cost estimation
A query optimizer, should not depend solely on heuristic rules. It should also estimate and compare the costs of executing a query using different execution strategies and should choose the strategy with the lower cost estimation.

7. Cost components for query execution
The cost of executing a query includes the following components
Access cost to secondary storage
Storage cost
Computation cost
Memory usage cost
Communication cost

8. DATABASE TUNING

9. Database tuning
Database tuning is the activity of making a database application run more quickly. Tuning is difficult because it requires global knowledge of an information system, from the hardware to the operating system to the transaction subsystem to the query language to the application.

10. The Goals of tuning database
To make applications run faster
To lower the response time of queries and transactions
To improve the overall throughput of transactions

11. DBMS can internally collect the following statistics:
Sizes of individual tables
Number of distinct values in a column
The number of times a particular query or transaction is submitted and executed in an interval of time
The times required for different phases of query and transaction.

12. I/O tuning
Hardware and software configuration of disk subsystems are examined
DBMS tuning
DBMS tuning refers to tuning of the DBMS and the configuration of the memory and processing resources of the computer running the DBMS.
Database maintenance
Database maintenance includes backups, column statistics updates, and defragmentation of data inside the database files.

13. Tuning of the database system parameters, as well as the higher level database design such as
Tuning of Hardware
Tuning of the schema
Tuning of indices
Tuning of physical design
Tuning of transactions