Query Processing

Query Processing

Presented by

Aung S. Win

Objectives

Query processing and optimization. Static versus dynamic query optimization. How a query is decomposed and

semantically analyzed. How to create a relational algebra tree to

represent a query. The rules of equivalence for the relational

algebra operations.

(Cont.)

Heuristic transformation rules. The types of database statistics required to

estimate the cost of operations. The different strategies for implementing the

relational algebra operations. The difference between materialization and

pipelining. The advantages of left-deep trees.

Query Processing

The activities involved in retrieving data from the database.

The aims of query processing (1)to transform a query written in a high-level language into a low-level language (2)to execute the strategy to retrieve the required data.

(Cont.)

Query processing can be divided into four main phases: decomposition, optimization, code generation, and execution.

Query Decomposition

The aims of query decomposition (1)to transform a high-level query into a

relational algebra query. (2)to check that the query is syntactically and

semantically correct.

(Cont.)

The typical stages of query decomposition are analysis, normalization, semantic analysis, simplification, and query restructuring.

Analysis

The query is lexically and syntactically analyzed using the techniques of programming language compilers.

Verifies that the relations and attributes specified in the query are defined in the system catalog.

Verifies that any operations applied to database objects are appropriate for the object type.

(Cont.)

On completion of the analysis, the high-level query has been transformed into some internal representation (query tree) that is more suitable for processing.

Root

Intermediate operations

leaves

Normalization

Converts the query into a normalized form that can be more easily manipulated.

There are two different normal forms, conjunctive normal form and disjunctive normal form.

Conjunctive normal form

A sequence of conjuncts that are connected with the and operator. Each conjunct contains one or more terms connected by the or operator.

for example (position=‘Manager’ V salary>20000) ^

branchNo = ‘B003’

Disjunctive normal form

A sequence of disjuncts that are connected with the or operator. Each disjunt contains one or more terms connected by the and operator.

for example (position=‘Manager’ ^ branchNo = ‘B003’) V

(salary>20000 ^ branchNo = ‘B003’)

Semantic analysis

The objective is to reject normalized queries that are incorrectly formulated or contradictory.

Simplification

To detect redundant qualifications, eliminate common subexpressions , and transform the query to a semantically equivalent but more easily and efficiently computed form.

Access restrictions, view definitions, and integrity constraints are considered at this stage.

Query restructuring

The final stage of query decomposition. The query is restructured to provide a more

efficient implementation.

Query optimization

The activity of choosing an efficient execution strategy for processing a query.

An important aspect of query processing is query optimization.

The aim of query optimization is to choose the one that minimizes resource usage.

(Cont.)

Every method of query optimization depend on database statistics.

The statistics cover information about relations, attribute, and indexes.

Keeping the statistics current can be problematic. If the DBMS updates the statistics every time a tuple

is inserted, updated, or deleted, this would have a significant impact on performance during peak period.

(Cont.)

An alternative approach is to update the statistics on a periodic basis, for example nightly, or whenever the system is idle.

Dynamic query optimization

Advantage: all information required to select an optimum strategy is up to date.

Disadvantage: the performance of the query is affected because the query has to be parsed, validated, and optimized before it can be executed.

Static query optimization

The query is parsed, validated, and optimized once that is similar to the approach taken by a compiler for a programming language.

Advantages1)The runtime overhead is removed2)More time available to evaluate a larger

number of execution strategies.

(cont.)

Disadvantage: the execution strategy that is chosen as being optimal when the query is compiled may no longer be optimal when the query is run.

Transformation Rules for the Relational Algebra Operations

By applying transformation rules, we can transform one relational algebra into an equivalent expression that is more efficient.

There are twelve rules that can be used to restructure the relational algebra tree generated during query decomposition.

Heuristics rules

Many DBMSs use heuristics to determine strategies for query processing.

Heuristics rules include -performing Selection and Projections as early as

possible.-combining Cartesian product with a subsequent

selection whose predicate represents a join condition into a join operation.

(Cont.)

-using associativity of binary operations to rearrange leaf nodes so that leaf nodes with the most restrictive Selections are executed first.

Cost estimation

Depends on statistical information held in the system catalog.

Typical statistics include the cardinality of each base relation, the number of blocks required to store a relation, the number of distinct values for each attribute, the selection cardinality of each attribute, and the number of levels in each multilevel index.

Join operation

Block nested loop join Indexed nested loop join Sort-merge join Hash join

Pipelining

In materialization, the output of one operation is stored in a temporary relation for processing by the next operation.

An alternative approach is to pipeline the results of one operation to another operation without creating a temporary relation to hold the intermediate result.

By using it, we can save on the cost of creating temporary relations and reading the results back in again.

Left – deep trees

A relational algebra tree where the right-hand relation is always a base relation.

Advantages: reducing the search space for the optimum strategy and allowing the query optimizer to be based on dynamic processing techniques.