Query processing

Query Processing and
Optimization

Introduction
• In this chapter we shall discuss the techniques
used by a DBMS to process, optimize and execute
high-level queries.
• The techniques used to split complex queries into
multiple simple operations and methods of
implementing these low-level operations.
• The query optimization techniques are used to
chose an efficient execution plan that will
minimize the runtime as well as many other types
of resources such as number of disk I/O, CPU
time and so on.

Query Processing
• Query Processing is a procedure of transforming a high-level
query (such as SQL) into a correct and efficient execution plan
expressed in low-level language.
• When a database system receives a query for update or
retrieval of information, it goes through a series of
compilation steps, called execution plan.
• Query processing goes through various phases:
• first phase is called syntax checking phase, the system parses
the query and checks that it follows the syntax rules or not.
• It then matches the objects in the query syntax with the view
tables and columns listed in the system table.

• In second phase the SQL query is translated in to an algebraic
expression using various rules.
• So that the process of transforming a high-level SQL query into a
relational algebraic form is called Query Decomposition.
• The relational algebraic expression now passes to the query
optimizer.
• In third phase optimization is performed by substituting equivalent
expression depends on the factors such that the existence of
certain database structures, whether or not a given file is stored,
the presence of different indexes & so on.
• Query optimization module work in tandem with the join manager
module to improve the order in which joins are performed.

• At this stage the cost model and several other
estimation formulas are used to rewrite the query.
• The modified query is written to utilize system
resources so as to bring the optimal performance.
• The query optimizer then generates an action plan also
called a execution plan.
• This action plans are converted into a query codes that
are finally executed by a run time database processor.
• The run time database processor estimate the cost of
each action plan and chose the optimal one for the
execution.

Query Analyzer
• The syntax analyzer takes the query from the
users, parses it into tokens and analyses the
tokens and their order to make sure they
follow the rules of the language grammar.
• Is an error is found in the query submitted by
the user, it is rejected and an error code
together with an explanation of why the query
was rejected is return to the user.

Query Decomposition
• In query decomposition the query processing aims are to transfer
the high-level query into a relational algebra query and to check
whether that query is syntactically and semantically correct.
• Thus the query decomposition is start with a high-level query and
transform into query graph of low-level operations, which satisfy
the query.
• The SQL query is decomposed into query blocks (low-level
operations), which form the basic unit.
• Hence nested queries within a query are identified as separate
query blocks.
• The query decomposer goes through five stages of processing for
decomposition into low-level operation and translation into
algebraic expressions.

Query Analysis
• During the query analysis phase, the query is
syntactically analyzed using the programming
language compiler (parser).
• A syntactically legal query is then validated,
using the system catalog, to ensure that all
data objects (relations and attributes) referred
to by the query are defined in the database.
• The type specification of the query qualifiers
and result is also checked at this stage.

• Example:
SELECT emp_nm FROM EMPLOYEE WHERE
emp_desg>100
This query will be rejected because the comparison
“>100” is incompatible with the data type of
emp_desg which is a variable character string.
• At the end of query analysis phase, the high-level query
(SQL) is transformed into some internal representation
that is more suitable for processing.
• This internal representation is typically a kind of query
tree.

• A Query Tree is a tree data structure that
corresponds expression.
• A Query Tree is also called a relational algebra
tree.
– Leaf node of the tree, representing the base
input relations of the query.
– Internal nodes result of applying an operation in
the algebra.
– Root of the tree representing a result of the query.

• SELECT (P.proj_no, P.dept_no, E.name, E.add,
E.dob)
• FROM PROJECT P, DEPARTMENT D, EMPLOYEE
E
• WHERE P.dept_no = D.d_no AND D.mgr_id =
E.emp_id AND P.proj_loc = ‘Mumbai’ ;

• The three relations PROJECT, DEPARTMENT,
EMPLOYEE are represent as a leaf nodes P, D
and E, while the relational algebra operations
of the represented by internal tree nodes.
• Same SQL query can have man different
relational algebra expressions and hence
many different query trees.
• The query parser typically generates a
standard initial (canonical) query tree.

Query Normalization
• The primary phase of the normalization is to avoid
redundancy.
• The normalization phase converts the query into a
normalized form that can be more easily manipulated.
• In the normalization phase, a set of equivalency rules
are applied so that the projection and selection
operations included on the query are simplified to
avoid redundancy.
• The projection operation corresponds to the SELECT
clause of SQL query and the selection operation
correspond to the predicate found in WHERE clause.
• The equivalency transformation rules that are applied.

Semantic Analyzer
• The objective of this phase of query processing is to reduce the number of
predicates.
• The semantic analyzer rejects the normalized queries that are incorrectly
formulated.
• A query is incorrectly formulated if components do not contribute to the
generation of result.
• This happens in case of missing join specification.
• A query is contradictory if its predicate cannot satisfy by any tuple in the relation.
• The semantic analyzer examine the relational calculus query (SQL) to make sure it
contains only data objects that is table, columns, views, indexes that are defined in
the database catalog.
• It makes sure that each object in the query is referenced correctly according to its
data type.
• In case of missing join specifications the components do not contribute to the
generation of the results, and thus, a query may be incorrect formulated.

Query processing

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