Query Execution
Chapter 15
Section 15.1
Presented by
Jiten Oswal
CS 257
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Agenda
 Query Processor and major parts of Query
processor
 Physical-Query-Plan Operators
 Scanning Tables
 Basic approaches to locate the tuples of a
relation R
 Sorting While Scanning Tables
 Computation Model for Physical Operator
 I/O Cost for Scan Operators
 Iterators
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What is a Query Processor
 Group of components of a DBMS that
converts a user queries and datamodification commands into a sequence of
database operations
 It also executes those operations
 Must supply detail regarding how the query
is to be executed
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Major parts of Query processor
Query Execution:
The algorithms
that manipulate the
data of the
database.
Focus on the
operations of
extended relational
algebra.
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Outline of Query Compilation
Query compilation
 Parsing : A parse tree for the
query is constructed
 Query Rewrite : The parse tree is
converted to an initial query plan
and transformed into logical query
plan (less time)
 Physical Plan Generation :
Logical Q Plan is converted into
physical query plan by selecting
algorithms and order of execution
of these operator.
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Physical-Query-Plan Operators
 Physical operators are implementations of the
operator of relational algebra.
 They can also be use in non relational
algebra operators like “scan” which scans
tables, that is, bring each tuple of some relation
into main memory
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Scanning Tables
 One of the basic thing we can do in a Physical
query plan is to read the entire contents of a
relation R.
 Variation of this operator involves simple
predicate, read only those tuples of the relation
R that satisfy the predicate.
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Scanning Tables
Basic approaches to locate the tuples of a relation R
 Table Scan
 Relation R is stored in secondary memory with its
tuples arranged in blocks
 It is possible to get the blocks one by one
 Index-Scan
 If there is an index on any attribute of Relation R, we
can use this index to get all the tuples of Relation R
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Sorting While Scanning Tables
 Number of reasons to sort a relation
Query could include an ORDER BY clause,
requiring that a relation be sorted.
Algorithms to implement relational algebra
operations requires one or both arguments
to be sorted relations.
Physical-query-plan operator sort-scan
takes a relation R, attributes on which the sort
is to be made, and produces R in that sorted
order
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Computation Model for Physical Operator
 Physical-Plan Operator should be selected
wisely which is essential for good Query
Processor .
 For “cost” of each operator is estimated by
number of disk I/O’s for an operation.
 The total cost of operation depends on the size
of the answer, and includes the final write back
cost to the total cost of the query.
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Parameters for Measuring Costs
 Parameters that affect the performance of a
query
Buffer space availability in the main
memory at the time of execution of the
query
Size of input and the size of the output
generated
The size of memory block on the disk and
the size in the main memory also affects
the performance
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Parameters for Measuring Costs
 B: The number of blocks are needed to hold all tuples
of relation R.
 Also denoted as B(R)
 T:The number of tuples in relationR.
 Also denoted as T(R)
V: The number of distinct values that appear in a
column of a relation R
 V(R, a)- is the number of distinct values of column for
a in relation R
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I/O Cost for Scan Operators
 If relation R is clustered, then the number of
disk I/O for the table-scan operator is = ~B disk
I/O’s
 If relation R is not clustered, then the number
of required disk I/O generally is much higher
 A index on a relation R occupies many fewer
than B(R) blocks
That means a scan of the entire relation R
which takes at least B disk I/O’s will require
more I/O’s than the entire index
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Iterators for Implementation of Physical
Operators
 Many physical operators can be implemented
as an Iterator.
 Iterator are methods by which the operators
compromising a physical query plan can pass
requests for tuples and answers among
themselves.
 Three methods forming the iterator for an
operation are:
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Iterators for Implementation of Physical
Operators
 Open ()
 GetNext()
 Close()
 1. Open( ) :
This method starts the process of getting
tuples
It initializes any data structures needed to
perform the operation
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 2. GetNext( ):
Returns the next tuple in the result
If there are no more tuples to return,
GetNext returns a special value NotFound
 3. Close( ) :
Ends the iteration after all tuples
It calls Close on any arguments of the
operator
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Thank You
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