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Query Execution
Section 15.1
Shweta Athalye
CS257: Database Systems
ID: 118
Section 1
Agenda
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Query Processor
Query compilation
Physical Query Plan Operators
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Scanning Tables
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Table Scan
Index scan
Sorting while scanning tables
Model of computation for physical operators
Parameters for measuring cost
Iterators
Query Processor
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The Query Processor is a group of
components of a DBMS that turns user
queries and data-modification commands
into a sequence of database operations and
executes those operations
Query processor is responsible for supplying
details regarding how the query is to be
executed
The major parts of the query
processor
Query compilation
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Query compilation itself is a multi-step
process consisting of :
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Parsing: in which a parse tree representing query
and its structure is constructed
Query rewrite: in which the parse tree is
converted to an initial query plan
Physical plan generation: where the logical query
plan is turned into a physical query plan by
selecting algorithms.
Outline of query compilation
Physical Query Plan Operators
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Physical query plans are built from operators
Each of the operators implement one step of
the plan.
They are particular implementations for one
of the operators of relational algebra.
They can also be non relational algebra
operators like “scan” which scans tables.
Scanning Tables
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One of the most basic things in a physical
query plan.
Necessary when we want to perform join or
union of a relation with another relation.
Two basic approaches to locating the
tuples of a relation R
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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
 This operation is called Table Scan
Two basic approaches to locating the
tuples of a relation R
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Index-scan
 there is an index on any attribute of
Relation R
 Use this index to get all the tuples of R
 This operation is called Index Scan
Sorting While Scanning Tables
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Why do we need sorting while scanning?
 the query could include an ORDER BY
clause requiring that a relation be sorted
 Various algorithms for relational-algebra
operations require one or both of their
arguments to be sorted relation
 physical-query-plan operator sort-scan
takes a relation R and a specification of the
attributes on which the sort is to be made,
and produces R in that sorted order
Model of Computation for Physical
Operators
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Choosing physical plan operators wisely is an
essential for a good query processor.
Cost for an operation is measured in number
of disk i/o operations.
If an operator requires the final answer to a
query to be written back to the disk, the total
cost will depend on the length of the answer
and will include the final write back cost to
the total cost of the query.
Improvements in cost
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Major improvements in cost of the physical
operators can be achieved by avoiding or
reducing the number of disk i/o operations
This can be achieved by passing the answer of
one operator to the other in the main
memory itself without writing it to the disk.
Parameters for Measuring Costs
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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
Iterators for Implementation of
Physical Operators
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Many physical operators can be implemented
as an iterator
It is a group of three functions that allows a
consumer of the result of the physical
operator to get the result one tuple at a time
Iterator
The three functions forming the iterator are:
Open:
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This function starts the process of getting
tuples.
 It initializes any data structures needed to
perform the operation
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Iterator
GetNext
 This function returns the next tuple in the
result
 Adjusts data structures as necessary to allow
subsequent tuples to be obtained
 If there are no more tuples to return, GetNext
returns a special value NotFound
Iterator
Close
 This function ends the iteration after all tuples
 it calls Close on any arguments of the
operator
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