Download Chapter 11

Survey
yes no Was this document useful for you?
   Thank you for your participation!

* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project

Document related concepts

Registry of World Record Size Shells wikipedia , lookup

DBase wikipedia , lookup

IMDb wikipedia , lookup

Tandem Computers wikipedia , lookup

Relational algebra wikipedia , lookup

Microsoft Access wikipedia , lookup

Entity–attribute–value model wikipedia , lookup

Oracle Database wikipedia , lookup

Functional Database Model wikipedia , lookup

Ingres (database) wikipedia , lookup

Microsoft Jet Database Engine wikipedia , lookup

Microsoft SQL Server wikipedia , lookup

SQL wikipedia , lookup

Database wikipedia , lookup

Concurrency control wikipedia , lookup

Extensible Storage Engine wikipedia , lookup

PL/SQL wikipedia , lookup

Open Database Connectivity wikipedia , lookup

ContactPoint wikipedia , lookup

Clusterpoint wikipedia , lookup

Relational model wikipedia , lookup

Database model wikipedia , lookup

Transcript
11.5 SQL Performance Tuning
• Evaluated from client perspective
– Most current relational DBMSs perform
automatic query optimization at the server end
– Most SQL performance optimization techniques
are DBMS-specific
• Rarely portable
• Majority of performance problems related to
poorly written SQL code
• Carefully written query usually outperforms a
poorly written query
Database Systems, 8th Edition
1
Index Selectivity
• Indexes are used when:
– Indexed column appears by itself in search
criteria of WHERE or HAVING clause
– Indexed column appears by itself in GROUP BY
or ORDER BY clause
– MAX or MIN function is applied to indexed
column
– Data sparsity on indexed column is high
• Index selectivity is a measure of how likely an
index will be used in query processing
Database Systems, 8th Edition
2
Index Selectivity (continued)
• General guidelines for indexes:
– Create indexes for each attribute in WHERE,
HAVING, ORDER BY, or GROUP BY clause
– Do not use in small tables or tables with low sparsity
– Declare primary and foreign keys so optimizer can
use indexes in join operations
– Declare indexes in join columns other than PK/FK
Database Systems, 8th Edition
3
Conditional Expressions
• Normally expressed within WHERE or HAVING
clauses of SQL statement
• Restricts output of query to only rows matching
conditional criteria
Database Systems, 8th Edition
4
• Common practices for efficient SQL:
– Use simple columns or literals in conditionals
• Avoid functions
– Numeric field comparisons are faster
• than character, date, and NULL comparisons
– Equality comparisons faster than inequality
• —the slowest is “LIKE” comparison
–
–
–
–
–
Transform conditional expressions to use literals
Write equality conditions first
AND: Use condition most likely to be false first
OR: Use condition most likely to be true first
Avoid NOT
Database Systems, 8th Edition
5
11.6 Query Formulation
• Identify what columns and computations are
required (p.459)
– Expressions
– Aggregate functions
– Granularity of raw required
• Identify source tables
• Determine how to join tables
• Determine what selection criteria is needed
– Simple comparison? IN? Nested Comparison?
HAVING
• Determine in what order to display output
Database Systems, 8th Edition
6
11.7 DBMS Performance Tuning
• Includes managing DBMS processes in primary
memory and structures in physical storage
• DBMS performance tuning at server end
focuses on setting parameters used for:
– Data cache: large enough
– SQL cache: same query may be submitted by many
users
– Sort cache
– Optimizer mode
Database Systems, 8th Edition
7
DBMS Performance Tuning
• Some general recommendations for creation of
databases:
– Use RAID (Redundant Array of Independent
Disks) to provide balance between performance
and fault tolerance
– Minimize disk contention
• At least with the following table spaces: system
table, user table, index table, temporary table,
rollback segment table
– Put high-usage tables in their own table spaces
– Assign separate data files in separate storage
volumes for indexes, system, high-usage tables
Database Systems, 8th Edition
8
DBMS Performance Tuning
• Some general recommendations for creation of
databases: (continued)
– Take advantage of table storage organizations
in database
• An indexed organized table stores the end user
table and the index table in consecutive locations
on permanent storage
– Partition tables based on usage
– Use denormalized tables where appropriate
– Store computed and aggregate attributes in
tables
Database Systems, 8th Edition
9
Common RAID Configurations
Database Systems, 8th Edition
10
11.8 Query Optimization Example
• Example illustrates how query optimizer works
• Based on QOVENDOR and QOPRODUCT
tables
• Uses Oracle SQL*Plus (Skip)
Database Systems, 8th Edition
11
Database Systems, 8th Edition
請參考以下 SQL Server 的講義
12
Database Systems, 8th Edition
13
Check the differences in query plan:
1.Before UPDATE STATISTICS QOVENDOR
2.After UPDATE STATISTICS QOVENDOR
3. CREATE INDEX QOV_NDX1 on QOVENDOR (V_AREACODE)
UPDATE STATISTICS QOVENDOR
4. CREATE INDEX QOV_NDX2 on QOVENDOR (V_NAME)
UPDATE STATISTICS QOVENDOR
Database Systems, 8th Edition
14
Database Systems, 8th Edition
15
Database Systems, 8th Edition
16
Database Systems, 8th Edition
17
Database Systems, 8th Edition
18
Check the differences in query plan:
1.Before UPDATE STATISTICS QOPRODUCT
2.After UPDATE STATISTICS QOPRODUCT
3. CREATE INDEX QOP_NDX2 ON QOPRODUCT(P_PRICE)
UPDATE STATISTICS QOPRODUCT
Database Systems, 8th Edition
19
Summary
• Database performance tuning
– Refers to activities to ensure query is processed
in minimum amount of time
• SQL performance tuning
– Refers to activities on client side to generate
SQL code
• Returns correct answer in least amount of time
• Uses minimum amount of resources at server end
• DBMS architecture represented by processes
and structures used to manage a database
Database Systems, 8th Edition
20
Summary (continued)
• Database statistics refers to measurements
gathered by the DBMS
– Describe snapshot of database objects’
characteristics
• DBMS processes queries in three phases:
parsing, execution, and fetching
• Indexes are crucial in process that speeds up
data access
Database Systems, 8th Edition
21
Summary (continued)
• During query optimization, DBMS chooses:
– Indexes to use, how to perform join operations,
table to use first, etc.
• Hints change optimizer mode for current SQL
statement
• SQL performance tuning deals with writing
queries that make good use of statistics
• Query formulation deals with translating
business questions into specific SQL code
Database Systems, 8th Edition
22