Transcript Chapter 11

Database Systems: Design,
Implementation, and
Management
Eighth Edition
Chapter 11
Database Performance Tuning and
Query Optimization
Objectives
• In this chapter, you will learn:
– Basic database performance-tuning concepts
– How a DBMS processes SQL queries
– About the importance of indexes in query
processing
– About the types of decisions the query optimizer
has to make
– Some common practices used to write efficient
SQL code
– How to formulate queries and tune the DBMS for
optimal performance
– Performance tuning in SQL Server 2005
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11.1 Database Performance-Tuning Concepts
• Goal of database performance is to execute
queries as fast as possible
• Database performance tuning
– Set of activities and procedures designed to
reduce response time of database system
• All factors must operate at optimum level with
minimal bottlenecks
• Good database performance starts with
good database design
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Performance Tuning: Client and Server
• Client side
– Generate SQL query that returns correct answer
in least amount of time
• Using minimum amount of resources at server
– SQL performance tuning
• Server side
– DBMS environment configured to respond to
clients’ requests as fast as possible
• Optimum use of existing resources
– DBMS performance tuning
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DBMS Architecture
• All data in database are stored in data files
• Data files
– Automatically expand in predefined increments
known as extends
– Grouped in file groups or table spaces
• Table space or file group:
– Logical grouping of several data files that store
data with similar characteristics
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Basic DBMS architecture
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DBMS Architecture (continued)
• Data cache or buffer cache: shared, reserved
memory area
– Stores most recently accessed data blocks in RAM
• SQL cache or procedure cache: stores most
recently executed SQL statements
– Also PL/SQL procedures, including triggers and
functions
• DBMS retrieves data from permanent storage and
places it in RAM
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DBMS Architecture (continued)
• Input/output request: low-level data access
operation to/from computer devices, such as
memory, hard disks, videos, and printers
• Data cache is faster than data in data files
– DBMS does not wait for hard disk to retrieve data
• Majority of performance-tuning activities focus on
minimizing I/O operations
• Typical DBMS processes:
– Listener, User, Scheduler, Lock manager, Optimizer
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Database Statistics
• Measurements about database objects and
available resources
– Tables, Indexes, Number of processors used,
Processor speed, Temporary space available
• Make critical decisions about improving query
processing efficiency
• Can be gathered manually by DBA or automatically
by DBMS
– UPDATE STATISTICS table_name [index_name]
– Auto-Update and Auto-Create Statistics option
• 資料庫屬性 -> 自動更新統計資料
• 資料庫屬性 -> 自動建立統計資料
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Ch08: dbcc show_statistics (customer, PK__CUSTOMER__24927208 )
Ch08: dbcc show_statistics (customer, CUS_UI1)
補充 SQL Server 2005
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11.2 Query Processing
• DBMS processes queries in three phases
– Parsing
• DBMS parses the query and chooses the most
efficient access/execution plan
– Execution
• DBMS executes the query using chosen
execution plan
– Fetching
• DBMS fetches the data and sends the result back
to the client
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Database Systems,
8th
Query Processing
Edition
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SQL Parsing Phase
• Break down query into smaller units
• Transform original SQL query into slightly
different version of original SQL code
– Fully equivalent
• Optimized query results are always the same as
original query
– More efficient
• Optimized query will almost always execute faster
than original query
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SQL Parsing Phase (continued)
• Query optimizer analyzes SQL query and finds
most efficient way to access data
– Validated for syntax compliance
– Validated against data dictionary
• Tables, column names are correct
• User has proper access rights
– Analyzed and decomposed into more atomic
components
– Optimized through transforming into a fully equivalent
but more efficient SQL query
– Prepared for execution by determining the execution or
access plan
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SQL Parsing Phase (continued)
• Access plans are DBMS-specific
– Translate client’s SQL query into series of
complex I/O operations
– Required to read the data from the physical data
files and generate result set
• DBMS checks if access plan already exists for
query in SQL cache
• DBMS reuses the access plan to save time
• If not, optimizer evaluates various plans
– Chosen plan placed in SQL cache
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SQL Execution and Fetching Phase
• All I/O operations indicated in access plan are
executed
– Locks acquired
– Data retrieved and placed in data cache
– Transaction management commands processed
• Rows of resulting query result set are returned
to client
• DBMS may use temporary table space to store
temporary data
– The server may send only the first 100 rows of 9000
rows
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Query Processing Bottlenecks
• Delay introduced in the processing of an I/O
operation that slows the system
–
–
–
–
–
CPU
RAM
Hard disk
Network
Application code
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SQL 敘述輸
入完成後
先不要執
行查詢,
請按下工
具列的顯
示估計執
行計劃
鈕
：
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11.3 Indexes and Query Optimization
• Indexes
– Crucial in speeding up data access
– Facilitate searching, sorting, and using
aggregate functions as well as join operations
– Ordered set of values that contains index key
and pointers
• More efficient to use index to access table than
to scan all rows in table sequentially
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Indexes and Query Optimization
• Data sparsity: number of different values a
column could possibly have
• Indexes implemented using: (課本 p. 453)
– Hash indexes
– B-tree indexes: most common index type. Used in
tables in which column values repeat a small number
of times. The leaves contain pointers to records It is
self-balanced.
– Bitmap indexes: 0/1
• DBMSs determine best type of index to use
– Ex: CUST_LNAME with B-tree and REGION_CODE
with Bitmap indexes
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B-tree and bitmap index representation 24
SELECT CUS_NAME
FROM CUSTOMER
WHERE CUS_STATE=‘FL’
Requires only 5 accesses to STATE_INDEX,
5 accesses to CUSTOMER
Index Representation for the
CUSTOMER table
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11.4 Optimizer Choices
• Rule-based optimizer
– Preset rules and points
– Rules assign a fixed cost to each operation
• Cost-based optimizer
– Algorithms based on statistics about objects
being accessed
– Adds up processing cost, I/O costs, resource
costs to derive total cost
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Example
SELECT P_CODE, P_DESCRIPT, P_PRICE, V_NAME, V_STATE
FROM PRODUCT P, VENDOR V
WHERE P.V_CODE=V.V_CODE
AND V.V_STATE=‘FL’;
• With the following database statistics:
–
–
–
–
The PRODUCT table has 7000 rows
The VENDOR table has 300 rows
10 vendors come from Florida
1000 products come from vendors in Florida
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Example
• Assume the PRODUCT table has the index
PQOH_NDX in the P_QOH attribute
SELECT MIN(P_QOH) FROM PRODUCT
could be resolved by reading only the first entry in
the PQOH_NDX index
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Using Hints to Affect Optimizer Choices
• Optimizer might not choose best plan
• Makes decisions based on existing statistics
– Statistics may be old
– Might choose less efficient decisions
• Optimizer hints: special instructions for the
optimizer embedded in the SQL command text
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Oracle 版本
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SQL Server Query Hints Example
select o.customerid,companyname
from orders as o inner MERGE join customers as c
on o.customerid = c.customerid
select o.customerid,companyname
from orders as o inner HASH join customers as c
on o.customerid = c.customerid
select o.customerid,companyname
from orders as o inner LOOP join customers as c
on o.customerid = c.customerid
select city, count(*)
from customers
group by city
OPTION (HASH GROUP)
MS SQL Server 的語法請參考：
http://msdn.microsoft.com/en-us/library/ms187713.aspx
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