从下列实验可以看出全表扫描是根据extent/block顺序去取数据, 第一次实验有
从下列实验可以看出全表扫描是根据extent/block顺序去取数据, 第一次实验有'db file scattered read'等待事件,,第二次实验已经把数据加载到databuffer中,所以没有等待实验。
SQL> create table test as select * from dba_objects;
SQL> select extent_id, file_id, block_id, blocks
2 from dba_extents
3 where segment_name = 'TEST'
4 order by 1;
EXTENT_ID FILE_ID BLOCK_ID BLOCKS
---------- ---------- ---------- ----------
0 6 83849 8
1 6 83857 8
2 6 83865 8
3 6 83873 8
4 6 83881 8
5 6 83889 8
6 6 83897 8
7 6 83905 8
8 6 83913 8
9 6 83921 8
10 6 83929 8
11 6 83937 8
12 6 83945 8
13 6 100409 8
14 6 100417 8
15 6 117121 8
16 6 83977 128
17 6 84105 128
18 6 84233 128
19 6 84361 128
20 6 84489 128
SQL> show parameter db_file_multiblock_read_count
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
db_file_multiblock_read_count integer 16
SQL> alter session set events 'immediate trace name flush_cache level 1';
SQL> alter session set events '10046 trace name context forever,level 12';
第一次实验:
SQL> select count(1) from test;
COUNT(1)
----------
51255
第一次实验查看10046文件:
ARSING IN CURSOR #5 len=27 dep=0 uid=61 oct=3 lid=61 tim=3414138868 hv=1143379599 ad='302eb520'
select count(1) from test
END OF STMT
PARSE #5:c=46875,e=236549,p=330,cr=59,cu=0,mis=1,r=0,dep=0,og=1,tim=3414138867
BINDS #5:
EXEC #5:c=0,e=45,p=0,cr=0,cu=0,mis=0,r=0,dep=0,og=1,tim=3414139031
WAIT #5: nam='SQL*Net message to client' ela= 2 driver id=1413697536 #bytes=1 p3=0 obj#=95737 tim=3414139067
WAIT #5: nam='SQL*Net message from client' ela= 211 driver id=1413697536 #bytes=1 p3=0 obj#=95737 tim=3414139327
WAIT #5: nam='db file scattered read' ela= 4594 file#=6 block#=83852 blocks=5 obj#=95737 tim=3414143992
WAIT #5: nam='db file scattered read' ela= 424 file#=6 block#=83857blocks=4 obj#=95737 tim=3414144545
WAIT #5: nam='db file scattered read' ela= 161 file#=6 block#=83862 blocks=3 obj#=95737 tim=3414144820
WAIT #5: nam='db file scattered read' ela= 288 file#=6 block#=83866 blocks=7 obj#=95737 tim=3414145211
WAIT #5: nam='db file scattered read' ela= 314 file#=6 block#=83873 blocks=8 obj#=95737 tim=3414145670
WAIT #5: nam='db file scattered read' ela= 404 file#=6 block#=83882 blocks=7 obj#=95737 tim=3414146232
WAIT #5: nam='db file scattered read' ela= 156 file#=6 block#=83889 blocks=3 obj#=95737 tim=3414146536
WAIT #5: nam='db file scattered read' ela= 195 file#=6 block#=83893 blocks=4 obj#=95737 tim=3414146840
WAIT #5: nam='db file scattered read' ela= 280 file#=6 block#=83898 blocks=7 obj#=95737 tim=3414147226
WAIT #5: nam='db file scattered read' ela= 1438 file#=6 block#=83905 blocks=8 obj#=95737 tim=3414148811
WAIT #5: nam='db file scattered read' ela= 340 file#=6 block#=83914 blocks=7 obj#=95737 tim=3414149308
WAIT #5: nam='db file scattered read' ela= 365 file#=6 block#=83921 blocks=8 obj#=95737 tim=3414149813
WAIT #5: nam='db file scattered read' ela= 285 file#=6 block#=83930 blocks=6 obj#=95737 tim=3414150250
WAIT #5: nam='db file scattered read' ela= 128 file#=6 block#=83937 blocks=2 obj#=95737 tim=3414150515
WAIT #5: nam='db file scattered read' ela= 219 file#=6 block#=83940 blocks=5 obj#=95737 tim=3414150826
WAIT #5: nam='db file scattered read' ela= 343 file#=6 block#=83946 blocks=7 obj#=95737 tim=3414151317
WAIT #5: nam='db file scattered read' ela= 382 file#=6 block#=100409 blocks=5 obj#=95737 tim=3414151838
WAIT #5: nam='db file scattered read' ela= 187 file#=6 block#=100415 blocks=2 obj#=95737 tim=3414152147
WAIT #5: nam='db file scattered read' ela= 190 file#=6 block#=100418 blocks=2 obj#=95737 tim=3414152418
WAIT #5: nam='db file scattered read' ela= 249 file#=6 block#=100421 blocks=4 obj#=95737 tim=3414152745

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Compared with other programming languages, MySQL is mainly used to store and manage data, while other languages such as Python, Java, and C are used for logical processing and application development. MySQL is known for its high performance, scalability and cross-platform support, suitable for data management needs, while other languages have advantages in their respective fields such as data analytics, enterprise applications, and system programming.

MySQL is worth learning because it is a powerful open source database management system suitable for data storage, management and analysis. 1) MySQL is a relational database that uses SQL to operate data and is suitable for structured data management. 2) The SQL language is the key to interacting with MySQL and supports CRUD operations. 3) The working principle of MySQL includes client/server architecture, storage engine and query optimizer. 4) Basic usage includes creating databases and tables, and advanced usage involves joining tables using JOIN. 5) Common errors include syntax errors and permission issues, and debugging skills include checking syntax and using EXPLAIN commands. 6) Performance optimization involves the use of indexes, optimization of SQL statements and regular maintenance of databases.

MySQL is suitable for beginners to learn database skills. 1. Install MySQL server and client tools. 2. Understand basic SQL queries, such as SELECT. 3. Master data operations: create tables, insert, update, and delete data. 4. Learn advanced skills: subquery and window functions. 5. Debugging and optimization: Check syntax, use indexes, avoid SELECT*, and use LIMIT.

MySQL efficiently manages structured data through table structure and SQL query, and implements inter-table relationships through foreign keys. 1. Define the data format and type when creating a table. 2. Use foreign keys to establish relationships between tables. 3. Improve performance through indexing and query optimization. 4. Regularly backup and monitor databases to ensure data security and performance optimization.

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