Oracle database knowledge points that cannot be learned (III)

Catalog

Locks and database objects

1. lock

2. Database objects

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Locks and database objects

1. lock

The mechanism used by database to control concurrent access to shared resources .

The type of lock ： Row-level locks , Table lock .

Row-level locks ： Lock the row being modified . Row lock is also called exclusive lock .

When using the following statements ,Oracle Will automatically apply row level lock ：

insert,update,delete,select…… for update

select……for update Allow users to lock multiple records for update at a time .

Use commit or rollback Release the lock .

Table lock ：

lock table user_tbl in mode mode;

Table lock type ：

Line sharing row share

OK, exclusive row exclusive

share share

Shared row exclusive share row exclusive

Exclusive exclusive

Deadlock ： Two or more transactions wait for each other to release resources , So it's a deadlock

2. Database objects

oracle Database objects are also called schema objects

Database objects are collections of logical structures , The most basic database objects are tables

Database objects ：

surface , Sequence , View , Indexes .

Sequences are used to generate unique , Objects with consecutive numbers .

Create Syntax ：

create sequence user_id_seq

start with 1000

increment by 1

maxvalue 2000

minvalue 1000

nocycle

cache 1000;-- Specify the sequence number preallocated in memory 

Access sequence ：

select user_id_seq.currval from dual;

select user_id-seq.nextval from dual;

Change delete sequence ：

alter sequence user_id_seq maxvalue 10000;-- It cannot be modified start with  value 

drop sequence user_id_seq;

stay Hibernate Medium access sequence ：

<generator class="sequence">

<param name="sequence">     

user_id_seq

</param>

</generator>

View

Display data from one or more tables in a customized way

Create view ：

create or replace view

user_tbl_view (vid,vname,vage)

as select id,user_name,age from user_tbl

[with check option]|[with read only];

Create a view with errors ：

create force view user_tbl_force_view as

select * from user_table;-- here user_table Can not exist 

Create an outer join view ：

create view user_stu_view as

select u.id,u.user_name,u.password,s.ddress

from user_tbl u,stu_tbl s

where u.s_id(+)=s.id;-- Which side has (+), Which side is secondary 

Delete view ：

drop user_stu_view;

The index is used to improve SQL The performance of statement execution

Index type ：

unique index , Bitmap index , Composite index , Function based index , Reverse key index

Create a standard index ：

create index user_id_index on user_tbl(id) tablespace schooltbs;

Rebuild index ：

alter index user_id_index rebuild;

Delete index ：

drop index user_id_index;

Create unique index ：

create unique index user_id_index on user_tbl(id);

Create a composite index ：

create index name_pass_index on user_tbl(user_name,password);

Create reverse key index ：

create index user_id_index on user_tbl(id) reverse;